Patent Publication Number: US-2022240014-A1

Title: Audio circuit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation under 35 U.S.C. § 120 of PCT/JP2020/037262, filed Sep. 30, 2020, which is incorporated herein by reference, and which claimed priority to Japanese Application No. 2019-191304, filed Oct. 18, 2019. The present application likewise claims priority under 35 U.S.C. § 119 to Japanese Application No. 2019-191304, filed Oct. 18, 2019, the entire content of which is also incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an audio circuit. 
     2. Description of the Related Art 
     Transmission methods for audio signals between audio Integrated Circuits (ICs) are broadly classified into analog transmission and digital transmission. An interface circuit is mounted on such an audio IC according to the transmission method. 
       FIG. 1A  and  FIG. 1B  are exemplary waveform diagrams showing a digital audio signal and an analog audio signal. As shown in  FIG. 1A , the digital audio signal is configured as a pulse signal that is switched between two levels, i.e., a high level (e.g., 3.3 V) and a low level (e.g., 0 V). For example, the digital audio signal may be configured as a combination of multiple pulse signals (e.g., a combination of a clock signal and serial data). Also, the digital audio signal may be configured as a single pulse signal (e.g., a Pulse Width Modulation (PWM) signal, a Pulse Density Modulation (PDM) signal, or a Direct Stream Digital (DSD) signal. In a case of using an analog audio signal for a Compact Disc (CD), for example, as shown in  FIG. 1B , the analog audio signal has an analog waveform having a maximum amplitude of 2 Vrms. 
       FIG. 2  is a diagram showing a conventional audio IC  900 . The audio IC  900  is configured to be capable of receiving both the inputs of an analog audio signal and a digital audio signal. The audio IC  900  includes an analog audio interface with two pins (Lch, Rch) and a serial input interface with four pins (SDATA, LRCLK, BCLK, MCLK). 
     The audio IC  900  includes a serial interface circuit (receiver)  902 , D/A converters  904  and  906 , and selectors  908  and  910 . 
     In a case in which the audio IC  900  is coupled to another Integrated Circuit (IC) or device provided with an analog interface, an analog audio signal is input to the two analog input pins (Lch, Rch). In this case, the four digital input pins (SDATA, LRCLK, BCLK, MCLK) are not used. The selectors  908  and  910  select the analog audio signals input via the Lch pin and the Rch pin, and output the analog signals thus selected to an unshown internal circuit. 
     In a case in which the audio IC  900  is coupled to another Integrated Circuit (IC) or device provided with a digital interface, the two analog input pins (Lch, Rch) are not used. In this case, a digital audio signal is input to the four digital input pins (SDATA, LRCLK, BCLK, MCLK). The serial interface circuit  902  separates the digital audio signal into an L-channel signal and an R-channel signal. The D/A converters  904  and  906  convert the L-channel digital signal and the R-channel digital signal into analog signals. The selectors  908  and  910  select the outputs of the D/A converters  904  and  906 ; and output the output signals thus selected to an unshown internal circuit. 
     The audio IC  900  shown in  FIG. 2  requires a total of six input pins, which is the sum total of the number of the analog input pins and the number of the digital input pins. Such an arrangement has a problem of requiring a large chip area and a large package area.  FIG. 2  shows only one input line. However, in some cases, the audio IC  900  has around four input lines. In this case, such an arrangement requires 24 (=6×4) pins. 
     SUMMARY 
     The present disclosure has been made in order to solve such a problem. 
     An embodiment of the present disclosure relates to an audio circuit. The audio circuit includes: N (N≥1) input pins via which a digital audio signal or an analog audio signal is to be input; an audio interface circuit structured such that, when the analog audio signals are input to the N input pins, a bias voltage is applied to each of the N input pins via a bias resistor; and an internal circuit structured to process the analog audio signal or the digital audio signal that has passed through the audio interface circuit. 
     Another embodiment of the present disclosure relates to an electronic device. The electronic device is provided with any one of the audio circuits described above. 
     Yet another embodiment of the present disclosure relates to an in-vehicle audio system. The in-vehicle audio system is provided with any one of the audio circuits described above. 
