Patent Publication Number: US-2021165055-A1

Title: Circuitry for detecting jack plug insertion or removal

Description:
FIELD OF THE INVENTION 
     This application relates to circuitry for detecting at least partial removal and/or insertion of an audio jack plug from or into a corresponding socket and, in particular, to circuitry for detecting disconnection of an accessory device from and/or connection of an accessory device to a connector of an extension cable, splitter or the like that is connected to a connector of a host device. 
     BACKGROUND 
     Many electronic devices include a suitable connector for removably connecting an accessory device to the electronic device. For example many electronic devices have a connector to connect audio accessories to the electronic device. Mobile telephones, tablets, laptop computers and the like are examples of electronic devices, also referred to as “host devices”, that are operable with audio accessory devices such as headphones, earphones, earbuds and headsets, for example, that are external to and distinct from the electronic device. Such audio accessories typically comprise mono or stereo speakers for audio playback and some audio accessories may also have a microphone for voice communication. 
     Such external accessory devices are often connected to the host electronic device via a mating connector arrangement such as a plug and socket. For instance, many audio accessories have a jack plug for connection to a suitable jack socket of the host electronic device. A well-known arrangement for a jack plug and its associated socket is the 3.5 mm 4-pole TRRS (Tip-Ring-Ring-Sleeve) configuration, which has four contacts, one contact for each of left audio, right audio, microphone, and ground. 
     In many devices it is desirable for the host electronic device to be able to detect when an accessory is connected to and/or disconnected from the host device. Thus commonly the host device may comprise circuitry such as a “jack detect” circuit to allow the host device to determine when a suitable accessory device connector has been connected to the host device (e.g. when a plug of an accessory device has been inserted into a socket of the host device). Various different types of jack detect arrangement are known. For instance, the presence of a suitable jack plug inserted fully into a socket may operate a mechanical switch to complete or disconnect a circuit coupled to a jack detect signal line. Monitoring the jack detect signal line, e.g. by comparing a voltage of the jack detect signal line with a known threshold voltage, gives an indication of whether a jack plug is inserted or not. In other arrangements the jack plug itself may form part of a jack detect circuit when inserted. Upon detection of the presence of a connector (e.g. a plug) of the audio accessory device in the connector (e.g. a socket) of an electronic device, the circuitry of the electronic device will be configured appropriately for operation with the accessory device e.g. to supply audio to the accessory device. 
     The detection circuitry also provides an indication of when the connector of the accessory device has been disconnected or removed from the connector of the host electronic device. This indication can cause the electronic device to react appropriately e.g. by suspending the generation and output of audio signals, thereby reducing power consumption of the electronic device, since audio output signals are not generated unnecessarily. 
     Some users may connect an accessory device such as a set of headphones, earphones, earbuds, a headset or the like to a host electronic device via a suitable extension cable. The extension cable may be formed of a suitable 3.5 mm TRRS jack plug at one end, with a corresponding TRRS jack socket at the other end. The plug of an accessory device can be received in the socket of the extension cable. 
     When an extension cable is used in this way, the jack detection circuitry of the host device will detect that a connector is received in the connector of the host electronic device, even when there is no accessory device connected to the extension cable. Thus audio signals may still be generated and output by the electronic device even when the accessory device is not connected to the connector of the extension cable, which can lead to unnecessary power consumption by the host electronic device. 
     Similarly, a splitter cable having a single jack plug at one end coupled to two or more parallel jack sockets at the other end may be used to connect two or more accessory devices to a host electronic device. The jack plug of the splitter cable is received in the socket of the host electronic device. Each socket of the splitter cable can receive the plug of a different accessory device. 
     When a splitter cable is used in this way, the jack detection circuitry of the host device will detect that a connector is received in the connector of the host electronic device, even when there is no accessory device connected to the splitter cable. Thus audio signals may still be generated and output by the electronic device even when no accessory device is connected to a connector of the splitter cable, which can lead to unnecessary power consumption by the host electronic device. 
     Also, when an extension cable or splitter cable is used in this way, the jack detection circuitry of the host device will not detect that a connector is removed from the far end of the extension or splitter cable (e.g. that a plug of an accessory device has been removed from a socket of the extension cable or splitter cable). Thus, audio signals may still be generated and output by the electronic device even when the accessory device is not connected to the connector of the extension cable or splitter cable, which can lead to unnecessary power consumption by the host electronic device. 
     Polling of the impedance of signal paths coupled to the left (L) and right (R) contacts of the socket of the host device can be employed in order to determine if such an accessory device is subsequently connected to the connector of the extension cable or splitter cable. Such polling typically involves temporarily connecting a DC voltage source to the relevant socket contact and measuring a parameter (e.g. a current) indicative of the impedance of the signal path. Whilst this approach is acceptable when audio signals are not being output to the connected accessory device, it is not viable when audio signals are being output, as the injection of DC voltages would give rise to unacceptable audio artefacts such as clicks and pops. 
     In some situations the plug of an extension cable or splitter cable may not correspond to the socket of the host device. For example, the host device may have a four-pole TRRS socket, whereas the extension cable or splitter cable may have a three pole tip, ring sleeve (TRS) plug at one end and a corresponding three pole TRS socket at the other end. Most host devices are capable of recognising if a three-pole plug is plugged into a four-pole socket and reconfiguring themselves as necessary for correct audio output. Thus, if an accessory device having a complementary TRS plug is connected to the TRS socket of the extension cable or splitter cable, audio output to the accessory device will typically be unaffected. However, if an accessory device having a four-pole plug (e.g. a TRRS plug) is connected to the three-pole (e.g. TRS) socket of the extension cable or splitter cable, the host device cannot recognise that this has happened and thus cannot reconfigure itself for correct audio output. Thus, in such circumstances audio playback on the accessory device may be distorted or attenuated, or may even not be possible. 
     There is thus a desire for a way to detect the disconnection of an accessory device from a host electronic device that can be used without degrading audio signals being output to the accessory device, even when the accessory device is connected to the host electronic device via an extension cable or splitter cable. 
     A desire also exists to detect mismatches between the plug of an accessory device and the socket of an extension cable, splitter cable or the like to which the accessory device is connected when the extension cable, splitter cable or the like is connected to a host device. In particular, a desire exists to detect when an accessory device having a four pole (e.g. TRRS) connector is connected to a three-pole connector of an extension cable, splitter cable or the like. 
     SUMMARY 
     According to a first aspect the invention provides circuitry for detecting connection of a plug of an audio accessory device to a socket of an intermediate cable that is connected to a host device, wherein the plug of the accessory device is of a different type than the socket of the intermediate cable, the circuitry comprising:
         a monitoring unit comprising:
           a first terminal configured to be electrically connected to a first socket contact of the socket that is in electrical contact with a first plug contact of the plug when the plug is fully received in the socket; and   a second terminal configured to be electrically connected to a second socket contact of the socket that is in electrical contact with a second plug contact of the plug when the plug is fully received in the socket,   
               

     wherein the monitoring unit is configured to:
         detect a first impedance of a first signal path from the first terminal;   detect a second impedance of a second signal path from the first terminal; and   detect a third impedance of a third signal path from the second terminal,       

     wherein the circuitry is configured to output a signal indicative of detection of connection of the plug to the socket in response to detection by the monitoring unit that the detected first, second and third impedances do not correspond to expected first, second and third impedances. 
     The second terminal may be configured to act as an end point of the first signal path. 
     The circuitry may further comprise a third terminal configured to be electrically connected to a third socket contact of the socket that is in electrical contact with a third plug contact of the plug when the plug is fully received in the socket, wherein the third terminal is configured to act as an end point of the second and third signal paths. 
     The circuitry may be configured to output the signal in response to detection by the monitoring unit that the sum of the second and third impedances does not correspond to the first impedance. 
     The plug may be a four-pole jack plug and the socket may be a three-pole socket. 
     For example, the plug may be a TRRS plug and the socket may be a TRS socket. 
     The intermediate cable may be an extension cable or a splitter cable, for example. 
     The monitoring unit may be configured to determine the first, second and third impedances in response to a request received by the host device. 
     The request may be a request for audio output, for example. 
     The monitoring unit may be configured to determine the first, second and third impedances periodically. 
     Alternatively, the monitoring unit may be configured to monitor the first, second and third impedances continuously. 
     The circuitry may be implemented as an integrated circuit. 
     A second aspect of the present disclosure provides an electronic device comprising circuitry according to the first aspect. 
     A third aspect of the present disclosure provides a method for detecting connection of a plug of an audio accessory device to a socket of an intermediate cable that is connected to a host device, wherein the plug of the accessory device is of a different type than the socket of the intermediate cable, the method comprising:
         detecting a first impedance of a first signal path from a first terminal of a monitoring unit, wherein the first terminal is configured to be electrically connected to a first socket contact of the socket that is in electrical contact with a first plug contact of the plug when the plug is fully received in the socket;   detecting a second impedance of a second signal path from the first terminal;   detecting a third impedance of a third signal path from a second terminal, wherein the second terminal is configured to be electrically connected to a second socket contact of the socket that is in electrical contact with a second plug contact of the plug when the plug is fully received in the socket; and   outputting a signal indicative of detection of connection of the plug to the socket in response to detection that the detected first, second and third impedances do not correspond to expected first, second and third impedances.       

     A fourth aspect of the present disclosure provides circuitry for detecting at least partial removal of a plug of an audio accessory device from a socket of an intermediate cable that is connected to a host device, the circuitry comprising: 
     a monitoring unit comprising:
         a first terminal configured to be electrically connected to a first socket contact of the socket that is in electrical contact with a first plug contact of the plug of the audio accessory device when the plug of the audio accessory device is fully received in the socket of the intermediate cable,   wherein the monitoring unit is configured to monitor a first impedance of a first signal path coupled to the first terminal, and   wherein the circuitry is configured to output a signal indicative of detection of at least partial removal of the plug of the audio accessory device from the socket of the intermediate cable in response to detection by the monitoring unit of a change in an impedance state of the first signal path from a first impedance state in which the impedance of the first signal path has a first impedance value to a second, high impedance state.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which: 
         FIG. 1  illustrates an example of circuitry for detecting at least partial removal of a jack plug from a corresponding socket; 
         FIGS. 2 a - c    illustrate an example of the removal of a four-pole jack plug from a corresponding socket over a sequence of different removal states; 
         FIGS. 3 a - d    illustrate another example of the removal of a four-pole jack plug from a corresponding socket over a sequence of different removal states; 
         FIGS. 4 a - d    illustrate a further example of the removal of four-pole jack plug from a corresponding socket over a sequence of different removal states; 
         FIG. 5 a    illustrates an example of circuitry for estimating the impedance of a signal path; 
         FIG. 5 b    illustrates another example of circuitry for estimating the impedance of a signal path; 
         FIGS. 6 a -6 c    illustrate an example of the removal of a three-pole jack plug from a corresponding socket over a sequence of different removal states; 
         FIG. 7 a -7 f    illustrate examples of different scenarios in which the removal of a jack plug from a corresponding socket can be detected; 
         FIG. 8  illustrates a situation in which a three-pole TRS jack plug (e.g. of an accessory device or an intermediate cable) is received in a four-pole TRRS socket of a host device; 
         FIGS. 9 a -9 f    illustrate circuitry for detecting the presence of a four-pole jack plug of an audio accessory device in three-pole sockets of different configurations of an intermediate cable; 
         FIGS. 10 a -10 j    illustrate examples of the removal of four-pole jack plug of an audio accessory device from three-pole sockets of different configurations of an intermediate cable over a sequence of different removal states; and 
         FIGS. 11 a -11 b    illustrate examples of different scenarios in which the insertion of a jack into, and the removal of a jack plug from, a mismatched socket of an intermediate cable can be detected. 
     
    
    
