Patent Publication Number: US-6658310-B1

Title: Method of entering audio signal, method of transmitting audio signal, audio signal transmitting apparatus, and audio signal receiving and reproducing apparatus

Description:
This is a division of application Ser. No. 08/921,191, filed on Aug. 29, 1997, and issued on Mar. 28, 2000 as U.S. Pat. No. 6,044,307. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of entering a digitized audio signal into a data processing apparatus, such as a personal computer, method of transmitting the audio signal, and audio signal transmitting apparatus and audio signal receiving and reproducing apparatus which permit transmission of the audio signal according to this signal transmitting method. 
     2. Prior Art 
     When a computer receives stereo audio signals, analog signals may be input via analog input terminals, and converted into PCM (Pulse Code Modulation) data in the inside of the computer, or digital audio signals may be input via serial ports, such as RS-232C, or SCSI (Small Computer System Interface) ports, for example. In the former method, however, the input signals in the analog format are likely to be affected by noise from the computer. The latter method has a problem of a slow input speed, and, in most cases, cannot be used for exclusively inputting audio signals. 
     In view of the above situation, an audio input interface has been developed which is designed to convert digitized audio signals into serial data, and transmit the serial data via an exclusive digital audio input terminal, such as SPDIF, as shown in FIG.  1 . 
     In the conventional audio interface as shown in FIG. 1, analog audio signals SL, SR of left (L) and right (R) channels are converted into digital signals by respective A/D converters  101 ,  102 , and packeted by a packet circuit  103  to be converted into serial data, which is then transmitted to a personal computer  200 . On the side of the personal computer  200 , the received serial data are synchronized by a PLL (phase-locked loop) circuit, and the resultant packet data is decoded by a decoding circuit  202  and reassembled into original digital data. Thus, the circuit arrangement tends to be complicated, with a result of an increased cost. 
     The other method of transmitting digitized audio signals may be roughly classified into a method of transmitting the input signals after converting them into analog signals, and a method of transmitting the digital signals as they are. FIG. 2 shows an example of the former method. In this example, digital audio signals of L channel and R channel which are reproduced in a reproducing system (not shown) are converted into corresponding analog audio signals by respective D/A converters  301 L and  301 R, and then amplified by respective amplifiers  302 L and  302 R. The analog audio signals of the two channels obtained from the amplifiers  302 L and  302 R are then transmitted over respective cables  303 L and  303 R, amplified by amplifiers  304 L and  304 R, and then generated from speakers  305 L and  305 R. 
     FIG. 3 shows one example of the method of transmitting audio signals in the form of digital signals. In the example of FIG. 3, a packet of digital audio signals of L channel and R channel is transmitted from a digital output circuit  401 . This packet is transmitted to a demodulator circuit  403  over a cable  402 , and is demodulated by this demodulator circuit  403  into the original digital audio signals of the L channel and R channel. These digital audio signals of the respective channels are converted into analog audio signals by D/A converters  404 L and  404 R, amplified by amplifiers  405 L,  405 R, and then generated from speakers  406 L and  406  R. 
     The above-described prior art method of FIG. 2, in which the audio signals are transmitted as analog signals, suffers from deterioration of the quality of sound reproduced from the speakers if noise is superposed on the analog signals during their transmission. Where this method is employed within digital equipment, such as a personal computer, in particular, noise involved in the digital signal system is likely to be superposed on analog audio signals during their transmission, whereby the quality of reproduced sound is considerably deteriorated. On the other hand, the method of FIG. 3 is advantageous in that the transmitted audio signal, which is a digital, signal, is highly resistant to noise. To implement this method, however, there is a need to provide the digital output circuit for assembling and transmitting a packet of digital audio signals, and the demodulator circuit for demodulating this packet, which may increase the size and cost of the whole apparatus. 
     SUMMARY OF THE INVENTION 
     It is therefore a first object of the present invention to provide a method of entering an audio signal, which method enables a simple circuit to use input terminals of analog audio signals to enter a digital audio signal into a data processing apparatus, and wherein a transmitted signal is unlikely to be affected by noise even in an environment having a lot of noise. 
     It is the second object of the invention to provide an audio signal transmitting method that permits an audio signal to be transmitted assuring high resistance to noise, without significantly changing the construction of the known apparatus, and to provide an audio signal transmitting apparatus and an audio signal receiving and reproducing apparatus for transmitting the audio signal according to this method. 