     It should be noted that any combination of the components described above, or any manifestation of the present disclosure may be mutually substituted between a method, apparatus and so forth, which are also effective as an embodiment of the present disclosure. The description of the items (means for solving the problems) is by no means intended to describe all the indispensable features of the present disclosure. That is to say, any sub-combination of the features as described above is also encompassed in the technical scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several FIGURES, in which: 
         FIG. 1A  and  FIG. 1B  are exemplary waveform diagrams showing a digital audio signal and an analog audio signal, 
         FIG. 2  is a diagram showing a conventional audio IC, 
         FIG. 3  is a circuit diagram showing a basic configuration of an audio circuit according to an embodiment, 
         FIG. 4  is an equivalent circuit diagram of the audio circuit when digital audio signals are input, 
         FIG. 5  is an equivalent circuit diagram of the audio circuit when analog audio signals are input, 
         FIG. 6  is a circuit diagram showing an audio circuit according to an example 1, 
         FIG. 7  is a circuit diagram showing an audio circuit according to an example 2, 
         FIG. 8  is a circuit diagram showing an audio circuit according to an example 3, 
         FIG. 9  is a circuit diagram showing an audio circuit according to an example 4, 
         FIG. 10  is a circuit diagram showing an audio circuit according to an example 5; and 
         FIG. 11A  and  FIG. 11B  are circuit diagrams each showing an audio interface circuit according to a modification 2. 
     
    
    
     DETAILED DESCRIPTION 
     Outline of Embodiments 
     Description will be made regarding an outline of several example embodiments of the present disclosure. In this outline, some concepts of one or more embodiments will be described in a simplified form as a prelude to the more detailed description that is presented later in order to provide a basic understanding of such embodiments. Accordingly, the outline is by no means intended to restrict the scope of the present invention or the present disclosure. Furthermore, this outline is not an extensive overview of all conceivable embodiments and is by no means intended to restrict essential elements of the embodiments. In some cases, for convenience, the term “one embodiment” may be used herein to refer to a single embodiment (example or modification) or multiple embodiments (examples or modifications) disclosed in the present specification. 
     An audio circuit according to an embodiment includes: N (N≥1) input pins via which a digital audio signal or an analog audio signal is to be input; an audio interface circuit structured such that, when the analog audio signals are input to the N input pins, a bias voltage is applied to each of the N input pins via a bias resistor; and an internal circuit structured to process the analog audio signal or the digital audio signal that has passed through the audio interface circuit. 
     With this configuration, the analog input pins and the digital input pins can be configured as shared input pins. This allows the number of pins to be reduced. It should be noted that the phrase “analog audio signals (digital audio signals) are input to the N input pins” does not mean a state in which all the N input pins are used. Rather, this means a state in which at least one of the N input pins is used. 
     With an embodiment, the audio interface circuit may be structured such that, when the digital audio signals are input to the N input pins, each of the N input pins is grounded via the bias resistor. 
     With an embodiment, the audio interface circuit may be structured such that, when the audio circuit is started up, each of the N input pins is grounded via the bias resistor. In a case in which an external circuit having a digital output is coupled to the N input pins, this is capable of preventing high voltage from being applied to the external circuit when the audio circuit is started up. 
     With an embodiment, the internal circuit may include: a D/A converter structured such that, when the digital audio signals are input to the N input pins, the D/A converter is set to an active state so as to convert the digital audio signals into analog signals; an output selector structured such that, when the digital audio signals are input to the N input pins, the output selector selects an output signal of the D/A converter, and when the analog audio signals are input to the N input pins, the output selector selects the analog audio signals; and an analog processing circuit structured to process an output of the output selector. 
     With an embodiment, the internal circuit may include: an A/D converter structured such that, when the analog audio signals are input to the N input pins, the A/D converter is set to an active state so as to convert the analog audio signals into digital signals; an output selector structured such that, when the digital audio signals are input to the N input pins, the output selector selects the digital audio signals, and when the analog audio signals are input to the N input pins, the output selector selects an output signal of the A/D converter; and a digital processing circuit structured to process an output of the output selector. 
     With an embodiment, the bias voltage may be ½ of a power supply voltage of the audio circuit. 
     With an embodiment, the audio interface circuit may include: a voltage dividing circuit structured to divide the power supply voltage into ½; a buffer structured to receive an output voltage of the voltage dividing circuit; N bias selectors each arranged such that a first input terminal thereof is coupled to an output of the buffer, and a second input terminal thereof is grounded; and N bias resistors each arranged such that one end thereof is coupled to an output terminal of the corresponding bias selector, and the other end thereof is coupled to the corresponding input pin. 