     DETAILED DESCRIPTION 
     The description below sets forth example embodiments according to this disclosure. Further example embodiments and implementations will be apparent to those having ordinary skill in the art. Further, those having ordinary skill in the art will recognize that various equivalent techniques may be applied in lieu of, or in conjunction with, the embodiments discussed below, and all such equivalents should be deemed as being encompassed by the present disclosure. 
       FIG. 1  illustrates circuitry  100  for detecting at least partial removal of an electrical plug  160  from a corresponding electrical socket  150 . In this example, the plug  160  is a conventional 3.5 mm TRRS (Tip-Ring-Ring-Sleeve) jack plug with four contacts, namely: a tip (T) contact  162 ; a first ring (R) contact  164 ; a second ring (R) contact  166 ; and a sleeve (S) contact  168 . The electrical socket  150  in this example is a 4-pole socket with TRRS contacts  152 - 158  (the sleeve contact  158  is shown in dashed outline  FIG. 1 ). As will be appreciated, when the plug  160  is fully received in the socket  150 , the T, R, R and S contacts  152 - 158  of the socket  150  are in electrical contact with the corresponding TRRS contacts  162 - 168  of the plug  160 . 
     The circuitry  100  may form part of a host electronic device such as a mobile phone, tablet or laptop computer or the like, and the plug  160  may form part of an accessory device such as a set of headphones, earphones, earbuds, a headset or the like. The insertion of the plug  160  into the socket  150  may therefore provide a mating connection between an accessory device and a host electronic device. 
     The circuitry  100  further comprises a monitoring unit  170 , which may comprise discrete circuitry, integrated circuitry, processor circuitry configured to execute suitable software instructions or any combination of discrete circuitry, integrated circuitry, processor circuitry and software. The monitoring unit  170  is configured to monitor a first impedance of a first signal path associated with the socket tip contact  152 , and a second impedance of a second signal path associated with the first socket ring contact  154 . To this end the monitoring unit  170  includes a first terminal  110 , a second terminal  120 , a third terminal  130  and a fourth terminal  140 . The first terminal  110  is electrically coupled to the tip contact  152  of the socket  150  via a conductor (e.g. a PCB trace, wire or the like)  112 , the second terminal  120  is electrically coupled to the first ring contact  154  of the socket  150  via a conductor  122 , the third terminal  130  is electrically coupled to the second ring contact  156  of the socket  150  via a conductor  132 , and the fourth terminal  140  is electrically coupled to the sleeve contact  158  of the socket  150  via a conductor  142 . 
     The monitoring unit  170  is configured to monitor, continuously or periodically, the impedance of the audio accessory via a first signal path between the first terminal  110  and the third terminal  130  and the impedance of the audio accessory via a second signal path between the second terminal  120  and the third terminal  130 , in order to detect a partial or complete removal of the plug  160  from the socket  150 . 
     For example, a stereo audio accessory comprising left and right speakers (represented in  FIG. 1  by impedances R L , R R  respectively) but no microphone may include a 3.5 mm TRRS plug  160  that can be received in the socket  150 . A left speaker of the audio accessory is typically connected to the tip contact  162  of the plug  164 , whilst a right speaker of the audio accessory is typically connected to the first ring contact  164  of the plug  164 . The second ring contact  166  and possibly also the sleeve contact  168  of the plug  160  typically provides a ground connection for the audio accessory. 
     Thus, when the plug  160  of such an audio accessory device is fully received in the socket  150  of a host device, the tip contact  162  of the plug  160  will be electrically connected to the first terminal  110  of the monitoring unit  170 , the first ring contact  164  of the plug  160  will be electrically connected to the second terminal  120  of the monitoring unit  170 , the second ring contact  166  of the plug  160  will be electrically connected to the third terminal  130  of the monitoring unit  170  and the sleeve contact  168  of the plug  160  will be electrically connected to the fourth terminal  140  of the monitoring unit  170 . 
     Accordingly, when the plug  160  of the audio accessory is fully received in the socket  150  of the host device the impedance of a first signal path between the first terminal  110  and the third terminal  130 , via the audio accessory, as measured by the monitoring unit  170  (which may be referred to as the first measured impedance), will be approximately equal to the impedance R L  of the left speaker of the audio accessory, and the impedance of a second signal path between the second terminal  120  and the third terminal  130 , via the audio accessory, as measured by the monitoring unit  170  (which may be referred to as the second measured impedance), will be approximately equal to the impedance R R  of the right speaker of the audio accessory. 
     Conversely, if no plug  160  is received in the socket  150  the first measured impedance will be very high, since the first signal path is open circuit, and the second measured impedance will also be very high, since the second signal path is also open circuit. 
     Thus, the impedance measurements made by the impedance monitoring unit  170  can be used to determine whether or not a plug  160  is received in the socket  150 . For example if the first and second measured impedances are relatively low, e.g. ˜16Ω, then it may be determined that a plug is present in the socket  150 , since such measured impedances may correspond to the impedances, or expected impedance ranges, of the first and second speakers of an audio accessory. If the first and second measured impedances are relatively high, e.g. &gt;10 kΩ, then it may be determined that there is no plug present in the socket  150 , since such measured impedances may be indicative of open circuit conditions in the first and second signal paths. 
     The circuitry  100  may thus use the measured impedance values of the first and second signal paths between the terminals of the monitoring unit, via the audio accessory, to determine whether or not a plug is received in the socket  150 , or may alternatively use relative impedance states of the first and second signal paths. In the example of  FIG. 1 , in which the plug  160  is fully received in the socket  150 , a first impedance state, detected at the first terminal  110 , and a second impedance state, detected at the second terminal  120 , will both be low impedance (since the measured impedances of the associated first and second signal paths are relatively low), thus indicating that the plug  160  is fully received in the socket  150 . 
     Thus, the monitoring unit  170  and associated circuitry  100  may implement “jack detect” functionality: when the monitoring unit  170  detects that both the first impedance state at the first terminal  110  and the second impedance state at second terminal  120  are low impedance (e.g. the first and second measured impedances correspond to left and right audio accessory speaker impedances respectively), then the monitoring unit  170  may determine that a plug is received in the socket  150 . 
     However, as the plug  160  is being removed from the socket  150 , the impedance states at the first and second terminals  110 ,  120  will change, as will be described in relation to  FIGS. 2 a - c , 3 a - d  and 4 a - d   . When the impedance state at at least the first terminal  110 , changes in accordance with a predetermined sequence, this will be indicative of at least partial removal of the plug  160  from the socket  150 . The circuitry  100  is therefore configured to output a signal S indicative of detection of at least partial removal of the plug  160  from the socket  150  in response to detection by the monitoring unit  170  of a predetermined sequence of impedance states detected at the first and/or second terminals  110 ,  120 . 
     This jack detect functionality may complement existing jack detect circuitry or functionality in a host device, such that in the event of failure of such existing jack detect circuitry (e.g. failure of a mechanical switch or contact used by the existing jack detect circuitry), the insertion of a plug into a socket of the host device and the removal of a plug from the socket of the host device can still be detected. 
     Alternatively, the jack detect functionality implemented by the monitoring unit  170  and associated circuitry  100  may replace other types of jack detect circuitry that may otherwise be provided in a host device. This may help to reduce a bill of materials cost of the host device, since, for example, a mechanical switch component that would otherwise be required for jack detect purposes can be omitted. 
     Moreover, the monitoring unit  170  and associated circuitry  100  are able to detect connection or disconnection of an audio accessory to or from a host device even when the audio accessory is connected indirectly to the host device, e.g. via an extension cable or splitter cable that remains connected to the host device when the accessory device is disconnected from the extension cable or splitter cable. 
       FIGS. 2 a - c    illustrate an example of an audio jack plug  260  being removed from a corresponding socket  150  over a sequence of different removal states. The elements in common between  FIG. 1  and  FIGS. 2 a - c    are given corresponding reference numerals. 
     In the illustrated example of  FIG. 2 , a plug  260  may comprise a TRRS jack plug to provide a connection to an audio accessory device such as a set of stereo headphones that does not include a microphone. A common configuration for the jack plug for such an accessory device is that the tip and first ring contacts  262 ,  264  provide connections for the left audio and right audio loads (e.g. left and right speakers), respectively, with the second ring and sleeve contact  266 ,  268  being connected together and providing a ground connection for the accessory device. Thus, as illustrated in  FIG. 2 a   , the plug tip (T) contact  262  provides a connection to the left audio load R L . Similarly, the first plug ring (R) contact  264  provides a connection to the right audio load R R . It will be appreciated that both the left audio load R L  and the right audio load R R  will be substantially the same and therefore the impedance of either load may be expressed as R LOAD . The second plug ring (R) contact  266  and the plug sleeve contact  268  of the plug  260  may both provide a connection to ground. 
     Therefore, as illustrated in  FIG. 2 a   , when the plug  160  is fully received in the socket  150 , the first terminal  110  of the monitoring unit  170  is electrically connected to the left load R L  at the plug tip contact  262  via the socket tip contact  152 , while the second terminal  120  of the monitoring unit  170  is electrically connected to the right load R R  at the first plug ring contact  264  via the first socket ring contact  154 . 
     The monitoring unit  170  further comprises a third terminal  130  and a fourth terminal  140 , which are electrically connected to the second socket ring contact  156  and the socket sleeve contact  158  respectively, via respective conductors such as PCB tracks, wires or the like. As illustrated in  FIG. 2 a   , when the plug  160  is fully received in the socket  150 , the second socket ring contact  156  is electrically connected to the second plug ring contact  266  and the socket sleeve contact  158  is electrically connected to the plug sleeve contact  268 . As described above, the second plug ring contact  266  and the plug sleeve contact  268  both provide a contact for connection to ground. Therefore, the third terminal  130  and the fourth terminal  140  connect the second plug ring contact  266  and the plug sleeve contact  268  to ground G, when the plug  160  is fully received in the socket  150 . 
     With both the third and fourth terminals  130 ,  140  connected to ground G, either of these terminals may provide a suitable reference from which impedance measurements may be taken. Therefore, impedance measurements may be taken for a first signal path from the first terminal  110  to either the third terminal  130  or the fourth terminal  140 , via the audio accessory, and for a second signal path from the second terminal  120  to either the third terminal  130  or the fourth terminal  140 , via the audio accessory. In other words, a first impedance state may be detected at the first terminal  110  for a first signal path between the first terminal  110  and either the third terminal  130  or the fourth terminal  140 , and a second impedance state may be detected at the second terminal  120 , for a second signal path between the second terminal  120  and either the third terminal  130  or the fourth terminal  140 . 
     In the illustrated example of  FIGS. 2 a - c   , the fourth terminal  140  is used as the end of the first and second signal paths for the purpose of detecting the first and second impedance states. 
       FIG. 2 a    illustrates the plug  260  and the socket  150  in an initial (or first) removal state, in which the plug  260  is fully inserted in the socket  150 . In this initial removal state, the impedance state at both the first terminal  110  and the second terminal  120  will be low, as the first and second terminals  110 , 120  are in electrical contact with the plug tip and first ring contacts  262 ,  264  via socket contacts  152 , 154 , respectively. The first signal path from the first terminal  110  to the fourth terminal  140 , via the audio accessory, includes the left load R L , and therefore the impedance of the first signal path will be measured as R LOAD . Accordingly, the first impedance state, detected at the first terminal  110 , is low impedance. Similarly, the second signal path from the second terminal  120  the fourth terminal  140 , via the audio accessory, includes the right audio load R R . Therefore, the impedance of the second signal path will also be measured as R LOAD  by monitoring unit  170 . Thus the second impedance state, detected at the second terminal  120 , is also low impedance. 
       FIG. 2 b    illustrates a second removal state of the plug  260  and the socket  150 , in which the plug  260  is partially removed from the socket  150 . In the second removal state, the plug  260  has been partially extracted from the socket  150 , such that the plug sleeve contact  268  is no longer received in the socket  150 . In the second removal state, the socket tip contact  152  is not in electrical contact with any of the plug contacts  262 - 268 . The signal path from the first terminal  110  is therefore open circuit. As such, the first impedance state, detected at the first terminal  110 , will be high impedance. 
     In the second removal state, the second terminal  120  is electrically connected to the plug tip contact  262  via the first socket ring contact  154 . The signal path between the second terminal  120  and the fourth terminal  140 , via the audio accessory, therefore includes the left audio load R L . As the impedances of the left audio load R L  and the right audio load R R  are substantially the same, the impedance of this signal path will therefore again be measured as R LOAD  in the second removal state, and thus the impedance state, detected at the second terminal  120 , when the plug  160  and the socket  150  are in the second removal state will be low impedance. 
       FIG. 2 c    illustrates the plug  260  and the socket  150  in a third removal state, which for the purposes of the present disclosure is equivalent to the full removal of the plug  260  from the socket  150 . In the third removal state, the plug tip contact  262  and the first plug ring contacts  264  are received in the socket  150 , in contact with the second socket ring contact  156  and the socket sleeve contact  158  respectively. However, the second socket ring contact  156  and the socket sleeve contact  158  are both grounded. The left and right audio contacts of the plug  260  (i.e. the socket tip contact  262  and the first socket ring contact  264 ) are therefore no longer in contact with the socket contacts of the socket  150  through which audio signals can be supplied to the left and/or right plug contacts (i.e. the socket tip contact  152  and the first socket ring contact  154 ). Therefore, neither of left audio load R L  and right audio load R R  can be driven in the third removal state. Accordingly, when the plug  260  and the socket  150  adopt the third removal state, the plug  260  will be considered to be removed from the socket  150  for the purposes of the present disclosure. 
     In the third removal state, the first terminal  110  is again not connected to any of the plug contacts  262 - 268 . Therefore the first impedance state, detected at the first terminal  110 , will again be high impedance. The first socket ring contact  154  is no longer in electrical contact with any of the plug contacts  262 - 268  in the third removal state. Therefore, the second impedance state, detected at the second terminal  120 , will also be high impedance. 
     As will be apparent from  FIG. 2 c   , in the third removal state a third signal path between the third and fourth terminals  130 ,  140  of the monitoring unit  170 , via the audio accessory, will include the left and right loads R L , R R  and thus the impedance of this third signal path will be approximately equal to 2R LOAD . The monitoring unit  170  could be configured to monitor the impedance of the third signal path, either continuously or in response to detection of high impedance states of the first and second signal paths, in order to detect or verify removal of the plug  260  from the socket  150  by detecting a change of the impedance of the third signal path to approximately 2R LOAD . 
     The sequence of values of the first and second impedance states as the plug  260  is removed from the socket  150  over the first to third removal states illustrated in  FIGS. 2 a - c    may therefore be expressed according to Table 1: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 First  
                 Second 
               
               
                   
                   
                 Impedance 
                 Impedance 
               
               
                   
                 Removal State 
                 State 
                 State 
               
               
                   
                   
               
             
            
               
                   
                 First 
                 Low-Z (R LOAD ) 
                 Low-Z (R LOAD ) 
               
               
                   
                 Second 
                 High-Z 
                 R LOAD   
               
               
                   
                 Third 
                 High-Z 
                 High-Z 
               
               
                   
                   
               
            
           
         
       