     To attain the first object, the present invention provides a method of entering an audio signal into a data processing apparatus, wherein analog audio signals of at least two channels as the audio signal are digitized by and transmitted from an audio signal output device on a side of an audio source to an audio signal input device on a side of the data processing apparatus, comprising the steps of a) supplying a clock signal from the audio signal input device to the audio signal output device; b) performing delta-sigma (ΔΣ) modulation on the analog audio signals of at least two channels in the audio signal output device so as to convert each of the analog audio signals of at least two channels into one-bit digital audio data for each of the at least two channels, based on the clock signal supplied to the audio signal output device, c) alternately selecting the one-bit digital audio data for the each channel, based on the clock signal, in the audio signal output device, to perform time-division multiplexing on the one-bit digital audio data for the at least two channels, to thereby produce one-bit multiplex audio data, and d) supplying the one-bit multiplex audio data from the audio signal output device to the audio signal input device. 
     Preferably, the multiplex audio data and the clock signal are respectively transmitted to the audio signal input device and the audio signal output device, through respective input lines provided in the data processing apparatus for receiving analog audio stereo signals. 
     Further preferably, the method of entering an audio signal further comprises the step of separating the one-bit multiplex audio data into at least two streams of data corresponding to respective ones of the at least two channels and converting each of the at least two streams of data into multiple-bit PCM data for the each channel in the audio signal input device. 
     To attain the first object, the present invention also provides a data processing apparatus having an interface comprising a) a block which generates a clock signal to an external device, b) a block which receives multiplex audio data produced by one-bit quantization of audio data of at least two channels in synchronization with the clock signal and time-division multiplexing of the quantized audio data based on the clock signal in the external device, and c) a block which processes the received multiplex audio data in synchronization with the clock signal. 
     To attain the second object, the present invention provides a method of transmitting an audio signal from an audio signal transmitting apparatus to an audio signal receiving apparatus, comprising the steps of a) performing delta-sigma (ΔΣ) modulation modulation and time-division multiplexing on digital audio signals of at least left channel and right channel as the audio signal in the audio signal transmitting apparatus, to produce bit stream data as a result of the time-division multiplexing of the digital audio signals, b) transmitting the bit stream data and a clock signal that is in synchronization with the bit stream data, from the audio signal transmitting apparatus to the audio signal receiving apparatus, and c) separating the bit stream data into at least two streams of data corresponding to respective ones of the at least left channel and right channel, based on the clock signal, in the audio signal receiving apparatus. 
     To attain the second object, the present invention also provides an audio signal transmitting apparatus comprising a one-bit D/A converter that performs delta-sigma (ΔΣ) modulation and time-division multiplexing on digital audio signals of at least left channel and right channel, to produce time division multiplexed bit stream data, a separator that separates the bit stream data into at least two streams of data corresponding to respective ones of the at least left channel and right channel, based on a clock signal that is in synchronization with the bit stream data, an analog signal reproducing device which converts the bit stream data of each of the at least left channel and right channel, into a corresponding analog signal, a plurality of signal output terminals that respectively correspond to the at least left channel and right channel, and a selecting device which selects one of first and second modes, the first mode being established by supplying the bit stream data and the clock signal to respective ones of the signal output terminals corresponding to the at least left channel and right channel, the second mode being established by supplying the analog signal of each of the at least left channel and right channel which is transmitted from the analog reproducing device, to one of the signal output terminals which corresponds to the each channel. 
     To attain the second object, the present invention further provides an audio signal receiving and reproducing apparatus comprising a plurality of signal input terminals that respectively correspond to at least left channel and right channel, a plurality of analog signal reproducing devices that respectively correspond to the at least left channel and right channel, a separator that separates time division multiplexed bit stream data, into at least two streams of data corresponding to respective ones of the at least left channel and right channel, in synchronization with a clock signal, and a selecting device that selects one of first and second modes, the first mode being established by transmitting input signals received by the signal input terminals corresponding to the at least left channel and right channel to the separator, the input signals being the bit stream data and the clock signal, and transmitting the at least two streams of data corresponding to the respective ones of the at least left channel and right channel, from the separator to the analog signal reproducing devices corresponding to the respective channels, the second mode being established by transmitting input signals received by the signal input terminals corresponding to the at least left and right channels, to the analog signal reproducing devices corresponding to the respective channels. 