     With an embodiment, an arrangement may be made in which N=4. Also, the digital audio signal may be transmitted in a serial format including serial data, an LR clock, a bit clock, and a master clock. 
     With an embodiment, an arrangement may be made in which N=3. Also, the digital audio signal may be transmitted in a serial format including serial data, an LR clock, and a bit clock. 
     With an embodiment, the digital audio signal may be a PWM signal. 
     With an embodiment, the digital audio signal may be a differential signal. 
     With an embodiment, the digital audio signal may be a single-ended signal. 
     With an embodiment, the audio circuit may be monolithically integrated on a single substrate. Examples of such an “integrated” arrangement include: an arrangement in which all the circuit components are formed on a substrate; and an arrangement in which principal circuit components are monolithically integrated. Also, a part of the circuit components such as resistors or capacitors may be arranged in the form of components external to such a substrate in order to adjust the circuit constants. By integrating the circuit on a single chip, such an arrangement allows the circuit area to be reduced and allows the circuit elements to have uniform characteristics. 
     Embodiments 
     Description will be made below regarding the present disclosure with reference to the drawings. In each drawing, the same or similar components, members, and processes are denoted by the same reference numerals, and redundant description thereof will be omitted as appropriate. The embodiments have been described for exemplary purposes only and are by no means intended to restrict the present disclosure. Also, it is not necessarily essential for the present disclosure that all the features or a combination thereof be provided as described in the embodiments. 
     In the present specification, a state represented by the phrase “the member A is coupled to the member B” includes a state in which the member A is indirectly coupled to the member B via another member that does not affect the electric connection between them, or that does not damage the functions of the connection between them, in addition to a state in which they are physically and directly coupled. Similarly, a state represented by the phrase “the member C is provided between the member A and the member B” includes a state in which the member A is indirectly coupled to the member C, or the member B is indirectly coupled to the member C via another member that does not affect the electric connection between them, or that does not damage the functions of the connection between them, in addition to a state in which they are directly coupled. 
       FIG. 3  is a circuit diagram showing a basic configuration of an audio circuit  300  according to an embodiment. The audio circuit  300  includes N (N≥1) input pins P 1  through P N , a power supply pin VCC, an audio interface circuit  310 , and an internal circuit  320 , which are integrated on a single semiconductor substrate. 
     A digital audio signal or an analog audio signal is exclusively and selectively input to the N input pins P 1  through P N . The audio interface circuit  310  receives digital or analog audio signals input via the N input pins P 1  through P N , and supplies the audio signals thus received to the internal circuit  320  configured as a downstream stage. 
     The audio interface circuit  310  is configured such that, when analog audio signals are input to the N input pins P 1  through P N , a bias voltage V B  is applied to the N input pins P 1  through P N  via bias resistors R 1   1  through R 1   N . For example, the bias voltage V B  is configured as a voltage (e.g., 7.2 V) obtained by dividing a power supply voltage V CC  (e.g., 14.4 V) supplied to the power supply pin VCC by 2 using a resistor dividing circuit. 
     Furthermore, the audio interface circuit  310  is configured such that, when digital audio signals are input to the N input pins P 1  through P N , the N input pins P 1  through P N  are respectively grounded via the bias resistors R 1   1  through R 1   N . 
     The audio interface circuit  310  includes a voltage dividing circuit  312 , a buffer  314 , N bias selectors SEL 1   1  through SEL 1   N , and N bias resistors R 1   1  through R 1   N . 
     The voltage dividing circuit  312  includes resistors R 21  and R 22  each having the same resistance value and divides the power supply voltage V CC  into ½. The buffer  314  receives the output voltage of the voltage dividing circuit  312 . In a case in which digital audio signals are input, the buffer  314  can be turned off. 
     The i-th (1≤i≤N) bias selector SEL 1   i  is arranged such that its input terminal ( 1 ) is coupled to the output of the buffer  314 , and its second input terminal ( 2 ) is grounded. 
     The i-th (1≤i≤N) bias resistor R 1   i  is arranged such that one terminal thereof is coupled to the output terminal (O) of the corresponding bias selector SEL 1   i , and the other terminal thereof is coupled to the corresponding input pin P i . 