     
     The changing sequence of the first and/or second impedance states detected at the first and/or second terminals  110 ,  120  respectively may therefore be indicative of the removal of the plug  260  from the socket  150 . As the plug  260  is removed from the socket  150 , the plug  260  and socket  150  will sequentially adopt the first, second and third removal states illustrated in  FIGS. 2 a - c   . The monitoring unit  170  is therefore configured to detect a sequence of first and/or second impedance states, and when the detected sequence of first and/or second impedance states corresponds to the relevant sequence(s) in Table 1, the monitoring unit  170  will detect that the plug  260  has been removed from socket  150 . 
     The circuitry  100  is configured to output a signal S indicative of detection of at least partial removal of the plug  260  from the socket  150  when the monitoring unit  170  detects this predetermined sequence. The signal S may be sent to a controller (not illustrated) of the host device, which may, in response to the signal S, suspend the generation and supply of audio signals to the socket  150 , thereby reducing power consumption of the host device, since audio signals are not unnecessarily generated and output. 
     The monitoring unit  170  may comprise a processor and/or circuitry configured to detect one or more of the predetermined sequences denoted in Table 1, indicative of removal of the plug  260  from socket  150 . In another example the monitoring unit  170  may detect the first and second impedance states (e.g. by measuring the impedances of signal paths from the first and second terminals  110 ,  120 , as described above) and transmit the detected impedance states to a downstream processor. The downstream processor may log the detected sequences of the first and/or second impedance states and, when one or more of the logged sequences of impedance states corresponds to a predetermined sequence, may output the signal S to a controller to suspend audio output by the host device. 
       FIGS. 3 a - d    illustrate another example of an audio jack plug  360  being removed from a corresponding socket  150  over a sequence of different removal states. The elements in common between  FIG. 1 ,  FIGS. 2 a - c    and  FIGS. 3 a - d    are given corresponding reference numerals. 
     In the example of  FIGS. 3 a - d   , a plug  360  may form part of an audio accessory device which has microphone capabilities, e.g. a stereo headset. One TRRS plug contact configuration for such an accessory device is left audio, right audio, microphone and ground, respectively. Therefore, as illustrated in  FIG. 3 a   , the plug tip and first plug ring contacts  362 ,  364  comprise the left audio contact and right audio contacts, illustrated by left load R L  and right load R R , respectively, in  FIG. 3 a   . As in the example of  FIG. 2 a   , the first terminal  110  is electrically connected to the plug tip contact  362  when the plug  360  is fully received in socket  150 . Similarly, the second terminal  120  is electrically connected to the first plug ring contact  364  when the plug  360  is fully received in the socket  150 . 
     The second plug ring contact  366  comprises the microphone contact of the plug  360 , which is illustrated by microphone MIC at the second plug ring contact  366 . When the plug  360  is fully received in the socket  150  as shown in  FIG. 3 a   , the third terminal  130  of the monitoring unit  170  is electrically connected to the second plug ring contact  366 . 
     The plug sleeve contact  368  provides a ground contact of the plug  360 . When the plug  360  is fully received in the socket  150  as shown in  FIG. 3 a   , the fourth terminal  140  of the monitoring unit  170  is electrically connected to the plug sleeve contact  368 . The fourth terminal  140  is also connected to a ground plane or rail of the circuitry  100 . 
     As described with reference to  FIGS. 2 a - c   , the fourth terminal  140  acts as the end of the first and second signal paths (which start at the first and second terminals  110 ,  120  respectively) for which the first and second impedances are measured by the monitoring unit  170 . 
     As illustrated in  FIG. 3 a   , with the plug  360  fully received in the socket  150  in an initial or first removal state, the first signal path between the first terminal  110  and the fourth terminal  140 , via the audio accessory, includes the left audio load impedance R L  (which is equal to R LOAD  as explained above with reference to  FIGS. 2 a -2 c   ) and the second signal path between the second terminal  120  the fourth terminal  140 , via the audio accessory, includes the right audio load impedance R R  (which is also equal to R LOAD  as explained above with reference to  FIGS. 2 a -2 c   ). Thus in this initial removal state, the first impedance state, detected at the first terminal  110 , and the second impedance state, detected at the second terminal  120 , will be both be R LOAD  (or low impedance), indicating a low measured impedance of both the first signal path between the first terminal  110  and the fourth terminal  140  and the second signal path between the second terminal  120  and the fourth terminal  140 . 
     The monitoring unit  170  may be further configured to monitor a third impedance state at the third terminal  130 . When the plug  360  is fully received in socket  150 , as shown in  FIG. 3 a   , the impedance of a third signal path between the third terminal  130  and the fourth terminal  140 , via the audio accessory, will be approximately equal to the impedance of the microphone MIC. For example, the measured impedance of the third signal path may be approximately 2.2 kΩ. 
     The changing value of the third impedance state at third terminal  130  may be used in conjunction with the changing values of the first and/or second impedance states to detect or confirm removal of the plug  360  from the socket  150 . 
     In the initial removal state, the second socket ring contact  156  is in electrical contact with the second plug ring contact  366 , which is connected to the microphone MIC. The signal path from the third terminal  130  to the fourth terminal  140 , via the audio accessory, therefore includes microphone MIC. Therefore, the impedance of the third signal path between the third terminal  130  the fourth terminal  140  will be measured as the impedance R MIC  of the microphone. 
       FIG. 3 b    illustrates a second removal state of the plug  360  and the socket  150 , in which the plug  360  is partially removed from socket  150 . In the second removal state, the first signal path from first terminal  110  is open circuit, as the socket tip contact  152  is not in contact with any of the plug contacts  362 - 368 . Therefore, the first impedance state, detected at the first terminal  110 , will be high impedance. 
     The second terminal  120  is electrically connected to the plug tip contact  362  via the first socket ring contact  154 . The second signal path from the second terminal  120  to the fourth terminal  140 , via the audio accessory, includes the left audio load impedance R L  and the microphone MIC. As the left audio load R L  impedance is substantially equal to right audio load impedance R R , the measured impedance of the second signal path will be approximately R LOAD +R MIC . The second impedance state, detected at the second terminal  120 , will therefore increase to approximately R LOAD +R MIC  in the second removal state. 
     The third signal path from the third terminal  130  to the fourth terminal  140 , via the audio accessory, includes the right audio load R R  and the microphone MIC, due to the contact between the second socket ring contact  156  and the second plug ring contact  366 . Therefore, in the second removal state the impedance of the third signal path between the third terminal  130  and the fourth terminal  140  will be measured as R LOAD +R MIC . Thus the detected impedance state at the third terminal  130  in the second removal state will be approximately R LOAD +R MIC . 
       FIG. 3 c    illustrates a third removal state of the plug  360  and the socket  150 , in which the plug  360  is partially removed from the socket  150 . In the third removal state, the first impedance state, detected at the first terminal  110 , is again high impedance, due to the absence of any electrical connection between the first terminal  110  and any of the plug contacts  362 - 368 . Similarly, in the third removal state, the second terminal  120  is not electrically connected to any of the plug contacts  362 - 368 . Therefore, the signal path from second terminal  120  is open circuit. As such, the second impedance state, detected at the second terminal  120 , is also high impedance. 
     In the third removal state, the signal path between the third terminal  130  and the fourth terminal  140 , via the audio accessory, includes the left audio load R L  and the right audio load R R . As the impedances of the left audio load R L  and the right audio load R R  are substantially the same, the measured impedance of the third signal path in the third removal state will be measured as R L +R R =2R LOAD , and thus the detected impedance state at the third terminal in the third removal state will be approximately 2R LOAD . 
       FIG. 3 d    illustrates a fourth removal state of the plug  360  from the socket  150 , which for the purposes of the present disclosure is equivalent to the full removal of the plug  360  from the socket  150 .  FIG. 3 d    illustrates that the plug tip contact  362  is received in the socket  150  and in contact with the socket sleeve contact  158  (which is grounded as a result of its connection to the fourth terminal  140 ), but none of the other plug contacts  364 - 368  are in contact with any of the socket contacts  152 - 158 . Therefore, when the plug  360  and the socket  150  adopt the fourth removal state, the plug  360  will be considered to have been removed from the socket  150  for the purposes of this disclosure. 
     In the fourth removal state illustrated in  FIG. 3 d   , the first terminal  110 , second terminal  120  and third terminal  130  are not in electrical contact with any of the plug contacts  362 - 368 . Therefore, the signal paths from the first, second and third terminals  110 - 130  are all open circuit and therefore the first, second and third impedance states will all be high impedance. 
     The sequence of values of the first, second and third impedance states as the plug  360  is removed from the socket  150  over the first to fourth removal states illustrated in  FIGS. 3 a - d    may therefore be expressed according to Table 2: 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 First  
                 Second 
                 Third  
               
               
                   
                   
                 Impedance 
                 Impedance 
                 Impedance 
               
               
                   
                 Removal state 
                 State 
                 State 
                 State 
               
               
                   
                   
               
             
            
               
                   
                 First 
                 R LOAD   
                 R LOAD   
                 R MIC   
               
               
                   
                 Second 
                 High-Z 
                 R LOAD  + R MIC   
                 R LOAD  + R MIC   
               
               
                   
                 Third 
                 High-Z 
                 High-Z 
                 2R LOAD   
               
               
                   
                 Fourth 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                   
                   
               
            
           
         
       
     
     When a TRRS plug with a left-right-microphone-ground configuration is removed from the socket  150 , the plug  360  and the socket  150  will adopt the first to fourth removal states illustrated in  FIGS. 3 a - d   . The first, second and third impedance states will therefore adopt the changing sequence of impedance states denoted in Table 2. 
     One or more of the changing impedance states of Table 2 may therefore form another predetermined sequence of impedance states indicative of at least partial removal of a plug  360  from the socket  150 . The monitoring unit  170  may therefore be configured to detect one or more of the predetermined sequences of impedance states in Table 2 and to output a signal S indicative of detection of removal of the electrical plug  360  from the electrical socket  150  in response to the detection. 
       FIGS. 4 a - d    illustrate another example of an audio jack plug  460  being removed from a corresponding socket  150  over a sequence of different removal states. The elements in common between  FIG. 1 ,  FIGS. 2 a - c   ,  FIGS. 3 a - d    and  FIG. 4 a - d    are given corresponding reference numerals. 
       FIG. 4 a    illustrates another example of a TRRS jack plug  460  providing a connection to an audio accessory device having stereo audio and microphone capabilities. The plug  460  comprises left audio, right audio, microphone and ground contacts, as described above with reference to  FIGS. 3 a - d   . However,  FIGS. 4 a - d    illustrate an alternative configuration of the contacts, in which the microphone MIC is connected to the plug sleeve contact  468  and the ground contact is provided at the second plug ring contact  466 . The TRRS contacts of the plug  460  therefore provide left audio, right audio, microphone and ground contacts, respectively. 
     The plug tip contact  462  therefore provides a connection to the left audio load R L  and first plug ring contact  464  provides a connection to the right audio load R R . The second plug ring contact  466  provides a connection to ground and the plug sleeve contact  468  provides a connection to the microphone MIC. In the example illustrated in  FIGS. 4 a - d   , the third terminal  130  will therefore act as the end of the first, second and third signal paths (which start at the first, second and fourth terminals  110 ,  120 ,  140  respectively) whose impedances are measured by the monitoring unit  170 . The third terminal  130  may also be connected to a ground plane or rail of the circuitry  100 . 
       FIG. 4 a    illustrates a first removal state of the plug  460  and the socket  150 , in which the plug  460  is fully received in the socket  150 . The socket tip contact  152  is therefore electrically connected to the plug tip contact  462 . The first signal path from the first terminal  110  to the third terminal  130 , via the audio accessory, therefore includes the left audio load R L . Therefore, the measured impedance of the first signal path will be approximately equal to the impedance of the left audio load, and so the first impedance state detected at the first terminal  110  will be ROAD. Similarly, the second signal path from the second terminal  120  the third terminal  130 , via the audio accessory, includes the right audio load R R , due to the connection between the first socket ring contact  154  and the first plug ring contact  464 . Therefore, in the first removal state, the measured impedance of the second signal path will be approximately equal to the impedance R R  of the right audio load, and so the second impedance sate detected at the second terminal  120  will also be R LOAD . 
       FIG. 4 a    illustrates that the fourth terminal  140  is connected to the microphone MIC through the electrical contact between the socket sleeve contact  158  and the plug sleeve contact  468 . The third signal path from the fourth terminal  140  to the third terminal  130 , via the audio accessory, includes the microphone MIC, which has an impedance of R MIC . As such the measured impedance of the third signal path between the fourth terminal  140  and the third terminal  130  will be approximately R MIC . Thus, the third impedance state, detected at the fourth terminal  140 , will be R MIC . 
       FIG. 4 b    illustrates a second removal state of the plug  460  from the socket  150 , in which the plug  460  is partially removed from the socket  150 . In the second removal state the first signal path from the first terminal  110  does not electrically contact any of the plug contacts  462 - 468  and is thus open circuit. Therefore, the first impedance state detected at the first terminal  110  will be high impedance. 
     In the second removal state, the second terminal  120  is electrically connected to the plug tip contact  462  and the third terminal  130  is electrically connected to the first plug ring contact  464 . The second signal path between the second terminal  120  and the third terminal  130 , via the audio accessory, therefore includes the left load R L  and the right load R R  (which each have an impedance of R LOAD ), and so the measured impedance of the second signal path will be approximately 2R LOAD . Thus the second impedance state detected at the second terminal  120  will be 2R LOAD . 
     The fourth terminal  140  is electrically connected to the second plug ring contact  466 . The third signal path between the fourth terminal  140  and the third terminal  130 , via the audio accessory, includes the right audio load R R , and so the measured impedance of this signal path will be R R . The third impedance state at the fourth terminal  140  will therefore be R LOAD . 
       FIG. 4 c    illustrates a third removal state of the plug  460  and the socket  150 , in which the plug  460  is partially removed from the socket  150 . In the third removal state neither the first terminal  110  nor the second terminal  120  is electrically connected to any of the plug contacts  462 - 468 . Therefore the first and second signal paths (between the first terminal  110  and the third terminal  130  and between the second terminal  120  and the third terminal  130  respectively) are both open circuit and so the first and second impedance states detected at the first and second terminals  110 , 120  will both be high impedance. 
     In the third removal state illustrated in  FIG. 4 c   , the third signal path between the fourth terminal  140  and the third terminal  130 , via the audio accessory, includes the left audio load R L  and the right audio load R R  (which each have an impedance of ROAD), such that the measured impedance of the third signal path is 2R LOAD . The third impedance state, detected at the fourth terminal  140 , will therefore be 2R LOAD . 
       FIG. 4 d    illustrates a fourth removal state of the plug  460  and the socket  150 , which, as described above with reference to  FIG. 3 d   , is equivalent to the full removal of the plug  460  from the socket  150  for the purposes of the present disclosure. 
     In the fourth removal state, the first and second terminals  110 , 120  are not electrically connected to any of the plug contacts  462 - 468 . Therefore, the first and second impedance states detected at the first and second terminals respectively will both be high impedance. 
     The fourth terminal  140  is electrically connected to the plug tip contact  462  in the fourth removal state. However, there is no plug contact connected to the third terminal  130 . Therefore, the third signal path from the fourth terminal  140  to the third terminal is open circuit. Therefore, the third impedance state, detected at the fourth terminal  140 , will be high impedance in the fourth removal state illustrated in  FIG. 4   d.    
     The sequence of values of the first, second and third impedance states as the plug  460  is removed from the socket  150  over the first to four removal states illustrated in  FIGS. 4 a - d    may therefore be expressed according to Table 3: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 First 
                 Second 
                 Third  
               