    
    
     The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a conventional audio signal input method; 
     FIG. 2 is a block diagram illustrating a conventional audio signal transmitting method; 
     FIG. 3 is a block diagram illustrating another conventional audio signal transmitting method; 
     FIG. 4 is a block diagram showing the construction of a system to which an audio signal input method according to one embodiment of the present invention is applied; 
     FIG. 5 is a block diagram showing the construction of a stereo A/D converter of the system of FIG. 4; 
     FIG. 6 is a timing chart showing the operation of the stereo A/D convertor of FIG. 5; 
     FIG. 7 is a circuit diagram showing the construction of a single-bit A/D converter used in the stereo A/D convert of FIG. 5; 
     FIG. 8 is a graph useful in explaining the effect of the circuit of FIG. 7; 
     FIG. 9 is a block diagram showing an audio input interface of a computer in the system of FIG. 4; 
     FIG. 10 is a block diagram showing the construction of a system to which an audio signal transmitting method according to one embodiment of the present invention is applied; 
     FIG. 11 is a view showing the construction of a single-bit D/A converter in the system of FIG. 10; 
     FIG. 12 is a view showing the construction of a L/R separator used in the system of FIG. 10; and 
     FIG. 13 is a view showing the construction of an analog output circuit in the system of FIG.  10 . 
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present invention will be described referring to the drawings. 
     Referring first to FIG. 4, there is shown the construction of a system to which an audio signal input method according to one embodiment of the present invention is applied. 
     A stereo A/D converter  2  performs one-bit A/D conversion and time-division multiplexing on analog audio signals SL, SR of left (L) and right (R) channels supplied from an audio source  1 , such as a microphone, audio reproducing device, electronic instrument, or a mixer, so as to produce a single-bit multiplex audio data L/R, which is in turn supplied to a personal computer  3 . On the other hand, a clock signal is supplied from the personal computer  3  to the stereo A/D converter  12  so as to control the timing of the one-bit A/D conversion and time-division multiplexing. 
     The stereo A/D converter  2  is equivalent to an audio signal output device provided on the side of the audio source  1 , and is constructed as shown in FIG. 5 by way of example. 
     The stereo A/D converter  2  includes a single-bit A/D converter  11  for converting the analog audio signal SL into a one-bit digital signal based on the clock signal φ, and a single-bit A/D converter  12  for converting the analog audio signal SR into a one-bit digital signal based on a clock signal (represented as “−φ”) into which the clock signal is inverted by an inverter  13 . The obtained one-bit audio data L, R of each channel is received by an input terminal of a corresponding one of AND gates  14 ,  15 . The AND gates  14 ,  15  alternately pass the audio data L, R based on the clock signals −φ, φ supplied to the other input terminals thereof. These audio data L, R are combined by an OR gate  16 , to produce one-bit multiplex audio data L/R. 
     FIG. 6 is a timing chart showing the above operation. The phases of one-bit digital data L, R of left and right channels are shifted from each other by a half cycle of the clock signal φ, and these data L, R are alternately selected so as to provide multiplex audio data L/R in which the data of the left and right channels are switched in a cycle that is twice the cycle of the clock signal. Where the cycle of the clock signal φ is set to around 12 MHz, the phase difference of the data of the right and left channels in the multiplex audio data is around 40 ns, which has absolutely no influence on the sense of hearing. 
     For example, well-known secondary delta-sigma (ΔΣ) modulators as shown in FIG. 7 may be used as the one-bit A/D converters  11 ,  12 . In the delta-sigma modulator, a two-stage integrator is constituted by an input-side inverter  21 , capacitors  22 ,  23  coupled in series with the input and output ends of the inverter  21 , a resistor  24  that is pulled down from a junction of the capacitors  22 ,  23 , and an inverter  25  provided on a feedback path, and a quantizer is constituted by an inverter  26  provided on the output side of the inverter  21 , while a delay holding circuit is constituted by a latch circuit  27  provided between the output of the inverter  26  and the feedback path. 
     The delta-sigma modulator is substantially equivalent to a modulator in which a low-boost integrator is provided on the input side thereof, and a low-cut differentiator is provided on the output side. This modulator is known as having a noise-shaping effect to improve S/N in an audible band, by shaping noise to concentrate on the high-frequency side, as shown in FIG.  8 . 
     FIG. 9 shows the construction of an audio signal input interface (audio signal input device) provided on the input side of the personal computer  3 . This circuit is constructed such that one of an audio signal input circuit for receiving multiplex audio data L/R and an input circuit for receiving analog audio signals SL, SR may be selected. The states of switches  31 ,  32  can be set by a CPU (not shown) in initializing mode or the like or by manual operation to select one of the above input circuits, and the selected input circuit is adapted to receive the multiplex audio data L/R or the audio signals. That is, both the multiplex audio data L/R and the analog audio signals SL, SR are input to the audio signal input interface of the personal computer  3  through analog audio signal input lines pre-installed in the computer  3 . 