     When analog audio signals are input to the N input pins P 1  through P N , the bias selectors SEL 1   1  through SEL 1   N  are each switched on to the first input terminal side. When digital audio signals are input, the bias selectors SEL 1   1  through SEL 1   N  are each switched on to the second input terminal side. 
     For example, the audio circuit  300  may include a register that stores data for specifying digital input and analog input. The states of the multiple bias selectors SEL 1   1  through SEL 1   N  may be switched based on the data thus stored. Alternatively, the audio circuit  300  may be provided with a setting pin for specifying digital input and analog input. With this, the states of the multiple bias selectors SEL 1   1  through SEL 1   N  may be switched based on the electrical state of the setting pin. 
     The audio interface circuit  310  may be configured such that, when the audio circuit  300  is started up, i.e., when the power supply is turned on, the N input pins P 1  through P N  are respectively grounded via the bias resistors R 1   1  through R 1   N  regardless of the state of the register or setting pin. For example, the audio circuit  300  includes a power-on reset circuit. The audio circuit  300  initializes the multiple bias selectors SEL 1   1  through SEL 1   N  such that they are switched on to the second input terminal side according to the output of the power-on reset circuit. Subsequently, after the completion of the startup of the audio circuit  300 , the audio circuit  300  may switch the states of the multiple bias selectors SEL 1   1  through SEL 1   N  based on the state of the register or the setting pin. 
     The above is the basic configuration of the audio circuit  300 . Next, description will be made regarding the operation thereof.  FIG. 4  is an equivalent circuit diagram of the audio circuit  300  when digital audio signals are input. A digital sound source  402  is coupled to M (1≤M≤N) input pins from among the multiple input pins P 1  through P N . M pulse signals D 1  through D M  are input from the digital sound source  402 . The pulse signals D 1  through D M  are supplied to the internal circuit  320  after they pass through the audio interface circuit  310 . 
       FIG. 5  is an equivalent circuit diagram of the audio circuit  300  when analog audio signals are input. An analog sound source  404  is coupled to K (1≤K≤N) input pins from among the multiple input pins P 1  through P N  via coupling capacitors C 1  through C K . K analog audio signals A 1  through A K  output from the analog sound source  404  are shifted by the audio interface circuit  310  such that their center levels each become equal to the bias voltage V BIAS . The K analog audio signals A 1  through A K  thus shifted are supplied to the internal circuit  320 . 
     The above is the operation of the audio circuit  300 . With the audio circuit  300 , this allows the input pins to be used as both analog input pins and digital input pins. This allows the number of pins to be reduced. 
     Furthermore, the audio interface circuit  310  is configured such that, when the audio circuit  300  is started up, N input pins are each grounded via a resistor. With this, in a case in which a digital sound source  402  with digital output is coupled to the input pins, this is capable of preventing a high voltage (V BIAS ) from being applied to the output pins of the digital sound source  402  when the audio circuit is started up. 
     The present disclosure encompasses various kinds of apparatuses and circuits that can be regarded as a circuit configuration shown in  FIG. 3  or otherwise that can be derived from the aforementioned description. That is to say, the present disclosure is not restricted to a specific configuration. More specific description will be made below regarding an example configuration for clarification and ease of understanding of the essence of the present disclosure and the circuit operation. That is to say, the following description will by no means be intended to restrict the technical scope of the present disclosure. 
     Example 1 
       FIG. 6  is a circuit diagram showing an audio circuit  300 A according to an example 1. Description will be made in this example regarding an arrangement in which N=4. The audio circuit  300 A supports input of a serial audio signal including serial data SDATA, LR clock LRCLK, bit clock BCLK, and master clock MCLK as digital audio signals. Examples of such a digital audio signal include PCM audio data in the I&#39;S format. The digital audio signals include two-channel audio signals (which will be referred to as an “L-channel audio signal” and an “R-channel audio signal” for convenience). In a case of employing a multi-channel audio signal, e.g., four-channel audio signal, the time division multiplexing (TDM) format may be employed. 
     Furthermore, the audio circuit  300 A supports differential input of a two-channel (L-channel and R-channel) audio signal in a differential (balanced) format. 
     In a case in which a digital sound source is connected, serial data SDATA, LR clock LRCLK, bit clock BCLK, and master clock MCLK are input to the first input pin P 1  through the fourth input pin P 4 . These signals each correspond to the pulse signal shown in  FIG. 1 . 