               
                   
                 Impedance 
                 Impedance 
                 Impedance 
               
               
                 Removal state 
                 State 
                 State 
                 State 
               
               
                   
               
             
            
               
                 First 
                 R LOAD   
                 R LOAD   
                 R MIC   
               
               
                 Second 
                 High-Z 
                 2R LOAD   
                 R LOAD   
               
               
                 Third 
                 High-Z 
                 High-Z 
                 2R LOAD   
               
               
                 Fourth 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                   
               
            
           
         
       
     
     When a TRRS plug with a left-right-ground-microphone configuration is removed from the socket  150 , the plug  460  and the socket  150  will adopt the first to fourth removal states of the plug  460  and socket  150  illustrated in  FIGS. 4 a - d   . The first, second and third impedance states will therefore adopt the changing sequence of values denoted in Table 3. 
     One or more of the changing impedance states of Table 3 may therefore form another predetermined sequence of impedance states indicative of at least partial removal of plug  460  from socket  150 . Monitoring unit  170  may therefore be configured to detect the predetermined sequence of impedance states in Table 3 and output a signal S indicative of detection of removal of the electrical plug  460  from the electrical socket  150  in response to the detection. 
     The circuitry  100  is therefore capable of detecting a jack extraction event, i.e. the removal of a plug  160 ,  260 ,  360 ,  460  from the socket  150 . The circuitry  100  may operate in conjunction with or as an alternative to conventional “jack detect” circuitry, such as the mechanical switch implementation described above. 
     It will be appreciated that the circuitry  100  is able to detect a jack extraction event even when an accessory device is connected to a host electronic device via an extension cable or a splitter cable that is plugged into the host electronic device, as the removal of a plug  160 ,  260 ,  360 ,  460  from a socket of the extension cable will generate a number of detectable sequences of impedance states. The circuitry  100  detects the removal of an accessory device from an extension cable by detecting a predetermined sequence of impedance states, which occurs regardless of whether the plug of the accessory device is received in the socket of the host device or the socket of an extension cable or splitter cable. Thus the predetermined sequence is detected by the circuitry  100  even when the plug  160 ,  260 ,  360 ,  460  is removed from the socket  150  of an extension cable, and so an extraction event can be detected and recorded even when an accessory device is connected to the host device via an extension cable. In contrast, with conventional jack detect circuitry, the removal of a plug from a socket of an extension cable would not result in the circuitry detecting an extraction event because the plug of the extension cable would remain in the socket of the host device. 
     The monitoring unit  170  is therefore configured to monitor the impedance states detected at the terminals  110 - 140  and when the impedance states correspond to a predetermined sequence e.g. any of the sequences denoted in Tables 1-3, the circuitry  100  outputs a signal indicative of removal of a plug  160 ,  260 ,  360 ,  460  from the socket  150 . It will be appreciated that the sequences denoted in Tables 1-3 are examples of predetermined sequences and the skilled person will understand that a predetermined sequence could be determined for any TRRS plug contact configuration, for different impedances states that may be detected during the extraction of the plug(s) of one or more accessory devices from one or more sockets of a splitter cable, or indeed for any configuration of different types of complementary plugs and sockets. 
     A memory may be associated with the monitoring unit  170  and may store the one or more predetermined sequences for detecting the removal of jack plugs of different configurations from a socket, and/or predetermined sequences indicative of the removal of the plug(s) of one or more accessory devices from one or more sockets of a splitter cable. The monitoring unit  170  may comprise a processor that logs the impedance states detected at terminals  110 - 140  and is configured to detect the removal of a plug from the socket  150  or from a socket of a splitter cable when the detected sequence of impedance states corresponds to any of the stored predetermined sequences. The monitoring unit  170  may be configured to transmit the detected impedance states to a downstream processor, which may detect the removal of a plug from the socket  150  or from a socket of a splitter cable when the detected impedance states correspond with any of the predetermined sequences stored in the memory. 
     The circuitry  100  may be configured to detect the type of plug received in a socket  150 . For example, using conventional microphone detection circuitry, the circuitry  100  may determine that the plug comprises a microphone contact and may further determine which contact of the plug comprises the microphone contact. In response to detecting the type of plug received in the socket  150 , a processor may select the corresponding predetermined sequence to detect the removal of the plug from the socket  150 . For example, in response to circuitry  100  determining that a TRRS plug with left-right-microphone-ground contacts is received in the socket  150 , the processor may select one or more of the sequences denoted in Table 2 to detect the removal of the plug from the socket  150 . 
     The circuitry  100  may output the signal S indicative of detection of removal of the plug from the socket  150  in response to the predetermined sequence of impedance states occurring within a predetermined time period. The predetermined time period may correspond to an average or expected removal time of a plug from socket  150  e.g. 500 ms. Detection of the plug  160 ,  260 ,  360 ,  460  moving from the first removal state to the second removal state in socket  150 , may act as trigger for monitoring unit  170  to detect the predetermined sequence of impedance states. 
     When the circuitry  100  or other jack detect circuitry determines that a plug is fully received in the socket  150 , the circuitry  100  may enter an idle mode in which the impedance of the signal paths connected to the terminals  110 - 140  is not continuously monitored and instead is periodically determined. For example, one of the terminals  110 - 140  may be periodically polled, e.g. every 500 ms, to measure the impedance of the associated signal path. This mode of operation may reduce power consumption compared to a continuous impedance measurement. 
     However, in response to determining that a trigger event or sequence has occurred e.g. the plug moving to the second removal state in socket  150 , the monitoring unit  170  may enter an “active” mode of operation in which the monitoring unit  170  continuously monitors the impedance states at terminals  110 - 140  to detect a predetermined sequence of impedance states. 
     Referring to Tables 1-3, a common feature of all the sequences is the first impedance state at the first terminal  110  being low impedance (e.g. R LOAD ) in the first removal state and high impedance in the second removal state. Therefore in one embodiment, the first impedance state transitioning from R LOAD  (or a low impedance state) in the first removal state to a high impedance state may act as a trigger for the monitoring unit  170  to enter the “active” mode to detect a predetermined sequence of impedance states within a predetermined time period. Detection of this trigger sequence may therefore act as the starting point for the predetermined time period. 
     In response to the trigger sequence or event, the monitoring unit  170  may operate in the “active” mode for the predetermined time period to detect a predetermined sequence of impedance states. In response to the monitoring unit  170  detecting a predetermined sequence of impedance states within the predetermined time period, the circuitry  100  outputs a signal S indicative of full removal of the plug from the socket  150 . 
     In some situations the monitoring unit  170  may not detect a predetermined sequence of impedance states within the predetermined time period. In one example, the plug may only be partially removed from the socket  150  within the predetermined time period e.g. the plug and socket  150  may not move beyond the second removal state within the predetermined time period. 
     In such examples, the circuitry  100  may output a signal S indicative of detection of partial removal of the plug from the socket  150 . The signal S may again be transmitted to a controller to cause the controller to suspend output of audio signals to the relevant contacts of the socket  150 . As the plug is partially removed from the socket  150 , the plug is not received in a manner that permits the accessory device to correctly receive the audio signals, and so outputting audio signals would result in unnecessary power consumption of the host device. 
     In some examples, the monitoring unit  170  may be configured to monitor the first and second impedance states detected at the first and second terminals  110 , 120  to detect a predetermined sequence indicative of at least partial removal of the plug from socket  150 , and to monitor the third impedance state detected at the third terminal  130  to determine whether a full or a partial removal of the plug from the socket  150  has occurred. 
     For example, referring again to  FIG. 4 a   , the plug  460  may be fully received in the socket  150  (i.e. the plug  460  and the socket are in the first removal state) and the monitoring unit  170  may continuously monitor the first and second impedance states at the first and second terminals  110 , 120 . The first and second impedance states will therefore be indicative of the load impedance ROAD. However, the monitoring unit  170  may not monitor the third impedance state at the fourth terminal  140  when the plug  460  and the socket  150  are in the first removal state. 
     Upon partial removal of the plug  460  from the socket  150 , the plug  460  will be moved to the second removal state illustrated in  FIG. 4 b   . In the second removal state, the monitoring unit  170  may again monitor the first and second impedance states at the first and second terminals  110 ,  120 . However, again, the monitoring unit  170  may be configured not to monitor the third impedance state at the fourth terminal  140  in the second removal state. The first impedance state is indicative of a measured high impedance value and the second impedance state is indicative of a measured impedance value of 2R LOAD  in the second removal state. 
     The plug  460  may be moved to the third removal state, in which the plug  460  is again partially removed from socket  150 . In this removal state the first and second impedance states are indicative of measured high impedance values. The sequence of the first and second impedance states over the first to third removal states may therefore be denoted according to Table 4: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 First 
                 Second 
               
               
                   
                   
                 Impedance 
                 Impedance 
               
               
                   
                 Removal state 
                 State 
                 State 
               
               
                   
                   
               
             
            
               
                   
                 First 
                 R LOAD   
                 R LOAD   
               
               
                   
                 Second 
                 High-Z 
                 2R LOAD   
               
               
                   
                 Third 
                 High-Z 
                 High-Z 
               
               
                   
                   
               
            
           
         
       
     
     In response to detecting the sequence of changing impedance states denoted in Table 4, the monitoring unit  170  may detect that at least a partial removal of the plug  460  from the socket  150  has occurred. To determine whether a partial removal has occurred (i.e. the plug  460  is received in socket  150  in the third removal state) or a full removal has occurred (e.g. the plug  460  is received in socket  150  in the fourth removal state), the monitoring unit  170  may begin to monitor the third impedance state of the third signal path from the fourth terminal  140  to the third terminal  130 . 
     Therefore, in response to detecting a first predetermined sequence of the first and second impedance states in Table 4, the monitoring unit  170  may monitor the third impedance state to determine if a partial or a full removal of the plug  460  from the socket  150  has occurred. 
     In the third removal state, the third impedance state is indicative of a measured impedance value of 2R LOAD . If the monitoring unit  170  detects this third impedance state for a predetermined period of time, then it will determine that a partial removal of plug  460  from socket  150  has occurred. The signal S will therefore be indicative that the plug  460  has been partially removed from socket  150 . 
     If the third impedance state transitions from 2R LOAD  to a high impedance state within the predetermined time period, then the circuitry  100  may determine that the plug  460  has moved to the fourth removal state illustrated in  FIG. 4 d   , i.e. that the plug  460  has been fully removed from the socket  150 . The third impedance state transitioning from a first value (e.g. 2R LOAD ) to a second value (e.g. high impedance) may therefore correspond to a second predetermined sequence of impedance states. In response to detection by the monitoring unit  170  of the second predetermined sequence of impedance states within the predetermined time period the signal S will be indicative that a full removal of the plug  460  from the socket  150  has occurred. In another example, when the third impedance state is initially detected as high impedance, then it may be determined that a full removal of the plug  460  from the socket  150  has occurred. 
     Monitoring the third impedance state therefore clarifies if a plug has been partially or fully removed from the socket  150 . Furthermore, monitoring the third impedance state only after an initial predetermined sequence of the first and second impedance states has occurred reduces power consumption, as the additional third impedance state is used less frequently compared to a continuous detection across all removal states, as described with reference to  FIGS. 3 a - d  and 4 a   - d.    
     In one embodiment, in response to receiving the signal S indicative of partial removal of a plug from the socket  150 , the controller may output an error message. In an example in which the circuitry  100  and the socket  150  are part of an electronic device, an error message, alert or warning may be output via a user interface of the electronic device. The error message, alert or warning may notify a user that the plug of an accessory device is not correctly received in the socket  150  correctly. 
     In some embodiments, the impedance states may not be measured as absolute values Instead the monitoring unit  170  may detect whether the impedance of the signal paths associated with the terminals  110 - 140  is high or low. The impedance of the signal paths associated with the terminals  110 - 140  may be compared to a threshold impedance using comparator circuitry. If the impedance associated with a terminal is greater than the threshold impedance, then the comparator circuitry may output the corresponding impedance state as a high value. This will therefore be indicative that the corresponding terminal is not electrically connected to a plug contact and the signal path from the corresponding terminal is open circuit. When the impedance from a terminal is less than the threshold impedance this will be indicative that a plug contact is electrically connected to the terminal. The corresponding impedance state will therefore be output as a low value. 
     Using only two values for the impedance states in this way may reduce the number of predetermined sequences stored in a memory by circuitry  100 . For example, the predetermined sequence for both the left-right-microphone-ground TRRS plug described with reference to  FIGS. 3 a - d    and the left-right-ground-microphone TRRS plug described with reference to  FIGS. 4 a - d   , may both be expressed according to Table 5: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 First  
                 Second 
                 Third  
               
               
                   
                 Impedance 
                 Impedance 
                 Impedance 
               
               
                 Removal state 
                 State 
                 State 
                 State 
               
               
                   
               
             
            
               
                 First 
                 Low 
                 Low 
                 Low 
               
               
                 Second 
                 High 
                 Low 
                 Low 
               
               
                 Third 
                 High 
                 High 
                 Low 
               
               
                 Fourth 
                 High 
                 High 
                 High 
               
               
                   
               
            
           
         
       
     
