Patent Publication Number: US-10779097-B2

Title: Audio connector and circuitry

Description:
The present application is a continuation of U.S. patent application Ser. No. 15/918,829, filed Mar. 12, 2018, now U.S. Pat. No. 10,306,387 issued May 28, 2019, which is a continuation of U.S. patent application Ser. No. 15/108,442, filed Jun. 27, 2016, now U.S. Pat. No. 9,955,272 issued Apr. 24, 2018, which is a 35 U.S.C. 371(c) National Stage Application of International Application No. PCT/CN2015/080823, filed on Jun. 4, 2015, each of which is incorporated herein by reference in its entirety. 
    
    
     This invention relates to a connector, e.g. a jack socket, for receiving a plug, and more specifically to a host device that includes such a connector, allowing an audio accessory device that has a suitable plug to be connected to the host device. 
     BACKGROUND 
     Many devices having audio outputs and/or inputs, such as mobile phones, tablets, computers and the like, are provided with a socket, allowing an audio accessory device with a suitable plug, such as a headset, to be connected to the device. 
       FIG. 1  illustrates an audio system, in which an audio accessory  20 , specifically a headset, is connected to a host device by means of a jack plug  25 . The host device comprises circuitry  11 , which connects to a number of contacts  26  in a jack socket.  FIG. 1  shows the situation in which the jack plug  25  is inserted into the jack socket, so that poles on the jack plug  25  make electrical contact with the contacts  26 . 
       FIG. 1  shows left and right driver amplifiers  102  and  104  driven from respective signal sources VinL and VinR. The signal sources VinL and VinR may for example comprise digital-to-analog converters (DACs) driven by received digital audio signals. These driver amplifiers  102 ,  104  are coupled to respective jack poles L and R by respective jack socket contacts. Signals from the driver amplifiers  102 ,  104  are then passed to left and right speakers respectively in the headset  20 . 
       FIG. 1  also shows an amplifier  106  serving as a microphone pre-amplifier coupled to a microphone pole, i.e. pole M, of the jack plug via a respective contact. The amplifier  106  thus receives a signal from a microphone in the headset  20 . 
     The pole G of the jack plug  25  provides a common ground return path for the signal paths through the other three poles L, R &amp; M. 
     In this example, the plug and socket arrangement is illustrated as being a 3.5 mm 4-Pole TRRS (Tip/Ring/Ring/Sleeve) jack plug and jack socket arrangement. The small size of such a socket allows only a small area of physical, and hence electrical, contact between the jack contact within the socket and the cylindrical pole of the inserted plug. Thus there may be an appreciable contact resistance between one or more respective jack contacts and respective poles, which may be further degraded due to corrosion or foreign matter. 
       FIG. 1  also illustrates that there may be various parasitic resistances, including a jack contact resistance, in the ground path between the ground jack pole G, i.e. node X 4 , and a ground reference point X 3  close to the circuitry  11 . These various parasitic resistances are illustrated as lumped together into a single resistance element R G . There may also be similar parasitic resistances between the microphone pole M of the jack plug and the microphone pre-amplifier  106  via the respective jack socket contact, and  FIG. 1  shows these parasitic resistances lumped similarly into a single resistance element R M . 
     The input signals VinL and VinR of the driver amplifier circuitry  102 ,  104  and the resulting buffered outputs VoutL and VoutR may be referenced to some ground point X 1  local to the driver amplifier circuitry  102 ,  104 . The microphone pre-amplifier  106  may be explicitly or implicitly referenced to a ground point X 2  local to the pre-amplifier  106 . By careful design of local ground planes or ground tracks on PCBs and/or in integrated circuits, these grounds may be maintained close in voltage to a common ground reference point, say the ground pin of an integrated circuit implementation of the circuitry  11 , which in turn may be closely coupled to some local ground reference point X 3 . However, the voltage signals VoutL and VoutR applied to the left and right speakers may give rise to corresponding ground return currents passing through the jack contact resistance and PCB trace resistance illustrated by R G , so it is likely that the ground voltage on the pole of the jack, node X 4 , will be significantly different from that at X 3  and will also be modulated by a.c. signal components of these ground return currents corresponding to the a.c. signal components of the applied voltages VoutL and VoutR. 
       FIG. 2  is a more detailed electrical circuit diagram showing the system of  FIG. 1 . 
     Thus,  FIG. 2  shows the left speaker  108 , the right speaker  110 , and the microphone  112  of the headset  20 . 
     The current I G  through the ground pole G of the jack plug comprises both the currents I SL  and I SR  through the left and right speakers  108 ,  110 . Thus, ignoring any ground return current corresponding to current through the microphone pole, i.e., M pole, of the plug:
 
 I   G   =I   SL   +I   SR .
 
     Given that the resistance R G  is non-negligible, the ground voltage on the ground pole, i.e. G pole, of the jack, at node X 4 , will differ from that at X 3  by a voltage V RG , where:
 
 V   RG   =I   G   *R   G .
 
     The input voltage V micA  to the microphone amplifier  106  will not be identical to the voltage V mic  generated by the microphone  112 , but will be modulated by the voltage V RG  as it varies with the variation of the total current I G  flowing through the left and right speakers  108 ,  110  into the ground return path. Thus there will be crosstalk of the electrical speaker signal into the sensitive microphone channel (which is especially relevant because a typical microphone signal might be only of the order of 10 mV). That is:
 
 V   micB   =V   micA   =V   mic   +V   RG .
 
     Also the respective voltages actually appearing across each respective speaker will be reduced by V RG . That is:
 