     In a case in which an analog sound source is connected, such an arrangement allows differential analog audio signals AL P  and AL N  for each channel (e.g., for the L channel) to be input via the first input pin P 1  and the second input pin P 2 . Furthermore, such an arrangement allows differential analog audio signals AR P  and AR N  for another channel (e.g., the R channel) to be input via the third input pin P 3  and the fourth input pin P 4 . 
     The internal circuit  320 A includes Schmitt buffers B 1   1  through B 1   4 , a serial audio interface circuit  322 A, D/A converters  324 L and  324 R, output selectors SEL 2   1  through SEL 2   4 , output buffers B 2   1  through B 2   4 , and an analog processing circuit  330 . 
     When digital audio signals are input to the input pins P 1  through P 4 , the serial audio interface circuit  322 A and the D/A converters  324 L and  324 R are set to the active state. In this state, the serial data thus received is separated into an L-channel digital signal and an R-channel digital signal. The D/A converter  324 L converts the L-channel digital signal into an analog signal. The D/A converter  324 R converts the R-channel digital signal into an analog signal. In the example 1, the D/A converters  324 L and  324 R each have a differential output. It should be noted that the D/A converters  324 L and  324 R are each configured to have a common digital unit. 
     When digital audio signals are input to the input pins P 1  through P 4 , the output selectors SEL 2   1  through SEL 2   4  select the output signals of the D/A converters  324 L and  324 R. When analog audio signals are input to the input pins P 1  through P 4 , the output selectors SEL 2   1  through SEL 2   4  select the analog audio signals thus input. 
     Specifically, the output selector SEL 2   1  is arranged such that its first input terminal ( 1 ) is coupled to the input pin P 1  and its second input terminal ( 2 ) is coupled to a positive electrode output of the D/A converter  324 L. The output selector SEL 2   2  is arranged such that its first terminal ( 1 ) is coupled to the input pin P 2  and its second input terminal ( 2 ) is coupled to a negative electrode output of the D/A converter  324 L. The output selector SEL 2   3  is arranged such that its first input terminal ( 1 ) is coupled to the input pin P 3  and its second input terminal ( 2 ) is coupled to a positive electrode output of the D/A converter  324 R. The output selector SEL 2   4  is arranged such that its first input terminal ( 1 ) is coupled to the input pin P 4  and its second input terminal ( 2 ) is coupled to a negative electrode output of the D/A converter  324 R. 
     The output buffers B 2   1  through B 2   4  receive the outputs of the output selectors SEL 2   1  through SEL 2   4 , and supply the outputs thus received to the analog processing circuit  330  configured as a downstream stage. It should be noted that the output buffers B 2   1  through B 2   4  may be omitted. 
     With the audio circuit  300 A, this allows digital audio signals in a serial format or differential analog audio signals to be received via the four input pins P 1  through P 4 . The number of input pins of the audio circuit  300 A is two less than that of the audio circuit  900  shown in  FIG. 2 . 
     It should be noted that, as an effective modification of the audio circuit  300 A shown in  FIG. 6 , the D/A converters  324 L and  324 R may be configured to support a single-ended output. Alternatively, single-ended analog audio signals may be input to two from among the input pins P 1  through P 4 . 
     Example 2 
       FIG. 7  is a circuit diagram showing an audio circuit  300 B according to an example 2. Description will be made in this example regarding an arrangement in which N=3. The audio circuit  300 B supports input of a serial audio signal including serial data SDATA, LR clock LRCLK, and bit clock BCLK as digital audio signals. Examples of such a digital audio signal include PCM audio data in the I 2 S format. The digital audio signals include two-channel audio signals (which will be referred to as an “L-channel audio signal” and an “R-channel audio signal” for convenience). In a case of employing a multi-channel audio signal, e.g., four-channel audio signal, the time division multiplexing (TDM) format may be employed. 
     Furthermore, the audio circuit  300 B supports input of two-channel (L-channel and R-channel) audio signals AL and AR in a single-ended (unbalanced) format. 
     In a case in which a digital sound source is connected, serial data SDATA, LR clock LRCLK, and bit clock BCLK are input to the first input pin P 1  through the third input pin P 3 . These signals each correspond to the pulse signal shown in  FIG. 1 . 
     In a case in which an analog sound source is connected, such an arrangement allows a single-ended analog audio signal AL for each channel (e.g., for the L channel) to be input via the first input pin P 1 . Furthermore, such an arrangement allows a single-ended analog audio signal AR for another channel (e.g., the R channel) to be input via the second input pin P 2 . 