     Therefore, the sequence of impedance states denoted in Table 5 may be used to detect removal of a left-right-microphone-ground configured TRRS plug and a left-right-ground-microphone TRRS configured plug from the socket  150 . A single predetermined sequence of impedance states may therefore be used to detect the removal of two types of plugs. Only one predetermined sequence of impedance states to detect the removal of a plurality of different types or configurations of plugs may therefore be stored in a memory associated with circuitry  100 . This may therefore reduce the amount of memory required compared to storing a plurality of predetermined impedance states for a corresponding plurality of plug types. 
     The circuitry described above with reference to  FIGS. 1-4  can be used to detect removal of the plug of an audio accessory from the socket of a host device or from the socket of an extension cable or splitter cable that is connected to the socket of the host device while audio signals are being output (via the socket of the host device) to the accessory device without injecting DC voltages that would give rise to undesirable audio artefacts such as clicks and pops. This is achieved by measuring the impedance of one or more signal paths as discussed above while the audio signals are being output. 
     The skilled person will be aware of several methods for measuring the impedance of a signal path as discussed above. For example, the monitoring unit  170  could be configured to measure the ground return current in the signal paths between terminals of the monitoring unit. As will be appreciated, for a given signal level in a signal path the ground return current in the signal path will vary depending upon the impedance state of the signal path. 
     For example, referring to  FIGS. 2 a -2 c   , in the first removal state illustrated in  FIG. 2 a    the ground return current in the first signal path between the first and fourth terminals  110 ,  140  for a given signal level will take a first value, and the ground return current in the second signal path between the second and fourth terminals  120 ,  140  for a given signal level will take a second value (which may be approximately equal to the first value, as the impedance of the left audio load R L  is equal to the impedance of the right audio load R R ). By measuring the return current in the first and second signal paths, the impedance of those signal paths can be determined by the monitoring unit  170  or a downstream processor. 
     When the plug  260  moves to the second removal state illustrated in  FIG. 2 b    the socket tip contact  152  is no longer connected to anything so no current flows in the first signal path. Thus, no ground return current is detected and so the monitoring unit  170  or downstream processor can determine that the impedance of the first signal path is high impedance. 
     In the second removal state, the second signal path includes the impedances of both the left audio load R L  and the right audio load R R . The return current in the second signal path will therefore be reduced, in comparison to the return current in the second signal path in the first removal state. By measuring the return current in the second signal path, the impedance of that signal path can be determined by the monitoring unit  170  or a downstream processor. 
     When the plug  260  moves to the third removal state illustrated in  FIG. 2 c    neither the socket tip contact  152  nor the first socket ring contact  154  is connected to anything so no current flows in the first signal path or the second signal path. Thus, no ground return current is detected in the first signal path or the second signal path and so the monitoring unit  170  or downstream processor can determine that the impedance of both the first signal path and the second signal path are high impedance. 
     In an alternative approach, the monitoring unit  170  or circuitry  100  may include current mirror circuitry, to mirror a proportion of the current through the loads (e.g. speakers of an audio accessory device) through a known resistance. By measuring a parameter associated with the mirrored current (e.g. the current or an associated voltage), the impedance of a signal path between terminals of the monitoring unit, via the audio accessory, can be determined. 
       FIG. 5 a    illustrates an example of circuitry  500  to mirror the load current I LOAD  passing through an audio load coupled to a jack socket of a host device. An audio output driver transistor  510  (e.g. a MOSFET) receives at a control terminal (e.g. a gate terminal) thereof an audio signal from an audio sub-system of the host device, and outputs an output voltage V OUT  to a circuit node  514 , which may be, for example, a tip or ring contact of a jack socket of the host device in which a corresponding jack plug of an audio accessory device is received. The output voltage V OUT  drives a load  512  such as a speaker of the audio accessory device, which has a nominal impedance R LOAD . 
     The circuitry  500  also includes current mirror circuitry comprising a current mirror transistor  520  (e.g. a MOSFET) coupled to a dummy resistance  522  having an impedance R DUM . A control terminal (e.g. a gate terminal) of the current mirror transistor  520  is coupled to the control terminal of the output driver transistor  510 , and thus also receives the audio signal from the audio sub-system of the host device. By appropriate selection of the ratio of the size of the current mirror transistor  520  to that of the output driver transistor  510 , or alternatively by appropriate selection of the ratio of the impedance R DUM  of the dummy resistance  522  to the nominal impedance R LOAD  of the audio load  512 , a current I SENSE  through the dummy resistance  522  can be set to be a suitable proportion of the load current I LOAD  through the audio load  512 . For example, if the nominal impedance R LOAD  of the audio load is 32Ω, then by setting the impedance R DUM  of the dummy resistance  522  to 3.2 kΩ and using a current mirror transistor  520  of the same size as the output driver transistor  510 , the current I SENSE  through the dummy resistance  522  can be set to be 1/100 of the load current I LOAD  through the audio load  512 . 
     The circuitry  520  also includes a comparator  520  having a first input that is coupled to the circuit node  514  and a second input that is coupled to a circuit node  524  between an output terminal of the current mirror transistor  520  and the dummy resistance  522 . The first input of the comparator thus receives the output voltage V OUT , and the second input of the comparator receives a voltage V SENSE  that develops across the dummy resistance  522  as a result of the mirrored current I SENSE . 
     The comparator  520  outputs a signal indicative of the difference between V OUT  and V SENSE . When the impedance R LOAD  of the audio load  512  changes due to full or partial removal of the jack plug of the audio accessory from the socket of the host device as described above with reference to  FIGS. 2-4 , the current I SENSE  will change, leading to a change in V SENSE  and a consequential change in the level of the signal output by the comparator  520 . This comparator output signal is thus indicative of the impedance R LOAD  of the audio load, and can be used by downstream processing circuitry to detect the different impedance states that occur as the jack plug of the audio accessory is removed from the socket of the accessory device. 
       FIG. 5 b    illustrates circuitry  550  implementing an alternative approach to detecting changes in the impedance states of signal paths between terminals of the monitoring unit  170 , via an audio accessory device. The elements in common between  FIGS. 5 a  and 5 b    are given corresponding reference numerals. 
     The circuitry  550  is similar to the circuitry  500  of  FIG. 5 a   , with the exception that there is no comparator  520 . Instead, the circuitry  550  includes an analog to digital converter (ADC)  560  having an input coupled to the circuit node  524 , such that the ADC  560  receives the voltage V SENSE  and outputs a digital signal representative of the voltage V SENSE  to downstream processing circuitry. As in the circuitry of  FIG. 5 a   , when the impedance R LOAD  of the audio load  512  changes due to full of partial removal of the jack plug of the audio accessory from the socket of the host device as described above with reference to  FIGS. 2-4 , the current I SENSE  will change, leading to a change in V SENSE . The digital signal output by the ADC  560  is thus indicative of the impedance R LOAD  of the audio load, and can be used by downstream processing circuitry to detect the different impedance states that occur as the jack plug of the audio accessory is removed from the socket of the accessory device. 
     The examples illustrated in  FIGS. 1-4  show a 4-pole TRRS plug being removed from a complementary 4-pole TRRS socket. It will be appreciated, however, that the techniques described above in relation to the examples illustrated in  FIGS. 1-4  are equally applicable to detecting the presence of a 3-pole TRS plug in a corresponding 3-pole TRS socket, and the removal of such a plug from such a socket. 
       FIGS. 6 a - c    illustrate an example of a three-pole jack plug  760  being removed from a corresponding socket  650  of a host device over a sequence of different removal states. 
     The host device includes circuitry  600  for detecting the present of a jack plug  760  in the socket. The circuitry  600  includes a monitoring unit having first, second and third terminals  610 ,  620 ,  630  which are connected, respectively, to tip, ring and sleeve contacts  652 ,  654 ,  656  of the socket  650  by respective conductors  612 ,  622 ,  632  such as printed circuit board (PCB) tracks, wires or the like. 
     In the illustrated example of  FIG. 6 , a plug  760  comprises a TRS (tip, ring, sleeve) jack plug to provide a connection to an audio accessory device such as a set of stereo headphones that does not include a microphone. A common configuration for the jack plug for such an accessory device is that the tip and ring contacts  762 ,  764  provide connections for the left audio and right audio loads (e.g. left and right speakers), respectively, with the sleeve contact  766  providing a ground connection for the accessory device. Thus, as illustrated in  FIG. 6 a   , the plug tip (T) contact  762  provides a connection to the left audio load R L . Similarly, the plug ring (R) contact  764  provides a connection to the right audio load R R . It will be appreciated that both the left audio load R L  and the right audio load R R  will be substantially the same and therefore the impedance of either load may be expressed as R LOAD . 
     Therefore, as illustrated in  FIG. 6 a   , when the plug  760  is fully received in the socket  650  of the host device, the first terminal  610  of the monitoring unit  670  is electrically connected to the left load R L  at the plug tip contact  762  via the socket tip contact  652 , while the second terminal  620  of the monitoring unit  670  is electrically connected to the right load R R  at the plug ring contact  764  via the socket ring contact  654 . 
     As illustrated in  FIG. 6 a   , when the plug  760  is fully received in the socket  650 , the socket sleeve contact  656  is electrically connected to the plug sleeve contact  766 . As described above, the plug sleeve contact  766  provides a contact for connection to ground. Therefore, the third terminal  630  connects the plug sleeve contact  266  to ground G, when the plug  760  is fully received in the socket  750 . 
     The third terminal  630  provides a suitable reference from which impedance measurements may be taken. Therefore, impedance measurements may be taken for a first signal path from the first terminal  610  to the third terminal  630 , via the audio accessory, and for a second signal path from the second terminal  620  to the third terminal  630 , via the audio accessory. In other words, a first impedance state may be detected at the first terminal  610  for a first signal path between the first terminal  610  and the third terminal  630 , and a second impedance state may be detected at the second terminal  620 , for a second signal path between the second terminal  620  and the third terminal  630 . 
       FIG. 6 a    illustrates the plug  760  and the socket  650  in an initial (or first) removal state, in which the plug  760  is fully inserted in the socket  650 . In this initial removal state, the impedance state at both the first terminal  610  and the second terminal  620  will be low, as the first and second terminals  610 , 620  are in electrical contact with the plug tip and ring contacts  762 ,  764  via socket contacts  652 , 654 , respectively. The first signal path from the first terminal  610  to the third terminal  630 , via the audio accessory, includes the left load R L , and therefore the impedance of the first signal path will be measured as R LOAD . Accordingly, the first impedance state, detected at the first terminal  610 , is low impedance. Similarly, the second signal path from the second terminal  620  the third terminal  630 , via the audio accessory, includes the right audio load R R . Therefore, the impedance of the second signal path will also be measured as R LOAD  by the monitoring unit  670 . Thus the second impedance state, detected at the second terminal  620 , is also low impedance. 
       FIG. 6 b    illustrates a second removal state of the plug  760  and the socket  650 , in which the plug  760  is partially removed from the socket  650 . In the second removal state, the plug  760  has been partially extracted from the socket  650 , such that the plug sleeve contact  766  is no longer fully received in the socket  650 . In the second removal state, the socket tip contact  652  is not in electrical contact with any of the plug contacts  762 - 766 . The signal path from the first terminal  610  is therefore open circuit. As such, the first impedance state, detected at the first terminal  610 , will be high impedance. 
     In the second removal state, the second terminal  620  is electrically connected to the plug tip contact  762  via the socket ring contact  654 . The signal path between the second terminal  620  and the third terminal  630 , via the audio accessory, therefore includes the left audio load R L  and the right audio load R R . As the impedances of the left audio load R L  and the right audio load R R  are substantially the same, the impedance of this signal path will therefore again be measured as approximately 2R LOAD  in the second removal state, and thus the impedance state, detected at the second terminal  620 , when the plug  760  and the socket  650  are in the second removal state will be approximately 2R LOAD . 
       FIG. 6 c    illustrates the plug  760  and the socket  650  in a third removal state, which for the purposes of the present disclosure is equivalent to the full removal of the plug  760  from the socket  650 . In the third removal state, the plug tip contact  762  is received in the socket  650 , in contact with the socket sleeve contact  656 . However, neither of the socket ring contact  764  and the socket sleeve contact  766  are connected to any plug contact. The left and right audio contacts of the plug  760  (i.e. the socket tip contact  762  and the socket ring contact  764 ) are therefore no longer in contact with the socket contacts of the socket  650  through which audio signals can be supplied to the left and/or right plug contacts (i.e. the socket tip contact  652  and the socket ring contact  654 ). Therefore, neither of left audio load R L  and right audio load R R  can be driven in the third removal state. Accordingly, when the plug  760  and the socket  650  adopt the third removal state, the plug  760  will be considered to be removed from the socket  650  for the purposes of the present disclosure. 
     In the third removal state, the first terminal  610  is again not connected to any of the plug contacts  762 - 766 . Therefore the first impedance state, detected at the first terminal  610 , will again be high impedance. The socket ring contact  754  is no longer in electrical contact with any of the plug contacts  762 - 266  in the third removal state. Therefore, the second impedance state, detected at the second terminal  620 , will also be high impedance. 
     The sequence of values of the first and second impedance states as the plug  760  is removed from the socket  650  over the first to third removal states illustrated in  FIGS. 6 a - c    may therefore be expressed according to Table 6: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                 First  
                 Second  
               
               
                   
                   
                 Impedance  
                 Impedance 
               
               
                   
                 Removal State 
                 State 
                 State 
               
               
                   
                   
               
             
            
               
                   
                 First 
                 Low-Z (R LOAD ) 
                 Low-Z (R LOAD ) 
               
               
                   
                 Second 
                 High-Z 
                 2R LOAD   
               
               
                   
                 Third 
                 High-Z 
                 High-Z 
               
               
                   
                   
               
            
           
         
       