 V   SL   =V out L−V   RG  and
 
 V   SR   =V out R−V   RG  
 
     Since V RG  is dependent on both I SR  and I SL , one effect is that the respective currents through each respective speaker influence the voltage that is generated across the other speaker, and so there is crosstalk from the right channel to the left channel, and from the left channel to the right channel, which can distort or blur the stereo image. 
     The ground return path via resistance R G  will also carry ground return current from any current taken through microphone pole M of the jack plug, and any modulation of this current may appear as audible crosstalk in the speaker outputs. The current through microphone pole M may comprise supply current for the microphone  112  which may vary with the microphone signal and possibly also if the microphone  112  is turned on or off or passes through different microphone operating modes, possibly autonomously due to some internal voice activity detector or suchlike. 
     Also in some applications current on this microphone supply line is used to signal to the host device, for example via one or more pushbutton switches coupled between the microphone pole M and ground pole G either directly or via resistances, illustrated by resistance Rpb  113 . Use of the one or more pushbutton switches may give rise to significant step changes in the current through poles M and G, which may in turn give audible artefacts through modulating V RG . 
     SUMMARY 
     According to an aspect of the present invention, there is provided a device for connection to an audio accessory comprising: 
     (a) a jack socket for connection to a jack plug of the audio accessory, wherein the jack socket comprises: 
     (i) first and second contacts, positioned to mate with a first pole of the jack plug, wherein one of the first and second contacts is coupled to ground, 
     (ii) a third contact positioned to mate with a second pole of the jack plug, and 
     (iii) a fourth contact, positioned to mate with a third pole of the jack plug, 
     (b) a first audio driver amplifier, comprising: 
     (i) a first input coupled to receive a first driver signal, 
     (ii) a second input coupled to an other of the first and second contacts, and 
     (iii) an output coupled to said third contact, and 
     (c) a second audio driver amplifier, comprising: 
     (i) a first input coupled to receive a second driver signal, 
     (ii) a second input coupled to the other of the first and second contacts, and 
     (iii) an output coupled to said fourth contact. 
     According to another aspect of the present invention, there is provided a device for connection to an audio accessory comprising: 
     (a) a jack socket for connection to a jack plug of the audio accessory, wherein the jack socket comprises, 
     (i) first and second contacts, positioned to mate with a first pole of the jack plug, wherein one of the first and second contacts is coupled to ground, 
     (ii) a third contact positioned to mate with a second pole of the jack plug, and 
     (iii) a fourth contact, positioned to mate with a third pole of the jack plug, 
     (b) a first audio driver amplifier, comprising: 
     (i) a first input coupled to receive a first driver signal; 
     (ii) a second input coupled to an other of said first and second contacts, and 
     (iii) an output coupled to said third contact, and 
     (c) a microphone pre-amplifier comprising: 
     (i) a first input coupled to said fourth contact, 
     (ii) a second input coupled to the other of the first and second contacts, and 
     (iii) an output coupled to output an amplified signal to a terminal of the device. 
     According to another aspect of the present invention, there is provided a first device for connection to a second device, comprising: 
     (a) a first device connector for connection to a connector of the second device, wherein the first device connector comprises, 
     (i) a first contact, coupled to ground, positioned to mate with a first pole of the connector of the second device, 
     (ii) a second contact positioned to mate with the first pole of the connector of the second device, 
     (iii) a third contact positioned to mate with a second pole of the connector of the second device, and 
     (iv) a fourth contact, positioned to mate with a third pole of the connector of the second device, 
     (b) a first amplifier wherein, 
     (i) a first input of the first amplifier is coupled to the second contact of the first device connector, 
     (ii) a second input of the first amplifier is coupled to receive a driver signal, and 
     (iii) an output of the first amplifier is coupled to the third contact of the first device connector, and 
     (c) a second amplifier wherein, 
     (i) a first input of the second amplifier is coupled to the second contact, 
     (ii) a first port of the second amplifier is coupled to the fourth contact of first device connector, and 
     (iii) a second port of the second amplifier is coupled to a terminal of the device. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  is a circuit diagram of an audio system. 
         FIG. 2  is a more detailed circuit diagram of the system of  FIG. 1 . 
         FIG. 3  illustrates an audio system. 
         FIG. 4 a    is a circuit diagram of an audio system as shown in  FIG. 3 . 
         FIG. 4 b    illustrates a plug and socket in the audio system of  FIG. 4 . 
         FIG. 5 a    is a circuit diagram of an alternative audio system as shown in  FIG. 3 . 
         FIG. 5 b    illustrates a plug and socket in the audio system of  FIG. 5 . 
         FIG. 6 a    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
         FIG. 6 b    illustrates a plug and socket in the audio system of  FIG. 6   a.    
         FIG. 6 c    illustrates an alternative form of a plug and socket in the audio system of  FIG. 6   a.    
         FIG. 6 d    illustrates an alternative form of a plug and socket in the audio system of  FIG. 6   a.    
         FIG. 6 e    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
         FIG. 7 a    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
         FIGS. 7 b  and 7 c    illustrate embodiments of a plug and socket in the audio system of  FIG. 7   a.    
         FIG. 8 a    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
         FIGS. 8 b  and 8 c    illustrate embodiments of a plug and socket in the audio system of  FIG. 8   a.    
         FIG. 9 a    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
         FIG. 9 b    is a circuit diagram of a further alternative audio system as shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  illustrates a host device  10 , which may take the form of a mobile phone or tablet computer or suchlike, provided with a socket  14  into which a plug  25  may be attached. In this case, the plug  25  is provided on a cable  21  that forms part of an audio accessory  20 . The audio accessory  20  is in the form of a headset, which comprises a pair of stereo speakers  22 ,  23 , a voice microphone  24  and a push-button module  26 . In this illustrated embodiment, plug  25  may be a jack plug, for instance a standard 3.5 mm diameter multi-pole audio jack plug, and socket  14  may be a compatible jack socket, but other forms of multi-pole connector may alternatively be implemented. 