     Where the switches  31 ,  32  are placed in the states as shown in FIG. 9, to select the input circuit for the multiplex audio data L/R, the multiplex audio data L/R is received via a terminal  34  of one of channels of an input terminal  33  on the analog audio signal input lines which also receive analog audio signals SL, SR, and the clock signal φ produced within the personal computer  3  is generated to the outside of the interface via a terminal  35  of the other channel of the input terminal  33 . 
     The received multiplex audio data L/R passes through the switch  31 , and is supplied to decimation filters  36 ,  37 . Each of the decimation filters  36 ,  37  is comprised of a digital low-pass filter (DLPF)  38 ,  39  for removing aliasing distortion from the audio data and producing multiple-bit data stream, and a decimation processor  40 ,  41  for thinning data so as to decrease the sampling frequency of the output from the low-pass filter to the sampling frequency for PCM data. To slightly shift the sampling timing with respect to switching of the left and right channels, a clock signal φ′ that is delayed relative to the clock signal φ about 20 ns by a delay circuit  42  is supplied to the digital low-pass filter (DLPF)  39 , and a clock signal—φ into which the clock signal φ′ is inverted by an inverter  43  is supplied to the digital low-pass filter (DLPF)  38 . The frequency of the clock signal φ is divided by a counter  44 , and the divided frequency clock signal φ″ having the sampling frequency of PCM is supplied to the decimation processors  40 ,  41 . In this manner, n-bit PCM data DL, DR are generated from the decimation filters  36 ,  37 , and entered into the inside of the personal computer  3 . 
     Where the analog audio signals SL, SR are received, the switches  31 ,  32  are switched so that the analog audio signals SL, SR are respectively directed to the A/D converters  45 ,  46 , and subjected to ordinary multiple-bit A/D conversion, based on the divided frequency clock signal φ″, to produce PCM data DL, DR. 
     In the present embodiment, as described above, the construction of the interface is simplified since special processes such as synchronizing, packeting and decoding, are not required to be performed, and the transmitted audio signal, which is comprised of digital data, is less likely to be affected by noise. Further, the clock signal and multiplex audio data can be transmitted though analog audio signal input lines capable of transmitting one-bit data, thus permitting the use of analog audio input lines or the like pre-installed on the computer, with no need to provide special input terminals. Moreover, the analog input lines and circuits used after conversion into PCM data may also be commonly used. 
     A method of transmitting an audio signal according to one embodiment of the present invention will be now described. 
     FIG. 10 shows the construction of a system to which the audio signal transmitting method of the present embodiment is applied. In FIG. 10, the present system is mainly comprised of an audio signal transmitting apparatus  50  and an audio signal receiving and reproducing apparatus  60 , which also constitute embodiments of the present invention. Reference numeral  70  denotes a known loudspeaker system equipped with amplifiers. 
     The audio signal transmitting apparatus  50  is provided in an output unit of a tone generator board of a personal computer, for example, for transmitting an audio signal. As shown in FIG. 10, the audio signal transmitting apparatus  50  of the present embodiment includes a one-bit D/A converter  51 , an L/R separator  52 , analog output circuits  53 L,  53 R, switches  54 L,  54 R, and signal output terminals  55 L,  55 R. 
     The one-bit D/A converter  51  performs oversampling and delta-sigma (ΔΣ) modulation on a digital audio signal reproduced by a reproducing system (not shown), and generates one-bit pulse density-modulated bit stream data. This type of one-bit D/A converter is often employed in analog output units of recent audio equipment. 
     FIG. 11 shows the construction of an example of the one-bit D/A converter  51 . In FIG. 11, FIFO (First-In First-Out)  511 L and FIFO  511 R occasionally take in 16-bit digital audio signals of L channel and R channel from the reproducing system, respectively, to hold them, and generate the digital audio signals in the order from the oldest to the latest, at a given sampling rate (48 kHz, for example). The D/A converter  51  also has a time-division multiplexer  512 , which performs time-division multiplexing on the digital audio signals generated from the FIFOs  511 L,  511 R, and alternately outputs the digital audio signal of the two channels at a fixed rate, such that the signal of the left channel is followed by that of the right channel, which is then followed by that of the left channel and so on. 
