Patent Abstract:
A backward compatible system and method for using 4P audio jack in an electronic device to provide power and signal to headset with active noise cancellation (ANC) as well as accessories that require an external power are disclosed. The method involves automatically deciding at the electronic device accessory type after accessory insertion detected and choosing proper accessory communication mode based at least on the decided accessory type and accessory input signal. The accessory communication mode may be an accessory power mode or an accessory microphone mode.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The application claims priority to U.S. Provisional Patent Application 62/120,457, entitled “Backward Compatible System and Method for Using 4P Audio Jack to Provide Power and Signal to Headset with Active Noise Cancellation,” filed Feb. 25, 2015, which application is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    A. Technical Field 
         [0003]    The present invention relates generally to accessory management and data communication via an audio port on an electronic device. 
         [0004]    B. Background of the Invention 
         [0005]    Audio sockets have been commonly used in various electronic devices, such as computers, laptops, media players, smart phones, etc. to communicate with audio accessories having audio jacks. The mostly common used jack plugs have 2.5 mm, 3.5 mm or 6.35 mm (¼ inch) configurations with 2, 3 or 4 conductors (2P, 3P or 4P) for mono, stereo or stereo plus microphone compatibility. Stereo 3.5 mm jacks may be used for line in/out, headset out, loudspeaker out, microphone in, etc. Three-conductor connectors are common on older electronic devices, while 4-conductor 3.5 mm connectors are more commonly used on modern electronic devices, including most smart phones. 
         [0006]    A 2-conductor jack is called TS connector with a tip and a sleeve for mono audio communication. A 3-conductor jack is called TRS connector with a tip, a ring and a sleeve for stereo audio communications. A 4-conductor jack is usually called TRRS connector with a tip, two rings and a sleeve for stereo plus microphone line communications. In certain circumstances, it is desirable to use a TRRS jack to transmit additional audio and/or data signal to host electronic devices. 
         [0007]    Beyond receiving audio signal input from electronic device, some audio accessory, such as headset with active noise control (ANC) also need power input to operate its noise control circuit. Traditionally, the ANC headset is powered by separate battery, which causes bulky size for the headset and limited operation time. 
         [0008]    Efforts have been done to explore further potential applications using audio jack connection. 5-conductor jack has been proposed and developed recently. However, it is very difficult to make those 5-conductor jacks backward compatible to most modern electronic devices. Additional cost will be needed for customer connector and hardware for 5-conductor jacks. Given the variety in the audio accessories of different characteristics and preferred settings, it would be desirable to provide improved accessory management and signal communication via 4P audio port with backward compatibility for supporting interactions between electronic devices and accessories. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention relates to accessory management and data communication, and more particularly, to backward compatible systems and methods for using 4P audio jack in an electronic device to provide power and signal to headset with active noise cancellation (ANC) as well as accessories that require an external power. 
         [0010]    The method involves automatically deciding at the electronic device accessory type after accessory insertion detected and choosing accessory communication mode based at least on the decided accessory type and accessory input signal. The accessory communication mode may be an accessory power mode or an accessory microphone mode. In the accessory power mode, the electronic device provides power to the accessory over a microphone line (MIC line) operated beyond traditional microphone bias voltage level. In the accessory microphone mode, the electronic device provides MIC bias and audio signals to the accessory. 
         [0011]    In certain embodiments, communication starts after the jack insertion is detected. The accessory&#39;s ID and audio jack configuration are initially checked. The audio jack configuration check verifies the type of the audio jack (TS, TRS or TRRS jack) and also the type of the accessory. The accessory type detection method includes impedance detection of audio lines as well as MIC line. 
         [0012]    If the accessory is detected to be a traditional 4P headset, the electronic device provides MIC bias and audio signals to the 4P headset. If the accessory is detected to be a 4P accessory requesting power support, the host electronic device provides power to the accessory over the microphone line (MIC line). 
         [0013]    In certain embodiments, some accessories have one audio line (L-audio or R-audio) multiplexed with the MIC line. The electronic device detects and monitors microphone bias on at least one audio line, such as L-audio or R-audio line. If the microphone bias is detected and is higher than V ref1  for duration of time longer than t deb1 , then the audio line is being used as a microphone and the electronic device routes the audio line as a microphone input and turn on an audio bypass switch. If the microphone bias is lower than V ref1  for duration of time longer than t deb1 , the electronic device latches the audio line for audio signal output from an audio drive circuit within the electronic device. 