     The host device  10  comprises circuitry  11  which may communicate with an attached accessory  20  via multiple wires  17  (which may be printed circuit board (PCB) tracks, or cables with conductors soldered on to a PCB or connected via edge connectors or suchlike) and via the socket  14 . The circuitry  11  may comprise driver amplifiers for driving the headset speakers  22 ,  23  and a microphone preamplifier for amplifying an analogue signal received from microphone  24 . Circuitry  11  may also comprise other analog or digital functions and may be an integrated circuit in the form of an audio codec and may be coupled to other circuitry  18 ,  19   a  and  19   b , for instance an applications processor or a Bluetooth modem or communications processor or suchlike. The location of circuitry  11  will be governed by many constraints, for example it may be preferable to situate circuitry  11  in close proximity to other circuitry  18  and  19   a  and  19   b  to reduce the length of the many connections between these circuits, possibly carrying high-speed digital signals, for instance three digital audio bus connections to or from an integrated circuit audio codec embodiment of circuitry  11  to or from an applications processor, an r.f. modem and a communications processor, in which case the circuitry  11  might not be located adjacent to the jack socket  14 . A PCB is likely to be tightly packed, and thus wires  17  that are in the form of PCB tracks  17  may be minimum or near minimum width. Thus there may be appreciable parasitic resistance of the metal tracks  17 . In some cases this resistance may even be desirable, or may comprise the impedance of added components, to help protect the circuitry  11  from ESD (Electro-Static Discharge) events occurring at the external jack socket or to help filter high frequency EMI (Electromagnetic Interference). 
     The circuitry  11  may be provided with a star ground reference point  12 . 
       FIG. 4 a    is an electrical circuit diagram of a system of the general type shown in  FIG. 3  comprising an embodiment. 
     Thus,  FIG. 4 a    shows the left speaker  402 , the right speaker  404 , and the microphone  406  of the headset  20 . The headset  20  is provided with a plug  25 , which in this example is a TRRS jack plug. As shown in more detail in  FIG. 4 b   , the left speaker  402  is connected between the tip (T)  408  and the sleeve (S)  410  of the plug  25 . The right speaker  404  is connected between the first ring (R 1 )  412  and the sleeve (S)  410  of the plug  25 . The microphone  406  is connected between the second ring (R 2 )  414  and the sleeve (S)  410  of the plug  25 . Thus, the sleeve (S)  410  acts as a common ground return. 
     The tip  408 , first ring  412 , second ring  414  and sleeve  410  of this plug may thus be regarded as the L, R, M and G poles respectively of the jack plug  25 . 
     In the circuitry  11 , as illustrated in  FIG. 4 a   , a left driver amplifier  416  is driven from a signal source VinL and produces a buffered output signal VoutL, and a right driver amplifier  418  is driven from a signal source VinR and produces a buffered output signal VoutR. The signal sources VinL and VinR may for example comprise digital-to-analog converters (DACs). Outputs of these driver amplifiers  416 ,  418  are coupled to respective jack socket contacts  420 ,  422 . 
       FIG. 4 a    also shows an amplifier  424  serving as a microphone pre-amplifier with an input coupled to a jack socket contact  426 . 
     A further jack socket contact  428  provides the common ground return path for the signal paths through the other three jack socket contacts. 
     Thus, when the jack plug is inserted into the jack socket, the tip contact  408  of the plug  25  contacts the jack socket contact  420 ; the first ring contact  412  of the plug  25  contacts the jack socket contact  422 ; the second ring contact  414  of the plug  25  contacts the jack socket contact  426 ; and the sleeve contact  410  of the plug  25  contacts the jack socket contact  428 . 
     As shown in  FIG. 4 a   , the jack socket is provided with an additional jack socket contact  432 . The additional jack socket contact  432  is positioned in the socket such that, when the plug  25  is inserted into the socket, the sleeve contact  410  of the plug  25  contacts the additional jack socket contact  432  as well as the jack socket contact  428 . 
     The additional jack socket contact  432  is connected to a second input of the microphone amplifier  424 . Preferably, amplifier  424  is designed to produce an output V micB  that depends on the difference between its two inputs V micA  and V micC . 
     The additional jack socket contact  432  is also connected to respective second inputs of the left driver amplifier  416  and the right driver amplifier  418 . The common voltage on these second inputs is denoted Vcom. Preferably, amplifier  416  is designed to produce an output VoutL which depends on the sum of VinL and Vcom, while amplifier  418  is similarly designed such that VoutR depends on the sum of VinR and Vcom. Preferably amplifiers  416  and  418  are identical in design or at least very similar, so as to provide well-matched signal paths from their respective inputs to their respective outputs. The respective second inputs of the amplifiers  416 ,  418  and  424  may be denoted the reference inputs for the amplifiers. 
       FIG. 4 a    illustrates the various parasitic resistances in the ground path between the jack pole G, i.e. node X 4 , and a ground reference point, i.e. node, X 3  close to the circuitry  11 , lumped into a single resistance element R G . These parasitic resistances may comprise the contact resistance of jack socket contact  428 , solder resistance associated with the connection of pins of an associated jack socket terminals to a PCB on which the socket is mounted, trace resistance of any PCB track in the path, resistance of any cabling in the path, contact resistance associated with any edge connectors or suchlike between cables and PCBs or between PCBs, or many other possible parasitic or added series elements. In some embodiments there may also be one or more switches in this path, to allow reconfiguration of the socket contacts to accommodate different types of accessory, and any such switches may have significant associated contact or active device on-resistance. 
     There may also be similar parasitic resistances, including a jack contact resistance component, between the jack pole M and the microphone pre-amplifier  424 , and  FIG. 4 a    illustrates these as lumped into a single resistance element R M . Similarly, any parasitic resistances associated with the path associated with the additional jack socket contact  432  are illustrated in  FIG. 4 a    lumped together as the resistance element R G2 . To explain the operation of the circuit it may be assumed that the second or reference inputs of the left driver amplifier  416  and the right driver amplifier  418  that are connected to Vcom, and the inputs of the microphone amplifier  424  that are connected to V micA  and V micC  are all high impedance inputs. Hence, there will be negligible currents I M  and I G2  flowing through the respective parasitic resistances R M  and R G2  associated with the tracks to these inputs, and thus the voltage drops V RM  and V RG2  will also be negligible. Similarly if there are any parasitic resistances (not illustrated) associated with any separate (i.e. not shared) segments of paths between the illustrated shared impedance R G2  and the driver or microphone amplifier inputs any resulting voltage drops will also be negligible. 
     The current I G  through the ground pole G of the jack plug comprises both the currents I SL  and I SR  through the left and right speakers  402 ,  404 . Thus, ignoring any current through the M pole of the plug that might return via pole G:
 