     
     The changing sequence of the first and/or second impedance states detected at the first and/or second terminals  610 ,  620  respectively may therefore be indicative of the removal of the plug  760  from the socket  650 . As the plug  760  is removed from the socket  650 , the plug  760  and socket  650  will sequentially adopt the first, second and third removal states illustrated in  FIGS. 6 a - c   . The monitoring unit  670  is therefore configured to detect a sequence of first and/or second impedance states, and when the detected sequence of first and/or second impedance states corresponds to the relevant sequence(s) in Table 6, the monitoring unit  670  will detect that the plug  760  has been removed from socket  650 . 
     The circuitry  600  is configured to output a signal S indicative of detection of at least partial removal of the plug  760  from the socket  650  when the monitoring unit  670  detects this predetermined sequence. The signal S may be sent to a controller (not illustrated) of the host device, which may, in response to the signal S, suspend the supply of audio signals to the socket  650 , thereby reducing power consumption of the host device, since audio signals are not unnecessarily generated and output. 
     The monitoring unit  670  may comprise a processor and/or circuitry configured to detect the predetermined sequence denoted in Table 6, indicative of removal of the plug  760  from the socket  650 . In another example the monitoring unit  670  may detect the first and second impedance states (e.g. by measuring the impedances of signal paths from the first and second terminals  610 ,  620 , as described above) and transmit the detected impedance states to a downstream processor. The downstream processor may log the detected sequence of the first and second impedance states and, when the logged sequence of impedance states corresponds to the predetermined sequence, may output the signal S to a controller to suspend audio output by the host device. 
     As will be appreciated by those skilled in the art, the circuitry described herein with reference to  FIGS. 1-6  can be used to detect removal of jack plugs from sockets in a number of different scenarios, as illustrated in  FIGS. 7 a   - 7   f.    
       FIG. 7 a    illustrates the removal of a 4-pole TRRS jack plug  260  of an audio accessory device from a corresponding 4-pole TRRS socket  150  of a host device. 
       FIG. 7 b    illustrates the removal of a 3-pole TRS jack plug  760  of an audio accessory device from a corresponding 3-pole TRS socket  650  of a host device. 
       FIG. 7 c    illustrates the removal of a 4-pole TRRS jack plug  260  of an audio accessory device from a corresponding 4-pole TRRS socket of an extension cable  810  having a 4-pole TRRS jack plug that is received in a 4-pole TRRS socket  150  of a host device. 
       FIG. 7 d    illustrates the removal of a 3-pole TRS jack plug  760  of an audio accessory device from a corresponding 3-pole TRS socket of an extension cable having a 3-pole TRS jack plug that is received in a 3-pole TRS socket  650  of a host device. 
       FIG. 7 e    illustrates the removal of a 4-pole TRRS jack plug  260  of an audio accessory device from a corresponding 4-pole TRRS socket of a splitter cable  830  having a 4-pole TRRS jack plug that is received in a 4-pole TRRS socket  150  of a host device. 
       FIG. 7 f    illustrates the removal of a 3-pole TRS jack plug  760  of an audio accessory device from a corresponding 3-pole TRS socket of a splitter cable  840  having a 3-pole TRS jack plug that is received in a 3-pole TRS socket  650  of a host device. 
     The description above relates to detecting removal of a jack plug from a complementary socket of the same type (e.g. removal of a four-pole TRRS plug from a complementary 4-pole TRRS socket, or removal of a 3-pole TRS plug from a complementary 3-pole TRS socket). The disclosed systems and techniques can detect removal of a plug from a socket of a host device or from a socket of an intermediate cable such as an extension cable, splitter cable or the like. 
     However, in some circumstances the plug of an intermediate cable such as an extension cable, splitter cable or the like may be mismatched with the socket of the host device, and/or the plug of an accessory device may be mismatched with the socket of the intermediate cable, i.e. the intermediate cable may have a plug of a different type than the socket of the host device to which it is connected, and/or may have a socket of a different type than the plug of the accessory device that is connected to the intermediate cable. For example, an extension cable or splitter cable may have a three-pole TRS jack plug at a first end, connected to one or more three-pole TRS jack sockets at a second end, whereas a host device may have a four-pole TRRS socket and an accessory device may have a four-pole TRRS plug. 
     Typically host devices such as mobile phones, tablet and laptop computers and the like include circuitry to detect a mismatch between the jack plug of an audio accessory device and the audio accessory socket of the host device when the jack plug is received in the socket and to configure or reconfigure internal connections to the socket to ensure compatibility between the host device and the accessory device. 
     For example,  FIG. 8  shows a situation in which a three-pole TRS jack plug (e.g. of an accessory device or an intermediate cable) is received in a four-pole TRRS socket  850  of a host device  800 . In this example both the second ring contact  856  and the sleeve contact  858  of the socket  850  are capable of providing either a microphone or a ground connection for an audio accessory device. When the plug  960  is received in the socket  850 , circuitry of the host device  850  detects that the second ring and sleeve contacts of the socket  850  are connected together (i.e. short-circuited) by the sleeve contact  966  of the plug  960 , and the host device  800  will thus configure itself so as to be suitable for use with an accessory device that uses a three-pole TRS plug, e.g. by configuring internal connections of circuitry of the host device to couple the second ring and sleeve contacts  856 ,  858  of the socket  850  to ground so as to provide a ground path for audio signals. 
     This configuration or reconfiguration of the internal connections of the host device  800  ensures that the host device  800  is able to output audio signals to an accessory device that uses a three-pole TRS jack plug correctly. However, if the jack plug  960  that is received in the socket  850  is not for an audio accessory device but is instead for an intermediate cable such as an extension cable or a splitter cable, the host device has no way to determine the type of device that is connected to connected at the second end of the intermediate cable (or even if any device at all is connected to the intermediate cable), and thus it is possible that the configuration or reconfiguration of the internal connections of the host device  800  will not provide the required connections for the accessory device that is connected to the intermediate cable, such that correct audio output to the audio accessory device is not possible. This can lead to a poor user experience, as audio output by the accessory device may be distorted or attenuated, or alternatively no audio may be output by the audio accessory at all. 
       FIG. 9 a    illustrates an example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . 
     In the example illustrated in  FIG. 9 a    the plug  1160  of the extension cable  1100  is a three-pole TRS plug and the socket  1150  of the extension cable  1100  is a three-pole TRS socket. Tip, ring and sleeve contacts  1162 ,  1164 ,  1166  of the plug  1160  are electrically connected to respective tip, ring and sleeve contacts  1152 ,  1154 ,  1156  of the socket  1150  by suitable conductors within the cable  1100 . 
     The plug  1260  of the accessory device  1200 , however, is a four-pole plug, having a tip contact  1262 , first and second ring contacts  1264 ,  1266  and a sleeve contact  1268 . Thus there is a mismatch between a plug type of the plug  1260  and a socket type of the socket  1150 . 
     In the example illustrated in  FIG. 9 a    the socket  1150  is configured such that when the plug  1260  is fully received in the socket  1150  the socket tip contact  1152  is in contact with the plug tip contact  1262 , the socket ring contact  1154  is in contact with the first plug ring contact  1264  and the socket sleeve contact  1156  is in contact with the plug sleeve contact  1568 . However, the position of the socket sleeve contact  1156  can vary according to different configurations of the socket  1150 , as discussed below with reference to  FIGS. 9 b -9 f   , and this can have an effect on which (if any) of the plug contacts is in contact with the socket sleeve contact  1156  when the plug  1260  is fully received in the socket  1150 . 
     In the example illustrated in  FIG. 9 a    the accessory device  1200  is configured such that a left audio load R L  (e.g. a left speaker of a set of headphones) is electrically connected to the plug tip contact  1262 , a right audio load R R  (e.g. a right speaker of a set of headphones) is electrically connected to the first plug ring contact  1264 , a microphone MIC is electrically connected to the second plug ring contact  1266  and a ground connection of the accessory device  1200  is electrically connected to the plug sleeve contact  1268 . 
     Thus, when the plug  1260  of the audio accessory device  1200  is fully received in the socket  1150  of the extension cable  1100 , the left audio load R L  is electrically coupled to the socket tip contact  1152  via the plug tip contact  1262 , the right audio load R R  is electrically coupled to the socket ring contact  1154  via the plug ring contact  1264 , and the ground connection of the accessory device  1200  is electrically coupled to the socket sleeve contact  1268 . As shown in  FIG. 9 a   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , the microphone MIC of the accessory device  1200  is not coupled to any socket contact of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are electrically connected to the left and right audio loads, respectively, of the audio accessory device  1200 , whilst the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are electrically connected to the ground connection of the audio accessory device  1200 . Thus in the arrangement illustrated in  FIG. 9 a    audio signals output by the host device via the socket  1050  can be correctly output by the audio accessory device  1200 . 
       FIG. 9 b    illustrates another example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . The elements in common between  FIGS. 9 a  and 9 b    are given corresponding reference numerals. 
     The host device and audio accessory device  1200  of the example illustrated in  FIG. 9 b    are similar to the corresponding devices of  FIG. 9 a   . However, the extension cable  1100  of  FIG. 9 b    differs from that of  FIG. 9 a    in that the socket sleeve contact  1156  is positioned such that, when the plug  1260  is fully received in the socket  1150 , the socket sleeve contact  1156  is in contact with the second plug ring contact  1266 , thus electrically coupling the socket sleeve contact  1156  to the microphone (MIC) of the audio accessory device  1200 . 
     Thus, as shown in  FIG. 9 b   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , the microphone MIC of the accessory device  1200  is coupled to the socket sleeve contact  1156  of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are again electrically connected to the left and right audio loads, respectively, of the audio accessory device  1200 , whilst the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are electrically connected to the microphone of the audio accessory device  1200 . Thus in the arrangement illustrated in  FIG. 9 b    audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200 , since the audio signal paths that include the left and right audio loads also include the microphone MIC, such that any audio signal transmitted via these signal paths will be at best attenuated, or worse, distorted by the additional impedance of the microphone, and the system may choose to respond appropriately. 
       FIG. 9 c    illustrates another example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . The elements in common between  FIGS. 9 a , 9 b  and 9 c    are given corresponding reference numerals. 
     The host device and audio accessory device  1200  of the example illustrated in  FIG. 9 c    are similar to the corresponding devices of  FIG. 9 a   . However, the extension cable  1100  of  FIG. 9 c    differs from that of  FIG. 9 a    in that the socket sleeve contact  1156  is positioned such that, when the plug  1260  is fully received in the socket  1150 , the socket sleeve contact  1156  is in contact with an insulator that is provided between the second plug ring contact  1266  and the plug sleeve contact  1268 . 
     Thus, as shown in  FIG. 9 c   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , neither the microphone MIC nor the ground connection of the accessory device  1200  is coupled to any socket contact of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are again electrically connected to the left and right audio loads respectively. However, the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are not electrically connected to anything, due to the position of the socket sleeve contact  1156  of the socket  1150  in contact with the insulator between the second plug ring contact  1266  and the plug sleeve contact  1268 . Thus in the arrangement illustrated in  FIG. 9 c    audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200  and the system may choose to respond appropriately. 
       FIG. 9 d    illustrates another example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . The elements in common between  FIGS. 9 a -9 d    are given corresponding reference numerals. 
     The host device and the extension cable  1100  of the example illustrated in  FIG. 9 d    are similar to the corresponding elements of  FIG. 9 a   . However, the audio accessory device  1200  differs from the audio accessory device  1200  of  FIG. 9 a    in that in the arrangement illustrated in  FIG. 9 d   , the microphone MIC of the audio accessory device  1200  is electrically connected to the plug sleeve contact  1268 . Thus, when the plug  1260  is fully received in the socket  1150 , the microphone MIC is electrically connected to the socket sleeve contact  1156  via the plug sleeve contact  1268 . As shown in  FIG. 9 d   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , the ground connection of the accessory device  1200  is not coupled to any socket contact of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are electrically connected to the left and right audio loads respectively, whilst the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are electrically connected to the microphone MIC of the audio accessory device  1200 . Thus in the arrangement illustrated in  FIG. 9 a    audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200 , since the audio signal paths that include the left and right audio loads also include the microphone MIC, such that any audio signal transmitted via these signal paths will be at best attenuated, or worse, distorted by the additional impedance of the microphone, and the system may choose to respond appropriately. 
       FIG. 9 e    illustrates another example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . The elements in common between  FIGS. 9 a -9 e    are given corresponding reference numerals. 
     The host device and audio accessory device  1200  of the example illustrated in  FIG. 9 e    are similar to the corresponding devices of  FIG. 9 d   . However, the extension cable  1100  of  FIG. 9 e    differs from that of  FIG. 9 d    in that the socket sleeve contact  1156  is positioned such that, when the plug  1260  is fully received in the socket  1150 , the socket sleeve contact  1156  is in contact with the second plug ring contact  1266 , thus electrically coupling the socket sleeve contact  1156  to the ground connection of the audio accessory device  1200 . 
     Thus, as shown in  FIG. 9 e   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , the microphone of the accessory device  1200  is not coupled to any socket contact of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are electrically connected to the left and right audio loads respectively, whilst the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are again electrically connected to the ground connection of the audio accessory device  1200 . Thus in the arrangement illustrated in  FIG. 9 e    audio signals output by the host device via the socket  1050  can be correctly output by the audio accessory device  1200 . 
       FIG. 9 f    illustrates another example of a situation in which a plug  1160  of an intermediate cable (in this case an extension cable  1100 ) is received in a socket  1050  of a host device, and a plug of an accessory device  1200  is received in a socket  1150  of the extension cable  1100 . The elements in common between  FIGS. 9 a -9 f    are given corresponding reference numerals. 
     The host device and audio accessory device  1200  of the example illustrated in  FIG. 9 f    are similar to the corresponding devices of  FIG. 9 d   . However, the extension cable  1100  of  FIG. 9 f    differs from that of  FIG. 9 d    in that the socket sleeve contact  1156  is positioned such that, when the plug  1260  is fully received in the socket  1150 , the socket sleeve contact  1156  is in contact with an insulator that is provided between the second plug ring contact  1266  and the plug sleeve contact  1268 . 
     Thus, as shown in  FIG. 9 f   , as a result of the mismatch between the plug type of the plug  1260  and the socket type of the socket  1150 , neither the microphone MIC nor the ground connection of the accessory device  1200  is coupled to any socket contact of the socket  1150 . 
     In this arrangement the socket tip contact  1052  and first socket ring contact  1054  of the socket  1050  of the host device are again electrically connected to the left and right audio loads respectively. However, the second socket ring contact  1056  and the socket sleeve contact  1058  of the socket  1050  of the host device are not electrically connected to anything, due to the position of the socket sleeve contact  1156  of the socket  1150  in contact with the insulator of the between the second plug ring contact  1266  and the plug sleeve contact  1268 . Thus in the arrangement illustrated in  FIG. 9 f    audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200 , and the system may choose to respond appropriately. 