         [0014]    In certain embodiments, when the electronic device provides power to the accessory over the microphone line (MIC line), the MIC line voltage is monitored. If the MIC line voltage is below the V ref2  for duration of time longer than t deb2 , the electronic device is not supporting the accessory power mode and the accessory operates in low power mode (legacy mode) and optional bypass mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Reference will be made to exemplary embodiments of the present invention that are illustrated in the accompanying figures. Those figures are intended to be illustrative, rather than limiting. Although the present invention is generally described in the context of those embodiments, it is not intended by so doing to limit the scope of the present invention to the particular features of the embodiments depicted and described. 
           [0016]      FIG. 1  is a schematic diagram of an electronic device in communication with an accessory in a system via an audio jack in accordance with an embodiment of the present invention. 
           [0017]      FIG. 2  is an exemplary schematic diagram of an accessory according to various embodiments of the invention. 
           [0018]      FIG. 3  is an exemplary schematic diagram of the accessory control IC according to various embodiments of the invention. 
           [0019]      FIG. 4  is an exemplary block diagram of the accessory in communication with an electronic device for host detection according to various embodiments of the invention. 
           [0020]      FIG. 5  is an exemplary schematic diagram of an electronic device according to various embodiments of the invention. 
           [0021]      FIG. 6  is an exemplary block diagram of the accessory in communication with an electronic device for accessory detection according to various embodiments of the invention. 
           [0022]      FIG. 7  is another exemplary block diagram of the accessory in communication with an electronic device for accessory detection according to various embodiments of the invention. 
       
    
    
       [0023]    One skilled in the art will recognize that various implementations and embodiments of the invention may be practiced in accordance with the specification. All of these implementations and embodiments are intended to be included within the scope of the invention. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    In the following description, for the purpose of explanation, specific details are set forth in order to provide an understanding of the present invention. The present invention may, however, be practiced without some or all of these details. The embodiments of the present invention described below may be incorporated into a number of different electrical components, circuits, devices, and systems. Structures and devices shown in block diagram are illustrative of exemplary embodiments of the present invention and are not to be used as a pretext by which to obscure broad teachings of the present invention. Connections between components within the figures are not intended to be limited to direct connections. Rather, connections between components may be modified, re-formatted, or otherwise changed by intermediary components. 
         [0025]    When the specification makes reference to “one embodiment” or to “an embodiment”, it is intended to mean that a particular feature, structure, characteristic, or function described in connection with the embodiment being discussed is included in at least one contemplated embodiment of the present invention. Thus, the appearance of the phrase, “in one embodiment,” in different places in the specification does not constitute a plurality of references to a single embodiment of the present invention. 
         [0026]    Various embodiments of the invention are used for accessory management and data communication via audio port in systems comprised of one or more integrated circuits (IC). An IC may be a memory, microcontroller, microprocessor, secure authenticator or any other devices within a system that communicates and/or receives information within the system. These systems, and the IC(s) therein, may be integrated on a single component or contain discrete components. Furthermore, embodiments of the invention are applicable to a diverse set of techniques and methods. 
         [0027]      FIG. 1  shows a schematic diagram of a system  100  according to various embodiments of the invention. The system  100  comprises an electronic device  200  in communication with an accessory  300 . Communication between the device and the accessory begins after an audio jack  310  on the accessory  300  inserts into an audio socket  210  on the electronic device  200 . The electronic device may detect the type of the accessory and communicates to the accessory in different operation modes accordingly. On the other hand, the accessory may detect the capacity and operation mode of the electronic device and adjust accessory configurations based on the detected operation mode. 
         [0028]    The electronic device  200  may be a computer device, a laptop, a portable media player, such as a MP3 player, a smart phone, etc. The accessory  300  may be an audio accessory such as a microphone, a headphone, loudspeakers, an audio amplifier, or be an electronic accessory having audio jack for voice and data communications. In an embodiment, the accessory  300  has a stereo or mono earphone and a microphone for audio input to the electronic device  200 . 
         [0029]    The audio socket  210  is a 4P audio socket. The audio jack  310  may have 2.5 mm, 3.5 mm or 6.35 mm (¼ inch) configurations with 2, 3, 4 or even 5 conductors for mono, stereo, stereo plus microphone, and stereo plus microphone with power compatibility. 