 I   G   =I   SL   +I   SR .
 
     Given that any current flowing through contact  432  may be negligible compared with that flowing through contact  428 , the whole of I G  will flow through parasitic resistance R G , and so the ground voltage on the G pole of the jack, at node X 4 , will differ from that at node X 3  by a voltage V RG , where:
 
 V   RG   =I   G   *R   G .
 
     The voltage actually appearing across each speaker  402 ,  404  will be the voltage generated by the respective drive amplifier  416 ,  418 , reduced by V RG . That is:
 
 V   SL   =V out L−V   RG and
 
 V   SR   =V out R−V   RG  
 
     As mentioned above, amplifiers  416  and  418  are configured such that VoutL=VinL+Vcom, and VoutR=VinR+Vcom. Moreover, as V RG2  is negligible, Vcom=V RG . 
     Thus:
 
 V   SL   =V out L−V   RG =( V in L+V com)− V   RG =( V in L+V   RG )− V   RG   =V in L , and
 
 V   SR   =V out R−V   RG =( V in R+V com)− V   RG =( V in R+V   RG )− V   RG   =V in R.  
 
     Thus, V SL =VinL and V SR =VinR, and there is no crosstalk signal at the speakers  402 ,  404 . 
     As also mentioned above amplifier  424  is configured such that:
 
 V   micB   =V   micA   −V   mic .
 
     Considering the voltages shown in  FIG. 4   a:  
 
 V   micA   =V   mic   +V   RG   −V   RM  and
 
 V   micC   =V   RG   −V   RG2 .
 
However, as mentioned above, V RM  and V RG2  are negligible, and so:
 
 V   micB   =V   micA   −V   mic =( V   mic   +V   RG )− V   RG   =V   mic .
 