     An oversampling unit  513  performs oversampling processing (including an interpolation process) on the thus generated digital audio signal for each of L, R channels, so as to convert the signal into a digital audio signal generated at a sampling rate that is  128  times that of the original signal. A delta-sigma (ΔΣ) modulator  514  performs delta-sigma modulation for each channel, on the digital audio signal generated from the oversampling unit  513 . Then, signals resulting from the delta-sigma modulation for each channel are subjected to time division multiplexing, to produce bit stream data, which is generated at a sampling rate (of 12.288 MHz, for example) which is far higher than that of the original digital audio signals. The bit stream data are generated in synchronization with the clock signal φ of a fixed frequency such that each bit data of the bit stream data which correspond to the L channel is generated upon a rise of the clock signal φ, and each stream data corresponding to the R channel is generated upon a fall of the clock signal. 
     The L/R separator  52  shown in FIG. 10 serves to separate the bit stream data supplied from the delta-sigma modulator  514  in the above manner, into data to be present in the L channel and data to be present in the R channel. 
     FIG. 12 shows the construction of an example of the L/R separator  52 . This L/R separator  52  is comprised of two flip flops  521 ,  522 , and an inverter  523  for inverting the clock signal φ. In this L/R separator  52 , when the clock signal φ falls, bit data of the L channel generated from the delta-sigma modulator  514  upon an immediately preceding rise of the clock pulse signal φ is written into the flip flop  521 . When the clock signal φ rises, bit data of the R channel generated from the modulator  514  upon an immediately preceding fall of the clock pulse signal is written into the flip flop  522 . In this manner, each bit data of the bit stream data for each channel is written into a corresponding one of the flip flops  521 ,  522 , and the bit stream data of the respective L, R channels are separately generated from the corresponding flip flops  521 ,  522 . 
     The analog output circuits  53 L,  53 R of FIG. 10 provide analog signal reproducing means for converting the bit stream data of respective channels obtained from the L/R separator  52  into corresponding analog audio signals. As shown in FIG. 13, each of the analog output circuits  53 L,  53 R is comprised of a low-pass filter  531  and an amplifier  532 . 
     The low-pass filter  531  receives the bit stream data from the L/R separator  52 . The received bit stream data contains shaping noise caused by the delta-sigma modulation, as well as the spectrum of the original audio signal. The low-pass filter  531 , which is provided for removing the shaping noise from the bit stream data to produce analog audio signals, is a first-order low-pass filter using one resistance and one capacitor. The spectrum of the shaping noise is concentrated at a frequency band around the sampling rate of the bit stream data, which is far away from the cut-off frequency (48 kHz, for example) of the original audio signals (refer to FIG.  8 ). Accordingly, even the above first-order low-pass filter may be able to attenuate the shaping noise with a sufficient attenuation factor of about 48 dB, so that audio signals corresponding to the original audio signals can be obtained from the bit stream data. 
     The switches  54 L and  54 R shown in FIG. 10 are used for selecting signals to be generated from the signal output terminals  55 L,  55 R, depending upon instructions of the user. More specifically, the audio signal transmitting apparatus  50  has an analog output mode and a digital output mode, one of which can be selected by the user. When the analog output mode is selected by the user, the switches  54 L,  54 R are placed in the positions for selecting the analog output circuits  53 L and  53 R, respectively, and the analog audio signals of the L, R channels generated from these circuits  53 L,  53 R are supplied to the signal output terminals  55 L,  55 R. Where the digital output mode is selected by the user, on the other hand, the switches  54 L,  54 R are placed in the positions for selecting the bit stream data and clock signal φ, respectively, and these bit stream data and clock signal are supplied to the signal output terminals  55 L and  55 R. 
     The audio signal receiving and reproducing apparatus  60  according to the present embodiment will be now explained. This apparatus  60  includes signal input terminals  61 L,  61 R for receiving signals supplied from the audio signal transmitting apparatus  50  of the present embodiment, or from other audio signal transmitting apparatus that are generally used. 
     The audio signal receiving and reproducing apparatus  60  includes an L/R separator  62  as means for separating the time division multiplexed bit stream data into those corresponding to respective channels. This L/R separator  62  is constructed similarly to the above-described L/R separator  52 . 