         [0030]    In one embodiment, the electronic device  200  has an audio socket  210 , an I2C interface  220 , a microprocessor  230 , a memory  240 , a power source  250  and an audio driver circuit  260 . The microprocessor  230  is configured to operatively connect to the I2C interface  220 , the memory  240 , the power source  250  and the audio driver circuit  260 . The I2C interface  220  is an Inter-Integrated Circuit used for attaching peripheral audio socket  210  to the microprocessor  230 . The memory  240  is configured to store a non-volatile computer readable logic or code for the implementation of desired function when the logic or code is executed by the microprocessor  230 . 
         [0031]    In one embodiment, the accessory  300  comprises an accessory control IC  320 , an audio jack  310  and an earphone  340 . In another embodiment, the accessory  300  has a microphone  350  coupled to the accessory control IC  320 . In yet another embodiment, the accessory  300  also has a secure authenticator  330  and an audio sensor  360  for noise control purpose. In some embodiment, the accessory control IC  320  may be an active noise control (ANC) circuit or a stereo speaker balance circuit, etc. The accessory control IC  320  couples to the audio jack  310 , the earphone  340 , audio sensor  360 , and the microphone  350 . In some embodiments, the audio sensor  360  is a stereo audio sensor comprising an L-audio sensor  362 , R-audio sensor  364 . 
         [0032]      FIG. 2  illustrates an exemplary schematic diagram of an accessory according to various embodiments of the invention. The accessory  300  has a TRRS audio jack  310 , a stereo earphone  340  (L-earphone  342  and R-earphone  344 ), a microphone  350 , an accessory control IC  320  and a secure authenticator  330 . The audio jack  310  comprises an L-audio conductor  311 , an R-audio conductor  312 , a ground conductor  313  and a microphone conductor  314 . The L-audio conductor  311 , R-audio conductor  312  and a microphone conductor  314  are connect to L-audio input port  321 , R-audio input port  322  and power input port (PWR)  323  of the accessory control IC  320  respectively. 
         [0033]    The accessory control IC  320  also comprises an L-Mic port  328 , a R-Mic port  329 , a L-audio output port  326 , R-audio output port  327  operatively connected to the L-audio sensor  362 , R-audio sensor  364 , L-earphone  342  and R-earphone  344 , respectively. During the operation of the accessory  300 , audio jack  310  is inserted to the audio socket  210  of the electronic device  200  for audio/MIC signal communication. The accessory control IC  320  receives audio signal inputs from the audio jack  310  and the audio sensor  360 , processes those signals via internal signal processing circuit (not shown in  FIG. 2 ) and generates outputs signals to the earphone  340 . 
         [0034]    In another embodiment, the accessory control IC  320  comprises a MIC input port  324  and a MIC bias port  325  to receive/output signal to the microphone  350 . The accessory control IC  320  may be operated to receive signal input from the microphone  350  and send the signal to the electronic device  200  through the microphone conductor  314 . The accessory control IC  320  may be alternatively operated to receive bias input from the electronic device  200  via the microphone conductor  314  and output the bias signal to the microphone  350  through the MIC bias port  325 . The accessory control IC  320  may also be operated to receive digital communication such as 1-wire or ultra sound input from the electronic device  200  via the microphone conductor  314  and output the bias signal to the microphone  350  through the MIC bias port  325 . 
         [0035]      FIG. 3  illustrates an exemplary schematic diagram of accessory control IC according to various embodiments of the invention. The accessory control IC  320  comprises a digital signal processing (DSP) circuit  371 , a R-audio/MIC selection switch  373 , a low dropout regulator (LDO)  378 , a LDO switch  379 , a MIC line voltage comparator  374  and a power line voltage comparator  377 . The DSP circuit  371  couples between signal input ports (L-audio input port  321 , R-audio input port  322 , L-Mic port  328  and R-Mic port  329 ) and audio output ports (L-audio output port  326  and R-audio output port  327 ). The DSP circuit  371  may be a DSP (digital signal processing) circuit for ANC or power accessory applications. In one embodiment, the accessory control IC  320  also comprises an optional button detection interface  372  to detect/receive button operations and send the detected operations to the DSP circuit  371  for further processing. In another embodiment, the LDO  378  comprises soft-start circuit to limit inrush current and control output voltage rising time during power-up. 