     Thus the net voltage appearing at the output of the microphone preamplifier  424  is equal to V mic  (possibly with a voltage gain applied, depending on the configuration of the amplifier  424 ). 
     Any other interference appearing at the jack pole G, i.e. any other additional component of V RG  superimposed on I G ·R G , due for instance to rectified EMI (electromagnetic interference) will also be removed from the net speaker drive and microphone sense signals. 
     The additional socket contact  432  may be implemented mechanically in various forms depending on the application.  FIG. 4 a    illustrates a construction where the additional contact  432  may be located on the same side of the jack plug as the contact  428 . In other embodiments, where space and the mechanical construction technology allow, the additional contact  432  may be substantially on the opposite side of the jack plug from the contact  428 , as illustrated in  FIG. 4 b   , and displaced further along the plug in a direction parallel with the axis of the plug. Other mechanical configurations are of course possible. 
       FIG. 5 a    is an electrical circuit diagram of a system similar to that shown in  FIG. 4 a    but without a microphone in the accessory. 
     Thus,  FIG. 5 a    shows the left speaker  502  and the right speaker  504  of the headset  20 . The headset  20  is provided with a jack plug  25   a , which in this example is a 3-pole TRS plug. As shown in  FIG. 5 b   , the left speaker  502  is connected between the tip (T)  506  and the sleeve (S)  508  of the plug  25 . The right speaker  504  is connected between the ring (R)  510  and the sleeve (S)  508  of the plug  25 . Thus, the sleeve  508  acts as a common ground return. 
     The tip  506 , ring  510  and sleeve  508  of this plug may thus be regarded as the L, R and G poles respectively of the jack plug  25   a.    
     In the circuitry  11 , a left driver amplifier  512  is driven from a signal source VinL and produces a buffered output signal VoutL, and a right driver amplifier  514  is driven from a signal source VinR and produces a buffered output signal VoutR. The signal sources VinL and VinR may for example comprise digital-to-analog converters (DACs). The driver amplifiers  512 ,  514  are coupled to respective jack socket contacts  516 ,  518 . 
     A third jack socket contact  520  provides the common ground return path for the signal paths through the other two jack socket contacts  516 ,  518 . 
     Thus, when the jack plug  25   a  is inserted into the jack socket, the tip contact  506  of the plug  25   a  contacts the jack socket contact  516 ; the ring contact  510  of the plug  25   a  contacts the jack socket contact  518 ; and the sleeve contact  508  of the plug  25   a  contacts the jack socket contact  520 . 
     As shown in  FIG. 5 a   , the jack socket is provided with an additional, i.e. a fourth, jack socket contact  522 . The additional jack socket contact  522  is positioned in the socket such that, when the plug  25   a  is inserted into the socket, the sleeve contact  508  of the plug  25   a  contacts the additional jack socket contact  522  as well as the jack socket contact  520 . 
     The additional jack socket contact  522  is connected to respective second, i.e. reference, inputs of the left driver amplifier  512  and the right driver amplifier  514 . The voltage on these second inputs is denoted Vcom. Preferably, amplifier  512  is designed to produce an output VoutL which depends on the sum of VinL and Vcom, and amplifier  514  is designed similarly to provide an output VoutR which depends on the sum of VinR and Vcom. 
       FIG. 5 a    illustrates the various parasitic resistances in the ground path between the jack pole G and a ground reference point X 3  close to the circuitry  11 , lumped into a single resistance element R G . 
     There may also be similar parasitic resistances between the jack plug pole G and the driver amplifiers via socket contact  522 , and  FIG. 5 a    illustrates these as lumped into a single resistance element R G2 . 
     It can be assumed that the inputs of the left driver amplifier  512  and the right driver amplifier  514  connected to Vcom are high impedance. Hence, there will be a negligible current I G2  flowing in the parasitic resistance R G2  associated with the tracks to these inputs, and thus the voltage drop V RG2  will also be negligible. 
     The current I G  through the ground pole G of the jack plug  25   a  is the sum of the currents I SL  and I SR  through the left and right speakers  502 ,  504 . That is:
 
 I   G   =I   SL   +I   SR .
 
     Given that any current flowing through contact  522  may be negligible compared with that flowing through contact  520  the whole of I G  will flow through parasitic resistance R G , and so the ground voltage on the pole G of the jack, at node X 4 , will differ from that at node X 3  by a voltage V RG , where:
 
 V   RG   =I   G   *R   G .
 
     The voltage actually appearing across each speaker  502 ,  504  will be the voltage generated by the respective drive amplifier  512 ,  514 , reduced by V RG . That is:
 
 V   SL   =V out L−V   RG  and
 
 V   SR   =V out R−V   RG  
 
     As mentioned above, driver amplifiers  512  and  514  are configured such that VoutL=VinL+Vcom, and VoutR=VinR+Vcom. Moreover, as V RG2  is negligible, Vcom=V RG . 
     Thus:
 
 V   SL   =V out L—V   RG =( V in L+V com)− V   RG =( V in L+V   RG )− V   RG   =V in L , and
 
 V   SR   =V out R−V   RG =( V in R+V com)− V   RG =( V in R+V   RG )− V   RG   =V in R.  
 
     Thus, V SL =VinL and V SR =VinR, and there is no crosstalk signal at the speakers  502 ,  504 . 
     Any other interference appearing at the jack pole, i.e. any other additional component of V RG  superimposed on I G ·R G , due for instance to rectified EMI (electromagnetic interference) or to ground return currents from other circuitry will also be removed from the net speaker drive signals. 
     The additional jack socket contact  522  may be implemented mechanically in various forms depending on the application.  FIG. 5 b    illustrates a construction where the additional contact  522  may be located on the same side of the jack plug as the contact  520 . In other embodiments where space and the mechanical construction technology allow, the additional contact  522  may be substantially on the opposite side of the jack plug from the contact  520 . Other mechanical configurations are of course possible. 
       FIG. 6 a    is an electrical circuit diagram of an alternative system of the type shown in  FIG. 3 , but again in which the accessory  20  currently attached does not have a microphone. The host device is however adapted so as to be able to operate in alternative configurations so as to co-operate with other accessories which may have microphones or other components coupled to poles of their respective plugs. These alternative configurations are enabled by means of a plurality of switches which may comprise switches illustrated as elements  626  and  624  and may also comprise other switches according to the configurations to be enabled. 
     Thus,  FIG. 6 a    shows the left speaker  602  and the right speaker  604 . The headset  20  is provided with a jack plug  25   b , which in this example is a 3-pole TRS plug. As illustrated in conjunction with  FIG. 6 b   , the left speaker  602  is connected between the tip (T)  606  and the sleeve (S)  608  of the plug  25   b . The right speaker  604  is connected between the ring (R)  610  and the sleeve (S)  608  of the plug  25   b . Thus, the sleeve  608  acts as a common ground return. 
     The tip  606 , ring  610  and sleeve  608  of this plug may thus be regarded as the L, R and G poles respectively of the jack plug  25   b.    
     In the circuitry  11 , a left driver amplifier  612  is driven from a signal source VinL and produces a buffered output signal VoutL, and a right driver amplifier  614  is driven from a signal source VinR and produces a buffered output signal VoutR. The signal sources VinL, VinR may for example comprise digital-to-analog converters (DACs). These driver amplifiers  612 ,  614  are coupled to respective jack socket contacts  616 ,  618 . Driver amplifiers  612  and  614  may be configured such that VoutL=VinL+Vcom, and VoutR=VinR+Vcom where Vcom is a voltage applied to a pair of respective second, i.e. reference, inputs of these driver amplifiers  612  and  614 . 
     The jack socket is mechanically configured such that when the jack plug  25   b  is inserted into the jack socket, the tip contact  606  of the plug  25   b  contacts the jack socket contact  616 ; the ring contact  610  of the plug  25   b  contacts the jack socket contact  618 ; and the sleeve contact  608  of the plug  25   b  contacts the jack socket contact  620 . As shown in  FIGS. 6 a  and 6 b   , the jack socket is provided with an additional jack socket contact  622 . The additional, i.e. fourth, jack socket contact  622  is positioned in the socket such that, when the plug  25   b  is inserted into the socket, the sleeve contact  608  of the plug  25   b  contacts the additional jack socket contact  622  as well as the jack socket contact  620 . 
     Thus, similarly to the embodiment illustrated in  FIGS. 5 a  and 5 b   , the jack socket comprises a plurality of jack contacts which are mechanically configured to mate with the common ground return pole G of a TRS plug when such a plug is inserted. However, in this particular embodiment, rather than one contact being permanently configured to provide a low impedance ground current return path to ground while the other is permanently dedicated to couple to driver amplifiers, in this embodiment each is connected to respective poles of each switch  624  and  626 . The other terminal of first switch  624  is coupled to ground at a ground reference point X 3 . The other terminal of second switch  626  may be coupled to driver amplifiers or other circuitry comprised in circuitry  11 , which may comprise microphone amplifiers or other functions. 
     When the host device is configured to co-operate with the TRS plug of  FIG. 6 b   , the first switch  624  may be controlled to couple the jack plug pole G via one socket contact  620  to ground reference point X 3  while the second switch  626  may be controlled to couple the jack plug pole G via the other socket contact  622  to the pair of respective second inputs of the left driver amplifier  612  and the right driver amplifier  614 , i.e. to the signal line Vcom. Alternatively the roles of switches  624  and  626  may be interchanged such that jack plug pole G is coupled to ground X 3  via switch  626  and to Vcom via switch  624 . 
       FIG. 6 a    illustrates the various parasitic resistances in the ground path from the jack pole G via socket contact  620  to the switches  624  and  626 , lumped into a single resistance element R G . Similarly  FIG. 6 a    illustrates the various parasitic resistances in the ground path from the jack pole G via socket contact  622  to the switches  624  and  626 , lumped into a single resistance element R M . 
     The parasitic resistances associated with the switch  624  are illustrated in  FIG. 6 a    lumped together as the resistance element R GS , and the resistances associated with the switch  626  are shown in  FIG. 6 a    lumped together as the resistance element R CS . 
     Assuming the case where ground pole G is coupled to ground reference point X 3  via the contact  620  and the switch  624 , and to Vcom via the contact  622  and the switch  626 , and assuming that the inputs of the left driver amplifier  612  and the right driver amplifier  614  connected to Vcom are high impedance, there will be negligible currents flowing in the parasitic resistances R M  and R CS  associated with the path from pole G to Vcom, and thus any associated voltage drop will be negligible. 
     The current I G  through the ground pole G of the jack plug is the sum of the currents I SL  and I SR  through the left and right speakers  602 ,  604 . That is:
 