     Amplifiers  63 L,  63 R and loudspeakers  64 L,  64 R constitute analog signal reproducing means for generating the analog audio signals of the respective L-channel and R-channel as sounds. Although these amplifiers  63 L,  63 R and loudspeakers  64 L,  64 R are constructed similarly to an ordinary loudspeaker system with amplifiers, the amplifiers  63 L and  63 R of the present embodiment receive bit stream data when the digital output mode is selected, and therefore a first-order low-pass filter (not shown) for removing shaping noise is provided in an input port of each amplifier. 
     Switches  65 L and  65 R are inserted between the signal input terminals  61 L and  61 R and the L/R separator  62 , and switches  66 L and  66 R are inserted between the L/R separator  62  and the amplifiers  63 L and  63 R. These switches are switched depending upon the mode selected by the user. 
     More specifically, the audio signal receiving and reproducing apparatus has an analog input mode and a digital input mode, one of which can be selected by the user. When the analog input mode is selected by the user, the switches  65 L,  65 R,  66 L and  66 R are placed in the positions for connecting the signal input terminals  61 L and  61 R to respective inputs of the amplifiers  63 L and  63 R. When the digital input mode is selected by the user, on the other hand, the switches  65 L,  65 R are placed in the positions for connecting the signal input terminals  61 L,  61 R to an input of the L/R separator  62  for receiving the bit stream data, and an input of the separator  62  for receiving the clock signal, respectively, and the switches  66 L and  66 R are placed in the positions for connecting outputs of the L/R separator  62  for generating the bit stream data of the respective channels, to corresponding inputs of the amplifiers  63 L and  63 R. 
     The loudspeaker system  70  equipped with amplifiers is comprised of signal input terminals  71 L and  71 R, amplifiers  72 L and  72 R, and loudspeakers  73 L and  73 R, which correspond to the L, R channels, respectively. 
     The operation of the present embodiment will be now explained. First, the operation in the case where a combination of the audio signal transmitting apparatus  50  and audio signal receiving and reproducing apparatus  60  of the present embodiment is used will be explained. In this case, the user connects the signal output terminal  55 L to the signal input terminal  61 L with a cable, and connects the signal output terminal  55 R to the signal input terminal  61 R with another cable. Then, the audio signal transmitting apparatus  50  is set in the digital output mode, and the audio signal receiving and reproducing apparatus  60  is set in the digital input mode. 
     As a result, bit stream data generated from the one-bit D/A converter  51  of the audio signal transmitting apparatus  50  and the clock signal φ that is in synchronization with the bit stream data are transmitted over respective cables, and received by the L/R separator  62  of the audio signal receiving and reproducing apparatus  60 . 
     The bit stream data are divided or separated by the L/R separator  62  into respective blocks of bit stream data corresponding to the L, R channels, based on the clock signal φ and the respective blocks of the bit stream data are fed to the amplifiers  63 L and  63 R. The low-pass filter provided at the input of each amplifier  63 L and  63 R removes shaping noise from a corresponding block of the bit stream data. The resultant analog audio signals are then amplified by the amplifiers  63 L,  63 R, and generated from the loudspeakers  64 L and  64 R. 
     In this manner, the audio signal is transmitted as a digital signal from the audio signal transmitting apparatus  50  to the audio signal reproducing apparatus  60 , assuring high resistance to noise. Further, since the conversion of the bit stream data into analog signals can be accomplished merely by directly passing the transmitted bit stream data through the analog low-pass filter, any bit synchronization or special modulation need not be performed on the side of the audio signal receiving and reproducing apparatus  60 . 
     While the most typical operation of the present embodiment has been described above, each apparatus according to the present embodiment may be used in combination with other known apparatus that have been generally used in the art. 
     Where the audio signal transmitting apparatus  50  of the present embodiment is used in combination with the known loudspeaker system  70  with amplifiers, the signal output terminal  55 L and signal input terminal  71 L, and the signal output terminal  55 R and signal input terminal  71 R of these apparatus are connected to each other with respective cables. By setting the audio signal transmitting apparatus  50  in the analog output mode, the switches  54 L,  54 R of the audio signal transmitting apparatus  50  are placed in the positions for selecting analog audio signals of L, R channels transmitted from the analog output circuits  53 L and  53 R, and the selected audio signals are transmitted to the loudspeaker system 7  with amplifiers over the respective cables. 
     In the case where the audio signal receiving and reproducing apparatus  60  of the present embodiment is used in combination with a general audio signal transmitting apparatus capable of generating only analog audio signals, signal output terminals of L, R channels of the transmitting apparatus are connected to the signal input terminals  61 L and  61 R of the audio signal receiving and reproducing apparatus  60 , and the apparatus  60  is set in the analog input mode.