         [0036]    The power line voltage comparator  377  has one input coupled to the power input port (PWR)  323  and compares the voltage at the power input port to a predetermined threshold voltage. The output of the power line voltage comparator  377  is coupled to the LDO switch  379  via a latch component  376  with a predetermined debounce time to switch ON/OFF the LDO switch  379 . In one embodiment, the predetermined threshold voltage is an input voltage required to operate the LDO  378 . In one embodiment, the predetermined threshold voltage is set at 2.8V. When the voltage at the power input port is larger or equal to the predetermined threshold voltage, the LDO switch  379  is switched ON to enable LDO  378  operation and thus provide power to the DSP circuit  371 . 
         [0037]    In one embodiment, the accessory control IC  320  comprises a bypass switch  375  coupled between signal input ports (L-audio input port  321 , R-audio input port  322 ) and output ports (L-audio output port  326  and R-audio output port  327 ). In one embodiment, the bypass switch  375  is a double pole, single throw (DPST) switch. When the bypass switch  375  is closed, the L-audio input port  321  and R-audio input port  322  are connected to the L-audio output port  326  and R-audio output port  327  directly, thus bypassing the DSP circuit  371 . 
         [0038]    In one embodiment, the R-audio/MIC selection switch  373  is a DPDT switch. When the R-audio/MIC selection switch  373  is switched ON (closed), the R-audio input port  322  connects to the MIC input port  324  directly and is used as a MIC communication port to transfer MIC input from the microphone  350  to the electronic device. At the same time, the L-audio input port  321  couples to both L-audio output port  326  and R-audio output port  327  for mono audio signal output. The earphone  340  (L-earphone  342  and R-earphone  344 ) thus operates in a mono mode. 
         [0039]    In one embodiment, the R-audio/MIC selection switch  373  is controlled by the MIC line voltage comparator  374 , which compares the voltage at the R-audio input port  322  to at least one reference voltage. In one embodiment, the R-audio/MIC selection switch  373  is controlled by the MIC line voltage comparator  374  and/or the power line voltage comparator  377 . 
         [0040]    One skilled in the art will recognize that the above-described accessory control IC is only one structure example. In one embodiment, the DSP circuit  371 , LDO  378 , stitches  373  and  375 , voltage comparators  374  and  377  may be integrated within one IC chip. In another embodiment, those components may be placed among a plurality of IC chips. The DSP circuit  371  may also comprise embedded memory storage module to store preloaded logic codes executable by the DSP circuit to implement desired active noise control processing. 
         [0041]      FIG. 4  is an exemplary block diagram of the accessory in communication with an electronic device for host detection according to various embodiments of the invention. 
         [0042]    Communication between the accessory  300  and the electronic device  200  starts after the audio jack  310  of the accessory  300  is inserted into the audio socket of an electronic device. At step  410 , the accessory control IC  320  checks whether the microphone conductor (MIC line)  314  has a power line voltage. If not, the accessory control IC  320  verifies that the electronic device  200  is a traditional host not providing power support to accessory via 4P audio socket at step  430 . If the microphone line (PWR) voltage is present (in step  420 ), the accessory control IC  320  is configured to draw no more than a minimum allowable current for a predetermined debounce time. Moreover, the DSP circuit  371  is in sleep mode. After the predetermined debounce time, the power line voltage at the microphone conductor (MIC line)  314  is compared to a predetermined voltage at step  440 . In one embodiment, the predetermined voltage is the working voltage for the LDO  378 . In another embodiment, the predetermined voltage is set as 94% of LDO working voltage based on I2C communication PIO level. For example, the predetermined voltage may be set at 2.5V. If the power line voltage is less than the predetermined voltage, the accessory control IC  320  verifies that the electronic device  200  does not provide power support over MIC line via 4P audio socket at step  450 . The electronic device may be a traditional host supporting stereo audio output and MIC signal input. If the power line voltage is larger than the predetermined voltage, the accessory control IC  320  verifies that the electronic device  200  supports power input over MIC line via 4P audio socket at step  460 . The power switch  379  is then switched to latch the LDO  378  to the power input port (PWR)  323  such that the DSP circuit  371  is powered up. 