 I   G   =I   SL   +I   SR .
 
     Considering the case where ground pole G is coupled to ground reference point X 3  via the contact  620  and the switch  624 , and given that any current flowing through contact  622  may be negligible compared with that flowing through contact  620  the whole of I G  will flow through parasitic resistance R G , and so the ground voltage on the pole of the jack, at node X 4 , will differ from that at X 3  by a voltage V RG , where:
 
 V   RG   =I   G *( R   G   +R   GS ).
 
     The voltage actually appearing across each speaker  602 ,  604  will be the voltage generated by the respective drive amplifier  614 ,  614 , reduced by V RG . That is:
 
 V   SL   =V out L−V   RG  and
 
 V   SR   =V out R−V   RG .
 
     As mentioned above, the driver amplifiers  612  and  614  may be configured such that VoutL=VinL+Vcom, and VoutR=VinR+Vcom. Moreover, as any voltage drop between plug pole G and Vcom is negligible, Vcom=V RG . 
     Thus:
 
 V   SL   =V out L−V   RG =( V in L+V com)− V   RG =( V in L+V   RG )− V   RG   =V in L , and
 
 V   SR   =V out R−V   RG =( V in R+V com)− V   RG =( V in R+V   RG )− V   RG   =V in R.  
 
     Thus, V SL =VinL and V SR =VinR, and there is no crosstalk signal at the speakers  602 ,  604 . 
     Any other interference appearing at the jack pole, i.e. any other additional component of V RG  superimposed on I G ·R G , due for instance to rectified EMI or to ground return currents from other circuitry will also be removed from the net speaker drive signals. 
     The additional jack socket contact  622  may be implemented mechanically in various forms depending on the application.  FIG. 6 a    illustrates a construction where the additional contact  622  may be located on the same side of the jack plug as the contact  620 . In other embodiments where space and the mechanical construction technology allow, the additional contact  622  may be substantially on the opposite side of the jack plug from the contact  620 . Other mechanical configurations are of course possible. 
       FIGS. 6 c  and 6 d    illustrate examples of accessories with which a host device similar to that discussed with respect to  FIGS. 6 a  and 6 b    might be configured to co-operate. 
       FIG. 6 c    in conjunction with  FIG. 6 e    illustrates an accessory which comprises speakers  602 ,  604  connected to the tip  708  and first ring  712  of the jack. However in this accessory the common ground return of the speakers is connected to the second ring  714  of the jack rather than the sleeve. The sleeve  710  is connected to an array of one or more push-buttons in a module  26 , which define a resistance Rpb between the sleeve and the common ground return. This resistance varies according to which buttons are pushed. In some cases one of the resistance values could essentially be zero, for example less than 1 ohm or less than 100 milohm or be essentially just the parasitic resistance of the cabling and switch contact resistance. 
     The tip  708 , first ring  712 , second ring  714  and sleeve  710  of this plug may thus be regarded as the L, R, G and M poles respectively of the jack plug  25   e.    
     In use the ground pole G of the jack may be coupled to ground via socket contact  622  and switch  624 . The sleeve of the jack plug, i.e. pole M, is coupled to circuitry  11  via socket contact  620  and switch  626 . In order for circuitry  11  to be able to measure the resistance Rpb without audio artefacts, an a.c. signal current as illustrated by a.c. current source  690  of frequency outside the audio bandwidth or outside the passband of the speaker frequency response may be injected onto Rpb via switch  626 , contact  620  and jack plug pole M and the resulting a.c. voltage monitored, for example by an ADC  691  as illustrated or by some analog amplitude detector. By means of circuitry  692  configured to compare the a.c. voltage or apparent impedance versus various predefined ranges consistent with the resistances corresponding to various push-button activations, the button or buttons activated may be detected, and control signals  693  generated to request corresponding action to be taken in the host device, for example to increase or decrease the playback volume or to start, pause or terminate playback. 
     By arguments similar to those above, when R pb  is set to a low value, of the same order as the parasitic resistances R G , R M  etc., and the driver amplifiers present an adequately high input impedance at Vcom then there will be negligible current or voltage drop associated with resistances R pb , R G  and R CS , and so the ground sense voltage monitored by Vcom will be equal to the voltage drop V RG  between jack ground node X 4  and the ground reference node X 3 . Thus there will be no crosstalk between the speakers. Even for higher values of R pb , say of the order of one kilohm, provided that the input impedance of the driver amplifiers and any other circuitry connected to Vcom is adequately high, say greater than 100 kilohm or than 1 Megohm, the current flowing from R pb  into Vcom will be low enough to still give greatly improved crosstalk over a circuit scheme without Vcom (i.e. a scheme equivalent to shorting Vcom to ground X 3 ). 
       FIG. 6 d    illustrates a similar accessory, but this accessory has poles G and M reversed (as illustrated by the dotted alternative connections  680  in  FIG. 6 e   ). Thus the sleeve  710  is the common ground or pole G and the second ring is used as the signalling pole M. 
     This variant of accessory can be accommodated merely by switching both switches to the other position. That is, the ground pole G of the jack may be coupled to ground via socket contact  620  and switch  624 . The second ring of the jack plug, i.e. pole M, is coupled to circuitry  11  via socket contact  622  and switch  624 . 
     Again for an adequately high input impedance at Vcom, then there will be negligible current or voltage drop associated with resistances R pb , R G  and R CS , and so Vcom will follow V RG  and be superimposed on VinL and VinR and speaker crosstalk will be greatly improved. 
     In a further variant of accessories illustrated in  FIGS. 6 c , 6 d  and 6 e   , the pushbutton resistance Rpb may be replaced by a short circuit. Thus either of the G and M poles of the plug may both carry the ground return current from the speakers and the other may be used to sense the ground voltage at the speakers. The parasitic resistance of the wires between plug and cable may be lumped together with RG or RM, and again the voltage drop across the sense path will be negligible, and the driver amplifiers will deliver output voltages with the sensed speaker ground voltage superimposed. 
       FIG. 7 a    is an electrical circuit diagram of a further alternative system of the general type shown in  FIG. 3  illustrating a further embodiment. 
     Thus,  FIG. 7 a    shows the left speaker  702 , the right speaker  704 , and the microphone  706  of the headset  20 . The headset  20  is provided with a jack plug, which in this example is a TRRS plug. The host device in this system comprises a socket, switches and circuitry  11  such that it can accommodate accessories that have plugs that are connected with different ground and microphone connections. 
       FIG. 7 b    shows a jack plug  25   e  connected according to the OMTP standard, and  FIG. 7 c    shows a jack plug  25   f  connected according to the CTIA standard, and both figures show the connections between mating contacts in these plugs and the socket arrangement as also illustrated in  FIG. 7   a.    
     More specifically, in the case of the OMTP jack plug  25   e  of  FIG. 7 b   , the common ground return connection is through the sleeve  710 , and the microphone is connected to the second ring  714 . The left speaker  702  is therefore connected between the tip (T)  708  and the sleeve (S)  710  of the plug  25   e . The right speaker  704  is connected between the first ring (R 1 )  712  and the sleeve (S)  710  of the plug  25   e . The microphone  706  is therefore connected between the second ring (R 2 )  714  and the sleeve (S)  710  of the plug  25 . Thus, the second ring  714  acts as a common ground return. 
     The tip  708 , first ring  712 , second ring  714  and sleeve  701  of this plug may thus be regarded as the L, R, M and G poles respectively of the jack plug  25   e.    
     In the case of the CTIA jack plug  25   f  of  FIG. 7 c   , the common ground return connection is through the second ring  714 , and the microphone is connected to the sleeve  710 . 
     The left speaker  702  is therefore connected between the tip (T)  708  and the second ring  714  of the plug  25   f . The right speaker  704  is connected between the first ring (R 1 )  712  and the second ring  714  of the plug  25   f . The microphone  706  is connected between the sleeve (S)  710  and the second ring (R 2 )  714  of the plug  25   f . Thus, the second ring  714  acts as a common ground return. 
     The tip  708 , first ring  712 , second ring  714  and sleeve  701  of this plug may thus be regarded as the L, R, G and M poles respectively of the jack plug  25   e.    
     In the circuitry  11 , a left driver amplifier  716  is driven from a signal source VinL and produces a buffered output signal VoutL, and a right driver amplifier  718  is driven from a signal source VinR and produces a buffered output signal VoutR. The signal sources VinL and VinR may for example comprise digital-to-analog converters (DACs). These driver amplifiers  716 ,  718  are coupled to respective jack socket contacts  720 ,  722 . Driver amplifiers  716  and  718  may be configured such that VoutL=VinL+Vcom, and VoutR=VinR+Vcom where Vcom is a voltage applied to a pair of respective second inputs of these driver amplifiers  716  and  718 . These second inputs are connectable via the switch  736  to the jack socket contact  728  or to the jack socket contact  726 . 
       FIG. 7 a    also shows an amplifier  724  serving as a microphone pre-amplifier coupled via two separate inputs to two jack socket contacts  726  and  728 . Amplifier  724  is designed to produce an output V micB  that depends on the difference between its two inputs V micA  and V micC . 
     The switch  734  is positioned to be able to connect the further jack socket contact  730  or further jack socket contact  732  to the ground point X 3 . 
     When a jack plug such as  25   e  or  25   f  is inserted into the jack socket, the tip contact  708  of the plug contacts the jack socket contact  720 ; the first ring contact  712  of the plug contacts the jack socket contact  722 ; the second ring contact  714  of the plug contacts the jack socket contacts  726  and  730 ; and the sleeve contact  710  of the plug contacts the jack socket contacts  728  and  732 . 
     The circuitry  11  may be provided with circuitry  740  for determining the type of jack plug that has been inserted. That is, the circuitry  11  is able to determine the positions of the ground and microphone connections in the inserted jack plug, and hence whether it is a jack plug  25   e  connected according to the OMTP standard as shown in  FIG. 7 b    or a jack plug  25   f  connected according to the CTIA standard as shown in  FIG. 7 c   . There are many techniques known for this, generally involving injecting current into one or more contacts of the jack socket and measuring voltages at one or more other contacts or vice versa to determine the impedances between various contacts due to the components connected to the corresponding poles of the inserted jack plug. Thus circuitry  740  may be coupled to various ones of jack socket contacts  720 ,  722 ,  726 ,  728 ,  730  or  732  via switches  734  or  736  or other wired paths or switches to drive or sense voltage or current signals comprising signal levels or tones on these contacts. 
     If the ground pole G is determined to be in the sleeve of the jack plug as shown in the jack plug  25   e , then switch  734  connects the contact  732  to the ground point X 3  and the switch  736  connects the contact  728  to the second inputs of the left and right driver amplifiers  716 ,  718 . 
     If the ground pole G is determined to be in the second ring (R 2 ) of the jack plug  25  as shown in the jack plug  25   f , then the switch  734  connects the contact  730  to the ground point X 3 , and the switch  736  connects the contact  726  to the second inputs of the left and right driver amplifiers  716 ,  718 . 
     The parasitic resistance associated with jack socket contact  732  is R G . Equivalently the parasitic resistance associated with the jack socket contact  730  is R M . However, the switch  734  also has a parasitic resistance R GS  associated with it. Therefore, the overall resistance of the ground path from the mating jack pole to the ground point X 3  when the switch is connected to contact  730  Is R M +R GS . Equivalently, the overall resistance of the ground path from the mating jack pole to the ground point X 3  when the switch is connected to the contact  732  is R G +R GS . 
     There may also be similar parasitic resistances between the microphone jack socket contacts  726  and  728 , and the microphone pre-amplifier.  FIG. 7 a    shows these lumped into respective single resistance elements R M2  and R G2 . 
     There is also a parasitic resistance R CS  which is associated with the switch  736 . However, as it can be assumed that the inputs of the left driver amplifier  716  and the right driver amplifier  718  connected to Vcom, and the inputs of the microphone amplifier  424  connected to V micA  and V micC  are high impedance inputs. Hence, there will be negligible currents I G2 , I CS  and I M2  flowing in the respective parasitic resistances R G2 , R Cs  and R M2  associated with the paths to these inputs, and thus the respective voltage drops V RG2 , V RCS  and V RM2  will also be negligible. 
     The current I G  through the ground pole G of the jack plug, whether this be positioned as shown in jack plug  25   e  or  25   f , comprises both the currents I SL  and I SR  through the left and right speakers  702 ,  704 . Thus, ignoring any current through the M pole of the plug:
 
 I   G   =I   SL   +I   SR .
 