     The host device includes circuitry  1000  configured to detect connection of a plug  1260  of an audio accessory device  1200  to a second  1150  of an intermediate cable  1100  that is connected to the socket  1050  of the host device, and to output a signal indicative of incompatibility between the accessory device  1200  and the intermediate cable  1100  or of a fault in the accessory device  1200  or intermediate cable. 
     The circuitry  1000  includes a monitoring unit  1070 , which may comprise discrete circuitry, integrated circuitry, processor circuitry configured to execute suitable software instructions or any combination of discrete circuitry, integrated circuitry, processor circuitry and software. 
     The monitoring unit  1070  is configured to determine a first impedance of a first signal path associated with the socket tip contact  1052 , a second impedance of a second signal path associated with the socket tip contact  1052  and a third impedance of a third signal path associated with the first socket ring contact  1054 . 
     To this end the monitoring unit  1070  includes a first terminal  1010 , a second terminal  1020 , a third terminal  1030  and a fourth terminal  140 . The first terminal  1010  is electrically coupled to the tip contact  1052  of the socket  1050  via a conductor (e.g. a PCB trace, wire or the like)  1012 , the second terminal  1020  is electrically coupled to the first ring contact  1054  of the socket  1050  via a conductor  1022 , the third terminal  1030  is electrically coupled to the second ring contact  1056  of the socket  1050  via a conductor  1032 , and the fourth terminal  1040  is electrically coupled to the sleeve contact  1058  of the socket  1050  via a conductor  1042 . 
     The monitoring unit  1070  is configured to determine, continuously, periodically, or in response to a particular event (e.g. when audio playback is requested by a user of the host device), the impedance of a first signal path between the first terminal  1010  and the second terminal  1020  via the audio accessory  1200 , the impedance of a second signal path between the first terminal  1010  and the third terminal  1030  or the fourth terminal  1040  via the audio accessory, and the impedance of a third signal path between the second terminal  1020  and the third terminal  1030  or the fourth terminal  1040  via the audio accessory  1200 . 
     As described above, when the plug  1260  of the audio accessory device  1200  is received in the socket  1150  of the extension cable  1100  in the arrangement illustrated in  FIG. 9 a   , audio signals can be correctly output by the audio accessory device  1200 . In this arrangement the impedance of the first signal path from the first terminal  1010  to the second terminal  1020  (via the extension cable  1100  and the accessory device  1200 ), as “seen” by the host device and determined by the monitoring unit  1070 , includes the left and right audio loads of the accessory device  1200 . Thus, in the arrangement illustrated in  FIG. 9 a    the impedance of the first signal path (also referred to as the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R , (where R L  and R R  are the impedances of the left and right audio loads respectively) plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . 
     In the arrangement illustrated in  FIG. 9 a   , the second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) includes only the left audio load. Thus, the impedance of the second signal path (also referred to as the second impedance state), as determined by the monitoring unit  1070 , will be approximately equal to R L , i.e. R XL =R L  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) includes only the right audio load. Thus, the impedance of the third signal path (also referred to as the third impedance state), as determined by the monitoring unit  1070 , will be approximately equal to R R , i.e. R XR =R R  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Accordingly, in the arrangement illustrated in  FIG. 9 a    the sum of the impedances R XL  and R XR  will be close to R TOT , i.e. R XL +R XR =R TO T+R TOL , where R TOL  is some tolerance value. 
     As described above, when the plug  1260  of the audio accessory device  1200  is received in the socket  1150  of the extension cable  1100  in the arrangement illustrated in  FIG. 9 b   , audio signals cannot be correctly output by the audio accessory device  1200 , since the second and third signal paths that include the left and right audio loads also include the microphone MIC. As in the arrangement of  FIG. 9 a   , in the arrangement of  FIG. 9 b    the impedance of the first signal path (i.e. the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R , plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . However, in the arrangement illustrated in  FIG. 9 b   , the second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  includes the left audio load and the microphone of the audio accessory device  1200 , and thus the impedance R XL  of the second signal path (i.e. the second impedance state) will be approximately R L +R MIC , where R MIC  is the impedance of the microphone. Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  includes the right audio load and the microphone of the audio accessory device  1200 , and thus the impedance R XR  of the second signal path (i.e. the third impedance state) will be approximately R R +R MIC . Accordingly, in the arrangement illustrated in  FIG. 9 d    the sum of the impedances R XL  and R XR  will be different than (e.g. greater than) R TOT . 
     In the arrangement illustrated in  FIG. 9 c   , audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200 , since the ground connection of the audio accessory device  1200  is open-circuit. In this arrangement the first signal path from the first terminal  1010  to the second terminal  1020  via the accessory device  1200  includes the left and right audio loads of the accessory device  1200 , and thus the impedance of the first signal path (i.e. the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R . However, as neither the microphone nor the ground connection of the accessory device is connected to any of the contacts of the extension cable socket  1150 , there is no complete signal path from either the first or the second terminal  1010 ,  1020  to the third or fourth terminal  1030 ,  1040 , and thus in the arrangement illustrated in  FIG. 9 c    the monitoring unit  1070  will determine that both the second and third signal paths are high impedance (i.e. the second and third impedance states are both high impedance). 
     In the arrangement illustrated in  FIG. 9 d   , audio signals output by the host device via the socket  1050  cannot be correctly output without attenuation and/or distortion by the audio accessory device  1200 , since the audio signal paths that include the left and right audio loads also include the microphone MIC. In this arrangement the first signal path from the first terminal  1010  to the second terminal  1020  via the accessory device  1200  includes the left and right audio loads of the accessory device  1200 , and thus the impedance of the first signal path (i.e. the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R . The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  includes the left audio load and the microphone of the audio accessory device  1200 , and thus the impedance R XL  of the second signal path (i.e. the second impedance state) will be approximately R L +R MIC , where R MIC  is the impedance of the microphone. Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  includes the right audio load and the microphone of the audio accessory device  1200 , and thus the impedance R XR  of the second signal path (i.e. the third impedance state) will be approximately R R +R MIC . Accordingly, in the arrangement illustrated in  FIG. 9 d    the sum of the impedances R XL  and R XR  will be different than (e.g. greater than) R TOT . 
     In the arrangement illustrated in  FIG. 9 e   , audio signals output by the host device via the socket  1050  can be correctly output by the audio accessory device  1200 . In this arrangement the first signal path from the first terminal  1010  to the second terminal  1020  via the accessory device  1200  includes the left and right audio loads of the accessory device  1200 , and thus the impedance of the first signal path (i.e. the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R . The second signal path, from the first terminal  1010  to the third or further terminal  1030 / 1040 , includes only the left audio load of the accessory device  1200 . Thus, the impedance of the second signal path (also referred to as the second impedance state), as determined by the monitoring unit  1070 , will be approximately equal to R L , i.e. R XL =R L  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) includes only the right audio load. Thus, the impedance of the third signal path (also referred to as the third impedance state), as determined by the monitoring unit  1070 , will be approximately equal to R R , i.e. R XR =R R  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Accordingly, in the arrangement illustrated in  FIG. 9 a    the sum of the impedances R XL  and R XR  will be close to R TOT , i.e. R XL +R XR =R TOT +R TOL , where R TOL  is some tolerance value. 
     In the arrangement illustrated in  FIG. 9 f   , audio signals output by the host device via the socket  1050  cannot be correctly output by the audio accessory device  1200 , since the ground connection of the audio accessory device  1200  is open-circuit. In this arrangement the first signal path from the first terminal  1010  to the second terminal  1020  via the accessory device  1200  includes the left and right audio loads of the accessory device  1200 , and thus the impedance of the first signal path (i.e. the first impedance state), as determined by the monitoring unit  1070 , is approximately equal to the combined impedance of the left and right audio loads of the accessory device, i.e. R TOT =R L +R R . However, as neither the microphone nor the ground connection of the accessory device is connected to any of the contacts of the extension cable socket  1150 , there is no complete signal path from either the first or the second terminal  1010 ,  1020  to the third or fourth terminal  1030 ,  1040 , and thus in the arrangement illustrated in  FIG. 9 c    the monitoring unit  1070  will determine that both the second and third signal paths are high impedance (i.e. the second and third impedance states are both high impedance). 
     The circuitry  1000  is configured to detect incompatibility between an intermediate cable  1100  that is connected to the host device and an accessory device  1200  that is connected to the intermediate cable  1100 , based on the determined impedances of the first, second and third signal paths associated with the first and second terminals  1010 ,  1020  of the monitoring unit  1070 . 
     As explained above with reference to  FIGS. 9 a  and 9 e   , when an audio accessory device  1200  having a plug  1260  that is compatible with the socket  1150  of an intermediate cable  1100  is connected to the intermediate cable  1100  and the intermediate cable  1100  is in turn connected to a host device, audio signals output by the host device via the socket  1050  of the host device can be correctly output by the audio accessory device  1200 . In this situation the impedance R TOT  of the first signal path, as detected by the monitoring unit  1070 , is approximately equal to R L +R R , and the sum of the impedances of the second and third signal paths is approximately equal to R TOT +R TOL . 
     In contrast, when an audio accessory device  1200  having a plug  1260  that is not compatible with the socket  1150  of an intermediate cable  1100  is connected to the intermediate cable  1100  and the intermediate cable  1100  is in turn connected to a host device, audio signals output by the host device via the socket  1050  of the host device cannot be correctly output by the audio accessory device  1200 . In this situation the impedance R TOT  of the first signal path, as detected by the monitoring unit  1070 , is again approximately equal to R L +R R , but the sum of the impedances of the second and third signal paths is different from (e.g. greater than) R TOT +R TOL . 
     The monitoring unit  1070  is configured to determine the impedances of the first, second and third signal paths, typically in response to an event such as a request for audio playback (but may alternatively or additionally determine these impedances periodically, or may monitor them continuously or in response to some other request). The monitoring unit  1070  may be further configured to compare the detected impedance of the first signal path to the sum of the detected impedances of the second and third signal paths. If the monitoring unit  1070  detects that the sum of the measured impedances of the second and third signal paths is different from the impedance of the first signal path (within some predefined tolerance), the monitoring unit  1070  can output a signal S indicative of incompatibility between the intermediate cable  1100  and the connected accessory device  1200 . In response to the signal S the host device may take appropriate action such as suspending audio output and/or issuing a warning or notification indicating that the audio accessory device  1200  is incompatible with the intermediate cable  1100 . 
     The circuitry  1000  and/or monitoring unit  1070  is also able to detect removal of a four-pole jack plug from a three-pole socket of an intermediate cable such as an extension cable or splitter cable that is connected to the socket of a host device, as will now be explained with reference to  FIGS. 10 a -10 i   , which show the removal of a four-pole jack plug from a three-pole jack socket of an intermediate cable over a sequence of removal states. 
       FIG. 10 a    shows a four-pole plug  1260  of an audio accessory device  1200  fully received in a three-pole socket  1150  of an intermediate cable  1100  in an initial or first removal state. The three-pole socket of  FIG. 10 a    has a first configuration in which its sleeve contact  1156  is in contact with a sleeve contact  1268  of the plug  1260  when the plug  1260  is fully received in the socket  1150 . A three-pole plug  1160  of the intermediate cable  1100  is received in a four-pole socket of a host device. The contacts  1262 - 1268  of the plug  1262  are connected as in the arrangement illustrated in  FIG. 9 a   , i.e. the tip contact  1262 , the first ring contact  1264 , the second ring contact  1266  and the sleeve contact  1268  are connected, respectively, to the left audio load, the right audio load, the microphone and the ground connection of the audio accessory device  1200 . 
     In the first removal state of the plug  1260  illustrated in  FIG. 10 a    the first signal path between the first terminal  1010  of the monitoring unit  1070  and the second terminal  1020 , via the audio accessory  1200 , includes the left audio load impedance R L  and the right audio load impedance R R . Thus, the impedance R TOT  of the first signal path (i.e. the first impedance state) in the first removal state as measured by the monitoring unit  1070  is approximately equal to R L +R R . The second signal path, between the first terminal  1010  and the third or fourth terminal  1030 / 1040  includes only the left audio load impedance R L . Thus, the impedance R XL  of the second signal path (i.e. the second impedance state) in the first removal state as measured by the monitoring unit  1070  is approximately equal to R L  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Similarly, the second signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) includes only the right audio load impedance R R . Thus, the impedance of the third signal path (i.e. the third impedance state) in the first removal state, as determined by the monitoring unit  1070 , will be approximately equal to R R  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . 
       FIG. 10 b    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a second removal state. In the illustrated second removal state the socket tip contact  1152  is no longer in contact with any plug contact, whilst the socket ring contact  1154  is in contact with the plug tip contact  1262  and the socket sleeve contact  1156  is in contact with the second plug ring contact  1266 . 
     In the second removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the second removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the second removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  includes the left audio load and the microphone, and thus the impedance of the third signal path (i.e. the third impedance state) in the second removal state, as determined by the monitoring unit  1070 , is approximately R L +R MIC . 
       FIG. 10 c    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a third removal state. In the illustrated third removal state the socket tip contact  1152  is again no longer in contact with any plug contact, whilst the socket ring contact  1154  is in contact with the plug tip contact  1262  and the socket sleeve contact  1156  is in contact with the first plug ring contact  1264 . 
     In the third removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the second removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the second removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  includes the left audio load and the right audio load, and thus the impedance of the third signal path (i.e. the third impedance state) in the third removal state, as determined by the monitoring unit  1070 , is approximately R L +R R . 
       FIG. 10 d    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a fourth removal state which, for the purposes of the present disclosure, is equivalent to the plug  1260  being fully removed from the socket  1150 . In the illustrated fourth removal state the socket tip contact  1152  is again not in contact with any plug contact, whilst the socket ring contact  1154  no longer in contact with any plug contact  1262 . The socket sleeve contact  1156  is in contact with the plug tip contact  1262 . 
     In the fourth removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the fourth removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the fourth removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  is also open-circuit, since only a single socket contact (i.e. the socket sleeve contact  1156 ) is electrically connected to a plug contact (i.e. the plug tip contact  1262 ) such that no complete signal path exists. Thus, the impedance of the third signal path (i.e. the third impedance state) in the fourth removal state, as determined by the monitoring unit  1070 , is high impedance. 
     The impedance states (as “seen” by the host device) of the first, second and third signal paths in the first, second, third and fourth removal states of the plug  1260  for the socket configuration shown in  FIGS. 10 a -10 d    are summarised in Table 7 below: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                 First  
                 Second 
                 Third  
               