     The internal circuit  320 B includes Schmitt buffers B 1   1  through B 1   3 , a serial audio interface circuit  322 B, D/A converters  324 L and  324 R, output selectors SEL 2   1  and SEL 2   2 , output buffers B 2   1  and B 2   2 , and an analog processing circuit  330 . 
     When digital audio signals are input to the input pins P 1  through P 3 , the serial audio interface circuit  322 B and the D/A converters  324 L and  324 R are set to the active state. In this state, the serial data thus received is separated into an L-channel digital signal and an R-channel digital signal. The D/A converter  324 L converts the L-channel digital signal into an analog signal. The D/A converter  324 R converts the R-channel digital signal into an analog signal. In the example 2, the D/A converters  324 L and  324 R each have a single-ended output. 
     When digital audio signals are input to the input pins P 1  through P 3 , the output selectors SEL 2   1  and SEL 2   2  select the output signals of the D/A converters  324 L and  324 R. When analog audio signals are input to the input pins P 1  and P 2 , the output selectors SEL 2   1  and SEL 2   2  select the analog audio signals thus input. 
     Specifically, the output selector SEL 2   1  is arranged such that its first input terminal ( 1 ) is coupled to the input pin P 1  and its second input terminal ( 2 ) is coupled to an output of the D/A converter  324 L. The output selector SEL 2   2  is arranged such that its first input terminal ( 1 ) is coupled to the input pin P 2  and its second input terminal ( 2 ) is coupled to an output of the D/A converter  324 R. 
     The output buffers B 2   1  and B 2   2  receive the outputs of the output selectors SEL 2   1  and SEL 2   2 , and supply the outputs thus received to the analog processing circuit  330  configured as a downstream stage. It should be noted that the output buffers B 2   1  and B 2   2  may be omitted. 
     With the audio circuit  300 B, this allows digital audio signals in a serial format or a single-ended analog audio signal to be received via the three input pins P 1  through P 3 . The number of input pins of the audio circuit  300 B is two less than that of the audio circuit  900  shown in  FIG. 2 . 
     Example 3 
       FIG. 8  is a circuit diagram showing an audio circuit  300 C according to an example 3. Description will be made in this example regarding an arrangement in which N=2. The audio circuit  300 C supports input of a signal in the S/PDIF (Sony Philips Digital InterFace) format as a digital audio signal. 
     Furthermore, the audio circuit  300 C supports input of two-channel (L-channel and R-channel) audio signals AL and AR in a single-ended (unbalanced) format. 
     In a case in which a digital sound source is connected, an S/PDIF signal is input to the first input pin P 1 . 
     In a case in which an analog sound source is connected, such an arrangement allows a single-ended analog audio signal AL for each channel (e.g., for the L channel) to be input via the first input pin P 1 . Furthermore, such an arrangement allows a single-ended analog audio signal AR for another channel (e.g., the R channel) to be input via the second input pin P 2 . 
     The internal circuit  320 C includes Schmitt buffer B 1   1  through B 1   4 , an S/PDIF circuit  322 C, D/A converters  324 L and  324 R, output selectors SEL 2   1  through SEL 2   2 , output buffers B 2   1  through B 2   2 , and an analog processing circuit  330 . 
     When an S/PDIF signal is input to the input pin P 1 , the S/PDIF circuit  322 C and the D/A converters  324 L and  324 R are set to the active state. In this state, the S/PDIF signal thus received is separated into an L-channel digital signal and an R-channel digital signal. The D/A converter  324 L converts the L-channel digital signal into an analog signal. The D/A converter  324 R converts the R-channel digital signal into an analog signal. The other configuration is the same as that of the internal circuit  320 B shown in  FIG. 7  (example 2). 
     With the audio circuit  300 C, this allows digital audio signals in a serial format or single-ended analog audio signals to be received via the two input pins P 1  and P 2 . 
     Example 4 
       FIG. 9  is a circuit diagram showing an audio circuit  300 D according to an example 4. Description will be made in this example regarding an arrangement in which N=2. The audio circuit  300 D supports direct input of pulse modulation signals DL and DR such as a PDM signal (DSD signal), PWM signal, or the like. 
     Furthermore, the audio circuit  300 D supports input of two-channel (L-channel and R-channel) audio signals AL and AR in a single-ended (unbalanced) format. 