     Given that substantially the whole of I G  will flow through R G  and R GS , or through R M  or and R GS  depending on the switch configuration, the ground voltage on the pole of the jack, at node X 4  (or node X 5  depending on the position of the ground connection in the jack plug), will differ from that at node X 3  by a voltage V RG , where:
 
 V   RG   =I   G *( R   G   +R   GS ) or  I   G *( R   M   +R   GS ).
 
     The voltage actually appearing across each speaker  702 ,  704  will be the voltage generated by the respective drive amplifier  716 ,  718 , reduced by V RG . That is:
 
 V   SL   =V out L−V   RG  and
 
 V   SR   =V out R−V   RG .
 
     As mentioned above, VoutL=VinL+Vcom, and VoutR=VinR+Vcom. Moreover, as the voltage drops across parasitic resistances R G2 , R Cs  and R M2  will also be negligible, Vcom=V RG . 
     Thus:
 
 V   SL   =V out L—V   RG =( V in L+V com)− V   RG =( V in L+V   RG )− V   RG   =V in L , and
 
 V   SR   =V out R−V   RG =( V in R+V com)− V   RG =( V in R+V   RG )− V   RG   =V in R.  
 
     Thus, V SL =VinL and V SR =VinR, and there is no crosstalk signal at the speakers  702 ,  704 . 
     As also mentioned above:
 
 V   micB   =V   micA   −V   mic .
 
     Considering the voltages shown in  FIG. 7   a:  
 
 V   micA   =V   mic   +V   RG   −V   RM2  and
 
 V   micC   =V   RG   −V   RG2 .
 
     However, as mentioned above, V RM2  and V RG2  are negligible, and so:
 
 V   micB   =V   micA   −V   mic =( V   mic   +V   RG )− V   RG .
 
     Thus, the microphone preamplifier  724  is connected across the two contacts  726 ,  728  that are not used for the ground current return in either position of the switches  734 ,  736 , and so it senses the microphone signal V mic  without any influence from the ground return current or any parasitic resistance in the socket contacts. The net voltage appearing at the output of the preamplifier  724  is therefore equal to V mic  (probably with a voltage gain applied by the preamplifier  724 ). 
     Any other interference appearing at the jack pole, i.e. any other additional component of V RG  superimposed on I G ·(R G +R GS ), due for instance to rectified EMI (Electromagnetic Interference) will also be removed from the net speaker drive and microphone sense signals. 
     The jack socket contacts  732  and  728  may be implemented mechanically in various forms depending on the application.  FIG. 7 a    illustrates a construction where the additional contacts  732  and  728  may be located on opposite sides of the jack plug. In other embodiments where space and the mechanical construction technology allow, the contacts  732  and  728  may be substantially on the same side of the jack plug, and displaced along the plug in a direction parallel with the axis of the plug. 
     The jack socket contacts  730  and  726  may be implemented mechanically in various forms depending on the application.  FIG. 7 a    illustrates a construction where the additional contacts  730  and  726  may be located on opposite sides of the jack plug. In other embodiments where space and the mechanical construction technology allow, the contacts  730  and  726  may be substantially on the same side of the jack plug, and displaced along the plug in a direction parallel with the axis of the plug. 
       FIG. 8 a    is an electrical circuit diagram of a further alternative system of the general type shown in  FIG. 3 . Specifically,  FIG. 8 a    shows the headset  20  as being a mono headset, having a left speaker  802  and a right speaker  804  connected in parallel, and also having a microphone  806 . 
     As shown in  FIG. 8 b    in conjunction with  FIG. 8 a    the headset  20  is provided with a jack plug  25   g , which in this example is a TRS plug. The left speaker  802  is connected between the tip (T)  808  and the sleeve (S)  810  of the plug  25   g . The right speaker  804  is similarly connected between the tip (T)  808  and the sleeve (S)  810  of the plug  25   g , so that the two speakers receive the same driving signal. The microphone  806  is connected between the ring (R)  812  and the sleeve (S)  810  of the plug  25   g . Thus, the sleeve  810  acts as a common ground return. 
     The tip  808 , ring  812 , and sleeve  810  of this plug may thus be regarded as the LR, M and G poles respectively of the jack plug  25   g.    
     In the circuitry  11 , a single driver amplifier  814  is driven from a signal source VinLR which may comprise a DAC and produces a buffered output signal VoutLR. An output of driver amplifier  814  is coupled to the jack socket contact  816 . 
       FIG. 8 a    also shows an amplifier  818  serving as a microphone pre-amplifier with an input coupled to a jack socket contact  820 . 
     A further, i.e. a fourth, jack socket contact  822  provides the common ground return path for the signal paths through the jack socket contacts  816  and  820 . 
     Thus, when the jack plug is inserted into the jack socket, the tip contact  808  of the plug  25   g  contacts the jack socket contact  816 ; the ring contact  812  of the plug  25   g  contacts the jack socket contact  820 ; and the sleeve contact  810  of the plug  25   g  contacts the jack socket contact  822 . 
     As shown in  FIG. 8 a   , the jack socket is provided with an additional jack socket contact  824 . The additional jack socket contact  824  is positioned in the socket such that, when the plug  25   g  is inserted into the socket, the sleeve contact  810  of the plug  25  contacts the additional jack socket contact  824  as well as the jack socket contact  822 . 
     The additional jack socket contact  824  is connected to a second input of the microphone amplifier  818 . Thus, the amplifier  818  is designed to produce an output V micB  that depends on the difference between its two inputs V micA  and V micC . 
     The additional jack socket contact  824  is also connected to a second input of the driver amplifier  814 . The voltage on this second input is Vcom. Preferably, amplifier  814  is designed to produce an output VoutLR which depends on the sum of VinLR and Vcom. 
       FIG. 8 a    illustrates the various parasitic resistances in the ground path between the jack pole G and a ground reference point X 3  close to the circuitry  11 , lumped into a single resistance element R G . 
     There may also be similar parasitic resistances between the jack plug pole M and the microphone pre-amplifier  818 , and  FIG. 8 a    shows these lumped into a single resistance element R M . 
     The parasitic resistances associated with the additional jack socket contact  824  are shown in  FIG. 8 a    as the resistance element R G2 . 
     It can be assumed that the second, i.e. reference, input of the driver amplifier  814  connected to Vcom, and the inputs of the microphone amplifier  818  connected to V micA  and V micC  are high impedance inputs. Hence, there will be negligible currents I M  and I G2  flowing in the respective parasitic resistances R M  and R G2  associated with the tracks to these inputs, and thus the voltage drops V RM  and V RG2  will also be negligible. 
     The current I G  through the ground pole G of the jack plug comprises both the currents I SL  and I SR  through the left and right speakers  802 ,  804  and also any current I M  through the M pole of the plug that might return via pole G:
 
 I   G   =I   SL   +I   SR   +I   M .
 