               
                   
                 impedance 
                 impedance 
                 impedance 
               
               
                 Removal state 
                 state 
                 state 
                 state 
               
               
                   
               
             
            
               
                 First 
                 R L  + R R  (approx.) 
                 R L  (approx.) 
                 R R  (approx.) 
               
               
                 Second 
                 High-Z 
                 High-Z 
                 R R  + R MIC  (approx.) 
               
               
                 Third 
                 High-Z 
                 High-Z 
                 R L  + R R  (approx.) 
               
               
                 Fourth 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                   
               
            
           
         
       
     
       FIG. 10 e    shows a four-pole plug  1260  of an audio accessory device  1200  fully received in a three-pole socket  1150  of an intermediate cable  1100  in an initial or first removal state. The three-pole socket of  FIG. 10 e    has a second configuration in which its sleeve contact  1156  is in contact with a second ring contact  1266  of the plug  1260  when the plug  1260  is fully received in the socket  1150 . A three-pole plug  1160  of the intermediate cable  1100  is received in a four-pole socket of a host device. The contacts  1262 - 1268  of the plug  1260  are connected to the elements of the audio accessory device  1200  as in  FIGS. 10 a   - 10   d.    
     In the first removal state of the plug  1260  illustrated in  FIG. 10 e    the first signal path between the first terminal  1010  of the monitoring unit  1070  and the second terminal  1020 , via the audio accessory, includes the left audio load impedance R L  and the right audio load impedance R R . Thus, the impedance R TOT  of the first signal path (i.e. the first impedance state) in the first removal state as measured by the monitoring unit  1070  is approximately equal to R L +R R . The second signal path, between the first terminal  1010  and the third or fourth terminal  1030 / 1040  includes only the left audio load impedance R L . Thus, the impedance R XL  of the second signal path (i.e. the second impedance state) in the first removal state as measured by the monitoring unit  1070  is approximately equal to R L  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) includes only the right audio load impedance R R . Thus, the impedance of the third signal path (i.e. the third impedance state) in the first removal state, as determined by the monitoring unit  1070 , will be approximately equal to R R  plus some additional impedance in the extension cable  1160  and the plugs  1160 ,  1260  and sockets  1050 ,  1150 . 
       FIG. 10 f    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a second removal state. In the illustrated second removal state the socket tip contact  1152  is no longer in contact with any plug contact, whilst the socket ring contact  1154  is in contact with the plug tip contact  1262  and the socket sleeve contact  1156  is in contact with an insulator that is provided between the plug tip contact  1262  and the first plug ring contact  1264 . 
     In the second removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the second removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the second removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  is also open circuit, due to the contact between the socket sleeve contact  1156  and the insulator between the plug tip contact  1262  and the first plug ring contact  1264 . Thus, the third impedance state is also high impedance. 
       FIG. 10 g    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a third removal state which, for the purposes of the present disclosure, is equivalent to the plug  1260  being fully removed from the socket  1150 . In the illustrated third removal state the socket tip contact  1152  is again not in contact with any plug contact, whilst the socket ring contact  1154  no longer in contact with any plug contact  1262 . The socket sleeve contact  1156  is in contact with the plug tip contact  1262 . 
     In the third removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the third removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the third removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  is also open-circuit, since only a single socket contact (i.e. the socket sleeve contact  1156 ) is electrically connected to a plug contact (i.e. the plug tip contact  1262 ) such that no complete signal path exists. Thus, the impedance of the third signal path (i.e. the third impedance state) in the third removal state, as determined by the monitoring unit  1070 , is high impedance. 
     The impedance states (as “seen” by the host device) of the first, second and third signal paths in the first, second and third removal states of the plug  1260  for the second socket configuration shown in  FIGS. 10 e -10 g    are summarised in Table 8 below: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                   
                 First 
                 Second 
                 Third  
               
               
                   
                 impedance 
                 impedance 
                 impedance 
               
               
                 Removal state 
                 state 
                 state 
                 state 
               
               
                   
               
             
            
               
                 First 
                 R L  + R R  (approx.) 
                 R L  (approx.) 
                 R R  (approx.) 
               
               
                 Second 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                 Third 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                   
               
            
           
         
       
     
       FIG. 10 h    shows a four-pole plug  1260  of an audio accessory device  1200  fully received in a three-pole socket  1150  of an intermediate cable  1100  in an initial or first removal state. The three-pole socket of  FIG. 10 g    has a third configuration in which its sleeve contact  1156  is in contact with an insulator between the second ring contact  1266  and the sleeve contact  1268  of the plug  1260  when the plug  1260  is fully received in the socket  1150 . A three-pole plug  1160  of the intermediate cable  1100  is received in a four-pole socket of a host device. The contacts  1262 - 1268  of the plug  1260  are connected to the elements of the accessory device  1200  as in  FIGS. 10 a   - 10   d.    
     In the first removal state of the plug  1260  illustrated in  FIG. 10 h    the first signal path between the first terminal  1010  of the monitoring unit  1070  and the second terminal  1020 , via the audio accessory, includes the left audio load impedance R L  and the right audio load impedance R R . Thus, the impedance R TOT  of the first signal path (i.e. the first impedance state) in the first removal state as measured by the monitoring unit  1070  is approximately equal to R L +R R . The second signal path, between the first terminal  1010  and the third or fourth terminal  1030 / 1040 , is open circuit, due to the contact between the socket sleeve contact  1156  and the insulator between the second plug ring contact  126  and the plug sleeve contact  1268 . Similarly, the third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  (via the extension cable  1100  and the accessory device  1200 ) is open circuit, due to the contact between the socket sleeve contact  1156  and the insulator between the second plug ring contact  126  and the plug sleeve contact  1268 . Thus, the second and third impedance states of the second and third signal paths, respectively, in the first removal state, as determined by the monitoring unit  1070 , will be high impedance. 
       FIG. 10 i    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a second removal state. In the illustrated second removal state the socket tip contact  1152  is no longer in contact with any plug contact, whilst the socket ring contact  1154  is in contact with the plug tip contact  1262  and the socket sleeve contact  1156  is in contact with the first plug ring contact  1156 . 
     In the second removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the second removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the second removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  includes the left and right audio loads. Thus, the impedance of the third signal path (i.e. the third impedance state) in the second removal state, as determined by the monitoring unit  1070 , is approximately R L +R R . 
       FIG. 10 j    shows the four-pole plug  1260  of an audio accessory device  1200  partially received in the three-pole socket  1150  of the intermediate cable  1100  in a third removal state which, for the purposes of the present disclosure, is equivalent to the plug  1260  being fully removed from the socket  1150 . In the illustrated third removal state the socket tip contact  1152  is again not in contact with any plug contact, whilst the socket ring contact  1154  no longer in contact with any plug contact  1262 . The socket sleeve contact  1156  is in contact with the plug tip contact  1262 . 
     In the third removal state, the first signal path from the first terminal  1010  of the monitoring unit  1070  to the second terminal  1020  is open-circuit, because the socket tip contact  1152  is not in contact with any plug contact. Thus, the impedance of the first signal path (i.e. the first impedance state) in the third removal state, as determined by the monitoring unit  1070 , is high impedance. The second signal path from the first terminal  1010  to the third or fourth terminal  1030 / 1040  is also open circuit, again due to the lack of any connection between the socket tip contact  1152  and any plug contact, and thus the impedance of the second signal path (i.e. the second impedance state) in the third removal state, as determined by the monitoring unit  1070 , is also high impedance. The third signal path from the second terminal  1020  to the third or fourth terminal  1030 / 1040  is also open-circuit, since only a single socket contact (i.e. the socket sleeve contact  1156 ) is electrically connected to a plug contact (i.e. the plug tip contact  1262 ) such that no complete signal path exists. Thus, the impedance of the third signal path (i.e. the third impedance state) in the third removal state, as determined by the monitoring unit  1070 , is high impedance. 
     The impedance states (as “seen” by the host device) of the first, second and third signal paths in the first, second and third removal states of the plug  1260  for the second socket configuration shown in  FIGS. 10 g -10 j    are summarised in Table 9 below: 
     
       
         
           
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                   
                 First  
                 Second 
                 Third 
               
               
                   
                 impedance 
                 impedance 
                 impedance 
               
               
                 Removal state 
                 state 
                 state 
                 state 
               
               
                   
               
             
            
               
                 First 
                 R L  + R R  (approx.) 
                 High-Z 
                 High-Z 
               
               
                 Second 
                 High-Z 
                 High-Z 
                 R L  + R R  (approx.) 
               
               
                 Third 
                 High-Z 
                 High-Z 
                 High-Z 
               
               
                   
               
            
           
         
       
     
     As will be apparent from the discussion above, regardless of which type of socket  1150  is provided on the intermediate cable  1100 , as the plug  1260  of the audio accessory device  1200  moves from the first removal state to the second removal state the impedance state of the first signal path changes from approximately R L +R R  to a high impedance state. The monitoring unit  1070  is configured to output a signal indicative of removal of the plug  1260  from the socket  1150  of the intermediate cable to downstream components (e.g. a processor or the like) of the host device on detection of this predetermined sequence of the impedance states of the first signal path. In response to receiving this signal the host device may take appropriate action, such as suspending the generation and output of audio signals and/or issuing an alert or notification to the user to advise the user of the disconnection of the audio accessory device  1200  from the intermediate cable  1100 , e.g. as a notification on a user interface of the host device. 
     It will be noted that in the examples discussed above in relation to  FIGS. 10 a -10 j    the contacts  1262 - 1268  of plug  1260  of the audio accessory device  1200  are connected to the elements of the audio accessory device in the manner illustrated in  FIG. 9 a   , i.e. the plug tip contact  1262 , first plug ring contact  1264 , second plug ring contact  1266  and plug sleeve contact  1268  are connected, respectively, to the left audio load, right audio load, microphone and ground connection of the audio accessory device  1200 . However, the change of the impedance of the first signal path described above will be observed when a plug  1260  that is connected as in  FIG. 9 b    (i.e. plug tip contact  1262 —left audio load, first plug ring contact  1264 —left audio load, second plug ring contact  1266 —audio accessory ground connection, plug sleeve contact  1268 —microphone) is removed from the socket  1150 . Thus, the plug removal mechanism described above is equally applicable to plugs that are connected as in  FIG. 9   b.    
     As will be appreciated by those skilled in the art, the circuitry described herein with reference to  FIGS. 8-10  can be used to detect the insertion of a jack into, and the removal of a jack plug from, a mismatched socket of an intermediate cable, as illustrated in  FIGS. 11 a   - 11   b.    
       FIG. 11 a    illustrates the removal of a 4-pole TRRS jack plug  1260  of an audio accessory device from a 3-pole TRS socket  1150  of an extension cable  1100  having a three-pole TRS jack plug that is received in a 4-pole socket  1050  of a host device. 
       FIG. 11 b    illustrates the removal of a 4-pole TRRS jack plug  1260  of an audio accessory device from a 3-pole TRRS socket of a splitter cable  1100  having a 3-pole TRS jack plug that is received in a 4-pole TRRS socket  1050  of a host device. 
     In the description above it has been assumed that the circuitry  100 ,  600 ,  1000  operates in isolation from other circuitry such as microphone detect circuitry that may be present in a host device. Thus, the impedance values and states provided in the description above are based on this assumption. As will be appreciated by those skilled in the art, where the circuitry  100 ,  600 ,  1000  is required to operate in conjunction with other circuitry such as microphone detection circuitry, the actual impedance values and states of the signal paths as the jack plug moves through the described removal states may differ from the impedance values and states described above. Those skilled in the art will readily be able to adapt the teachings of the present disclosure to detect impedance values and states that are appropriate to the requirements of a particular host device or other implementation, and thus it will be understood that the impedance values and states provided in the above description are examples used to illustrate the principles of the present disclosure, and are not limitations of the scope of the present disclosure. 
     The description above has presented the present disclosure in the context of an audio accessory connected via a jack plug  160 ,  260 ,  360 ,  460 ,  760  to be received in socket  150 ,  650 . However, the skilled person will appreciate that a wide variety of different accessory devices or apparatus may comprise a jack plug for a mating connection to a corresponding socket. The skilled person will therefore understand that the teaching in accordance with the present disclosure may be applied to any such accessory apparatus or device. 
     Circuitry according to embodiments of the present invention may be implemented as an integrated circuit and may be implemented in a host device. The term host device is used in this specification to refer to any electronic or electrical device which is removably connectable to an external accessory apparatus. The host device may especially be a portable and/or battery powered host device such as a mobile telephone, an audio player, a video player, a PDA, a mobile computing platform such as a laptop computer or tablet and/or a games device for example. A removable accessory apparatus is any apparatus which may be connected to and used with a host device. The accessory apparatus may, for instance, be a set of headphones, earphones, earbuds or the like, possibly including a microphone, or a headset. 
     The skilled person will thus recognise that some aspects of the above-described apparatus and methods may be embodied as processor control code, for example on a non-volatile carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications embodiments of the invention will be implemented on a DSP (Digital Signal Processor), ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog™ or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re)programmable analogue array or similar device in order to configure analogue hardware 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference numerals or labels in the claims shall not be construed so as to limit their scope. 
     As used herein, when two or more elements are referred to as “coupled” to one another, such term indicates that such two or more elements are in electronic communication or mechanical communication, as applicable, whether connected indirectly or directly, with or without intervening elements. 
     This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative. Accordingly, modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the disclosure. For example, the components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     Although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described above. 
     Unless otherwise specifically noted, articles depicted in the drawings are not necessarily drawn to scale. 
     All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the disclosure and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present disclosure have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure. 
     Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the foregoing figures and description. 
     To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. § 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.