         [0043]    In some embodiments, the power line voltage is further verified to check whether the voltage is the same as a first reference voltage V Ref1  at step  470 . The first reference voltage V Ref1  is higher than the predetermined voltage. If yes, the accessory enters a mono mode (or a call mode for a phone electronic device, such as a smart phone) at step  472  and the accessory control IC  320  is configured to switch ON the R-audio/MIC selection switch  373  to connect the R-audio input port  322  to the MIC input port  324  directly. At the same time, the L-audio input port  321  couples to both L-audio output port  326  and R-audio output port  327  for mono audio signal output. In one embodiment, the first reference voltage V Ref1  is set as 3.15V. 
         [0044]    In some embodiments, the power line voltage is also verified to check whether the voltage is the same as a second reference voltage V Ref2  at step  480 . The second reference voltage V Ref2  is higher than the predetermined voltage but lower than the first reference voltage V Ref1 . If yes, the accessory enters a stereo mode at step  482 , wherein the R-audio input port  322  is coupled to the R-audio output port  327  to provide a stereo audio output together with the L-audio output port  326 . In one embodiment, the second reference voltage V Ref2  is set as 2.8V. 
         [0045]      FIG. 5  is an exemplary schematic diagram of an electronic device according to various embodiments of the invention. The electronic device  200  has an audio socket  210  to receive an audio jacket from an accessory, an I2C circuit  220 , a microprocessor  230 , and an audio driver circuit  260 . The electronic device also has other components (not shown in  FIG. 5 ) including memory, power source and at least one I/O interface, such as touch screen, keyboard, pin pad, etc. The microprocessor  230  is configured to couple to the I2C circuit  220  and the audio driver circuit  260  (and also the memory, the power source and I/O interfaces). 
         [0046]    The audio socket  210  comprises an L-audio socket conductor  211 , a R-audio socket conductor  212 , a socket ground conductor  213  and a socket microphone conductor  214 , which respectively contact the L-audio conductor  311 , R-audio conductor  312 , ground conductor  313  and a microphone conductor  314  after the insertion of the audio jack  310 . In one embodiment, the audio socket  210  comprises an audio insertion detection conductor  215  to contact the L-audio conductor  311  after the audio jack  310  insertion. The audio insertion detection conductor  215  couples to the I2C circuit  220  via pin  222  and is configured to detect audio jack insertion. 
         [0047]    The L-audio socket conductor  211  couples to an L-A (left audio output) port  261  of the audio driver circuit  260  to receive audio signal. The R-audio socket conductor  212  couples to either an R-audio output port  262  or a MIC-bias port  263  of the audio driver circuit  260  via a controllable switch  270 . In one embodiment, the controllable switch  270  is a DPDT (double pole double through) switch such that when the R-audio socket conductor  212  is able to couple to both the MIC-bias port  263  and an MIC port  264  of the audio driver circuit  260  simultaneously. The controllable switch  270  is controlled by the microprocessor  230  or by a MIC Bias and Power LDO  221  within the I2C circuit  220 . The MIC Bias and Power LDO  221  sends an INT signal to the controllable switch  270  through pin  226  for switch control according to electronic device audio setup preferences. 
         [0048]    The MIC Bias and Power LDO  221  may couple to the socket microphone conductor  214  via a MIC IN pin  224  to send power or to receive signal from the microphone conductor  314 . The MIC Bias and Power LDO  221  may also couple to the socket microphone conductor  214  via a RES (reset) pin  225  to send power to or to receive signal from the accessory  300  via the microphone conductor  314 . The voltage output from the MIC Bias and Power LDO  221  to the RES (reset) pin  225  is configurable to be different levels according to electronic device audio setup preferences. 
         [0049]      FIG. 6  is an exemplary block diagram of the accessory in communication with an electronic device for accessory detection according to various embodiments of the invention. At step  610 , the electronic device detects the types of accessory after audio jack insertion detected. The accessory detection may be done by checking the impedance at the L-Audio line  311  and/or the MIC line  314  and/or the secure authenticator  330 . The secure authenticator  330  could be also a digital ID as well as a secure authenticator. 