     Given that any current flowing through contact  824  may be negligible compared with that flowing through contact  822 , the whole of I G  will flow through parasitic resistance R G , and so the ground voltage on the pole of the jack, at node X 4 , will differ from that at X 3  by a voltage V RG , where:
 
 V   RG   =I   G   *R   G .
 
     The voltage actually appearing across each speaker  802 ,  804  will be the voltage generated by the driver amplifier  814 , reduced by V RG . That is:
 
 V   SL   =V   SR   =V out LR−V   RG .
 
     As mentioned above, amplifier  814  is configured such that VoutLR=VinLR+Vcom. 
     Moreover, as V RG2  is negligible, Vcom=V RG . 
     Thus:
 
 V   SL   =V   SR   =V out LR−V   RG =( V in LR+V com)− V   RG =( V in LR+V   RG )− V   RG   =V in LR  
 
     Thus, V SL =VinLR and V SR =VinLR, and any microphone signal cannot cause crosstalk at the speakers  802 ,  804 . 
     As also mentioned above:
 
 V   micB   =V   micA   −V   mic  
 
     Considering the voltages shown in  FIG. 8   a:  
 
 V   micA   =V   mic   +V   RG   −V   RM  and
 
 V   micC   =V   RG   −V   RG2  
 
     However, as mentioned above, V RM  and V RG2  are negligible, and so:
 
 V   micB   =V   micA   −V   mic =( V   mic   +V   RG )− V   RG   =V   mic .
 
     Thus the net voltage appearing at the output of the preamplifier  818  is equal to V mic  (probably with a voltage gain applied by the preamplifier). 
     Any other interference appearing at the jack pole, i.e. any other additional component of V RG  superimposed on I G ·R G , due for instance to rectified EMI (electromagnetic interference) will also be removed from the net speaker drive and microphone sense signals. 
     The additional jack socket contact  824  may be implemented mechanically in various forms depending on the application.  FIG. 8 c    illustrates a construction where the additional contact  824  is located on the opposite side of the jack plug from the contact  822 , and  FIG. 8 b    illustrates a construction where the additional contact  824  is substantially on the same side of the jack plug as the contact  822 , and displaced further along the plug in a direction parallel with the axis of the plug. 
       FIG. 8 a    illustrates an example where the speakers  802 ,  804  are connected in parallel. However, exactly the same circuitry  11  can be provided in the case where the speakers  802 ,  804  are connected in series, or in the case where there is only one speaker. 
     It will also be noted that, while  FIG. 8 a    shows the second input of the driver amplifier  814  being connected to the additional contact  824 , in examples where the speaker drive gain accuracy or microphone-to-speaker crosstalk is not considered important, the second input of the driver amplifier  814  need not be connected to the additional contact  824  and may instead be connected to a ground closely coupled to ground reference point X 3 . 
     Embodiments described so far relate to a device having a socket that is configured to receive a cylindrical plug. However, other connectors may be used. Thus, in further embodiments, the mechanical configuration of the plug may be different from the cylindrical plug with poles distributed along a common axis described hitherto. Various mechanical configurations are possible for a socket to still provide a plurality of contacts to mate with a signal pole of an inserted plug. For instance a receptacle for a USB Type C plug may comprise two separate contacts in the area normally occupied by a single contact, or in an area facing a contact of an inserted USB Type C plug. The USB Type C plug may be wired to an adapter or captive accessory according to the USB Type C Audio Adapter Accessory Mode. Preferably the multi-pole contact is on one or both terminals assigned to MIC/GND (M/G) or GND/MIC (G/M) in this mode. 
     In embodiments above, the accessory has been illustrated as comprising a plug or male connector, inserted into a socket or female connector on the device. In further embodiments, the device may comprise a male connector and the accessory a female connector. In some embodiments the connectors may be sex-less, for example each being a coplanar array of contact pads or suchlike, mechanically held together by means such as magnetic elements. 
     The terms host device and accessory device have been used to denote respectively a first device containing circuitry  11  connected to a second device containing some acoustic load or transducer connected together by some detachable connector means. 
     In some cases the first device containing the circuitry  11  may be more naturally be considered an accessory and the second device containing some acoustic load or transducer may be considered a host device. 
     Switches coupled to the M and G poles have been illustrated as single-pole double-throw switches, where the pole of the switch may be connected to one of two other terminals. These may be implemented as a pair of elements, for example MOS transistor switches, connected between the common switch pole to respective other terminals, with one of the two switches being in a conductive state and the other in a non-conductive state at any one time. However in some embodiments the switches may be controlled so that at some times both switches may be on at the same time or neither of the two switches is on at the same time. 
       FIG. 9 a    and  FIG. 9 b    illustrate further embodiments. These illustrate embodiments similar to  FIGS. 6 a  and 6 e    respectively, with like numbered elements denoting equivalent components, but in this case the device may incorporate a different receptacle and plug arrangement, such as a USB Type C arrangement, interposed between the switches and amplifiers and the socket contacts. The accessory plug may be inserted into a 3.5 mm audio jack-to-USB adapter comprising a four pole jack socket and a 24-pin USB-C plug wired according to the USB Type C Audio Adapter Accessory Mode annex, which is incorporated herein by reference, of which the six most relevant contacts are shown. This plug may be attached in either rotational polarity to a 24-pin USB Type C receptacle, wired to couple to circuitry  11  as shown. In this case the combination of the host device and the adapter may be considered as a device coupled to the accessory. It will be appreciated by those skilled in the art that while a 24-pin USB Type C arrangement has many more contacts than a traditional 3 or 4 pole audio socket and plug arrangement that are allocated to other non-audio functionality, and therefore more capability and/or flexibility, the USB-C contacts other than those associated with audio accessory functionality have been ignored for reasons of clarity. 
     In each case the operation associated with  FIGS. 9 a  and 9 b    is similar to that described with respect to  FIGS. 6 a  and 6 e    respectively. For analysis, any relevant parasitic resistances associated with the USB socket connections or any cabling between the sockets and the circuitry  11  may be lumped together with R G  or R M  and crosstalk due to all components of these resistances may be shown to be reduced by similar analysis to that of  FIGS. 6 a    and  6   e.    
     Note that as used herein the term module shall be used to refer to a functional unit or block which may be implemented at least partly by dedicated hardware components such as custom defined circuitry and/or at least partly be implemented by one or more software processors or appropriate code running on a suitable general purpose processor or the like. A module may itself comprise other modules or functional units. A module may be provided by multiple components or sub-modules which need not be co-located and could be provided on different integrated circuits and/or running on different processors. 
     Embodiments may be implemented in a host device, especially 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. Embodiments of the invention may also be implemented wholly or partially in accessories attachable to a host device, for example in active speakers or headsets or the like. Especially in more complex devices, there may be more than one connector and associated signal paths per aspects of the invention. 
     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. Terms such as amplify or gain include possibly applying a scaling factor of less than unity to a signal.