     In a case in which a digital sound source is connected, such an arrangement allows an L-channel pulse modulation signal DL and an R-channel pulse modulation signal DR to be input via the input pins P 1  and P 2 , respectively. 
     In a case in which an analog sound source is connected, such an arrangement allows a single-ended analog audio signal Al for each channel (e.g., for the L channel) to be input via the first input pin P 1 . Furthermore, such an arrangement allows another single-ended analog audio signal AR for another channel (e.g., the R channel) to be input via the second input pin P 2 . 
     The internal circuit  320 D includes Schmitt buffers B 1   1  and B 1   2 , a decoder circuit  322 D, D/A converters  324 L and  324 R, output selectors SEL 2   1  and SEL 2   2 , output buffers B 2   1  and B 2   2 , and an analog processing circuit  330 . 
     When pulse modulation signals DL and DR are input to the input pins P 1  and P 2 , the decoder circuit  322 D is set to the active state. In this state, the pulse modulation signals DL and DR thus received are decoded. The D/A converter  324 L converts the L-channel digital signal into an analog signal. The D/A converter  324 R converts the R-channel digital signal into an analog signal. The decoder circuit  322 D and the D/A converters  324 L and  324 R may be replaced by an analog lowpass filter. The other configuration is the same as that of the internal circuit  320 B shown in  FIG. 7  (example 2). 
     With the audio circuit  300 D, this allows pulse-modulated digital audio signals or single-ended analog audio signals to be received via the two input pins P 1  and P 2 . 
     Example 5 
     Description has been made in the examples 1 through 4 regarding an audio circuit provided with the analog processing circuit  330 . Also, the present disclosure is applicable to an audio circuit provided with a digital processing circuit  340 .  FIG. 10  is a circuit diagram showing an audio circuit  300 E according to an example 5. The audio circuit  300 E is capable of receiving the same signals as those that can be received by the audio circuit  300 A shown in  FIG. 6 . Instead of the analog processing circuit  330  and the D/A converters  324 , the audio circuit  300 E includes a digital processing circuit  340  and A/D converters  326 . 
     When analog audio signals are input to the input pins P 1  through P 4 , the A/D converters  326 L and  326 R are set to the active state. In this state, the A/D converters  326 L and  326 R each convert the analog audio signal to a digital signal. Specifically, the A/D converter  326 L has a differential input and converts L-channel differential analog signals AL P  and AL N  input via the input pins P 1  and P 2  into a digital signal. Furthermore, the A/D converter  326 R has a differential input and converts R-channel differential analog signals AR P  and AR N  input via the input pins P 3  and P 4  into a digital signal. 
     When analog signals are input to the input pins P 1  through P 4 , the digital processing circuit  340  processes the outputs of the A/D converters  326 L and  326 R. When digital signals are input to the input pins P 1  through P 4 , the digital processing circuit  340  processes the output of the serial audio interface circuit  322 E. 
     For example, the digital processing circuit  340  may include output selectors SEL 3   1  and SEL 3   2  configured such that, when digital signals are input to the input pins P 1  through P 4 , the selectors SEL 3   1  and SEL 3   2  select the digital audio signals, and when analog signals are input to the input pins P 1  through P 4 , the selectors SEL 3   1  and SEL 3   2  select the outputs of the A/D converters  326 . 
     Modification 1 
     In the same manner as in the example 5, in the examples 2 through 4, the analog processing circuit  330  can be replaced by the digital processing circuit  340 . 
     Modification 2 
     The configuration of the audio interface circuit  310  is not restricted to an arrangement shown in  FIG. 3 .  FIG. 11A  and  FIG. 11B  are circuit diagrams each showing the audio interface circuit  310  according to a modification 2.  FIG. 11A  and  FIG. 11B  show only a portion that corresponds to a single pin. In a modification shown in  FIG. 11A , two bias resistors R 1   #  are provided on the input side of a bias selector SEL 1   # . In a modification shown in  FIG. 11B , instead of the bias selector SEL 1   # , a switch SW 1   #  is provided. When an analog audio signal is input, the switch SW 1   #  is turned on. In this state, the bias voltage V BIAS  is applied to the input pin P #  via the bias resistor R 1   #  and the switch SW 1   # . The bias resistor R 1   #  and the switch SW 1   #  may be swapped.