         [0050]    At step  620 , the electronic device configures the control setup for the controllable switch  270  based on preferred audio setting. In some embodiments, the electronic device is a smart phone and the controllable switch  270  may be configured as mono mode for phone calls and stereo mode for music playing. At mono mode, the R-audio socket conductor  212  is latched both the MIC-bias port  263  and an MIC port  264  of the audio driver circuit  260 . At stereo mode, the R-audio socket conductor  212  is latched to R-audio output port  262  of the audio driver circuit  260 . In some embodiments, the electronic device also configures the MIC bias and power LDO  221  based on preferred audio setting. For example, the LDO  221  may output a voltage of V Ref1  when the electronic device is in mono mode for phone calls and output a voltage of V Ref2  when the electronic device is in stereo mode for music playing. In some embodiments, the first reference voltage V Ref1  is higher than the second reference voltage V Ref2 . For example, the first reference voltage V Ref1  may be set as 3.15V and the second reference voltage V Ref2  may be set as 2.8V. 
         [0051]    At step  630 , the accessory control IC  320  compares the power line voltage at the microphone conductor (MIC line)  314  to a predetermined voltage. If the power line voltage is larger than the predetermined voltage, the accessory control IC  320  verifies that the electronic device  200  supports power input over MIC line via 4P audio socket. The power switch  379  is then switched to latch the LDO  378  to the power input port (PWR)  323  such that the DSP circuit  371  is powered up. The power line voltage is further verified to check whether the voltage is equal to the first reference voltage V Ref1  at step  632 . If yes, the accessory enters a mono mode (or a call mode for a phone electronic device, such as a smart phone) at step  636  and the accessory control IC  320  is configured to switch ON the R-audio/MIC selection switch  373  to connect the R-audio input port  322  to the MIC input port  324  directly. At the same time, the L-audio input port  321  couples to both L-audio output port  326  and R-audio output port  327  for mono audio signal output. In one embodiment, the first reference voltage V Ref1  is set as 3.15V. 
         [0052]    At step  634 , the power line voltage is further verified to check whether the voltage is equal to the second reference voltage V Ref2 . If yes, the accessory enters a stereo mode at step  638 , wherein the R-audio input port  322  is coupled to the R-audio output port  327  to provide a stereo audio output together with the L-audio output port  326 . Therefore, by controlling the MIC bias and power LDO  221  output voltage to the socket microphone conductor  214 , the electronic device may set the operation mode of the accessory according to its preferred audio setting. 
         [0053]    At step  640 , the accessory control IC  320  checks whether the power line signal at the microphone conductor (MIC line)  314  is a MIC input signal, which is typically smaller than the predetermined voltage. If yes, then the accessory control IC  320  configures to latch the LDO power switch  379  to MIC input, latch the R-A/MIC switch  373  to R-audio signal and close the bypass switch  375  to bypass the DSP circuit  371 . 
         [0054]      FIG. 7  is another exemplary block diagram of the accessory in communication with an electronic device for accessory detection according to various embodiments of the invention. Similar to  FIG. 6 , at step  630 , the accessory control IC  320  compares the power line voltage at the microphone conductor (MIC line)  314  to a predetermined voltage. If the power line voltage is larger than the predetermined voltage, the accessory control IC  320  verifies that the electronic device  200  supports power input over MIC line via 4P audio socket. Then, the accessory control IC  320  checks the signal at the R-audio input port  322 . At step  710 , the accessory control IC  320  checks if the signal at the R-audio input port  322  has a MIC bias. If yes, the accessory enters a mono mode (or a call mode for a phone electronic device, such as a smart phone) at step  712  and the accessory control IC  320  is configured to switch the R-audio/MIC selection switch  373  to latch the R-audio input port  322  to the MIC input port  324  directly. At the same time, the L-audio input port  321  couples to both L-audio output port  326  and R-audio output port  327  for mono audio signal output. 
         [0055]    At step  720 , the accessory control IC  320  checks if the signal at the R-audio input port  322  is a regular audio signal, which typically has a lower voltage than the MIC bias voltage. If yes, the accessory enters a stereo mode at step  722 , wherein the R-audio input port  322  is coupled to the R-audio output port  327  to provide a stereo audio output together with the L-audio output port  326 . 
         [0056]    The foregoing description of the invention has been described for purposes of clarity and understanding. It is not intended to limit the invention to the precise form disclosed. Various modifications may be possible within the scope and equivalence of the application.

Technology Classification (CPC): 6