GB2262868A

GB2262868A - Multiplex broadcasting systems

Info

Publication number: GB2262868A
Application number: GB9225606A
Authority: GB
Inventors: Byoung Kon Min
Original assignee: Gold Star Co Ltd
Current assignee: LG Electronics Inc
Priority date: 1991-12-16
Filing date: 1992-12-08
Publication date: 1993-06-30
Anticipated expiration: 2012-12-08
Also published as: KR930015785A; KR940005936B1; GB9225606D0; GB2262868B; DE4242548A1

Abstract

A circuit for automatically tuning audio signals of multiplex broadcasting systems which is capable of automatically demodulating a digital audio signal of a multiplex broadcasting signal on the basis of the type of the multiplex broadcasting system to which the digital audio signal belongs, by discriminating the type of the multiplex broadcasting system through detection of a FM carrier of the multiplex broadcasting signal. The circuit comprises a circuit (60) for discriminating the type of the multiplex broadcasting system of a received broadcasting signal on the basis of a FM audio signal of the received broadcasting signal and generating a control signal in accordance with the discriminated result, and a circuit (80) for detecting a digital audio signal of the broadcasting signal in response to the control signal from the broadcasting system type discriminating circuit and demodulating the digital audio signal in response to a PCM clock signal which is controlled by the control signal from the broadcasting system type discriminating circuit. <IMAGE>

Description

CIRCUIT FOR AUTOMATICALLY TUNING AUDIO SIGNALS OF MULTIPLEX BROADCASTING SYSTEMS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to a circuit for automatically tuning audio signals cf multiplex broadcasting systems, and more particularly to a circuit for automatically tuning audio signals of multiplex broadcasting systems in a video cassette recorder, referred to hereinafter as VCR, or a television receiver, referred to hereinafoer as TV which 5 capable of receiving multiplex broadcasting signals, wherein a digital audio signal of -the multiplex broadcastnng signal can automatically be tuned on the basis of the type of the multiplex brcadcsting system to which the digi t l audio signal belongs.

Descriptlon of the Prior Art A Near-Instantaneous Companding Audio Multiple (NICAM) system is well-known as one of multiplex broadcasting systems.

A multiplex broadcasting signal of the NICAM system contains an analog frequency modulation (FM) audio signal and a digital audio signal as a subaudio signal. Figs. 1A and 1B illustrate base band spectra of the NICAM multiplex broadcasting signal according to the PAL-B/G and PAL-I systems, respectively.

In Fig. 1A, a color carrier C.C is present within the range of 5MHz on the basis of a picture carrier P.C. The FM audio signal is placed at a 5.5MHz carrier band and a pulse code modulation (PCM) audio signal as the digital audio signal is placed at a 5.85MHz carrier band.

In Fig. 1B, the color carrier C.C is present within the range of 5MHz on the basis of a picture carrier P.C. The FM audio signal is placed at a 6.0MHz carrier band and the PCM audio signal as the digital audio signal is placed at a 6.552MHz carrier band.

In the NICAM broadcasting signal, as mentioned above with reference to Figs. 1A and 1B, the FM audio signal and the PCM audio signal have the carriers different according to the PAL B/G and PAL-I systems. This results in the following problem.

Namely, in the case where a TV capable of receiving multiplex broadcasting signals is to receive the multiplex broadcasting signals of the two types of multiplex broadcasting systems and to obtain desired audio components from the received broadcasting signals, the digital audio output cannot be obtained due to an interference between the broadcasting signals and a discordance between time constants resulting from the different systems. For this reason, the received broadcasting signals must be processed separately on the basis of the types of the multiplex broadcasting systems, in order to restore the FM audio signals and the PCM audio signals from the received broadcasting signals, respectively.

That is, in the conventional circuit for tuning the audio signals of the multiplex broadcasting systems in the multiplex broadcasting signal receiving system such as the TV, audio intermediate frequency signals detected after process of video signals of the received multiplex broadcasting signals are filtered such that the 5.5MHz carrier in the PAL-B/G system and the 6.0MHz carrier in the PAL-I system are detected therefrom, respectively, on the basis of the types of the multiplex broadcasting systems. The detected carriers are filtered such that a single carrier resulting from their mixing with a local oscillating frequency is detected therefrom. In result, the FM audio signals are detected from the single carrier, for demodulation process thereof.

For the restoration of the PCM audio signal as the digital audio signal, there are required two sets of digital audio signal processing circuits which filter the audio intermediate frequency signals to detect therefrom carriers different on the basis of the types of the multiplex broadcasting systems and detect the PCM audio signals from the detected carriers in response to PCM clock signals different on the basis of the types of the multiplex broadcasting systems. A path of the audio intermediate frequency signal detected after the video signal process to one of the two digital audio signal processing circuits is selected by a control signal which is generated according to a user's selection on the basis of the types of the multiplex broadcasting systems.A demodulation process of the PCM audio signal is performed on the basis of the selected system type by the digital audio signal processing circuit on the selected path.

However, the above-mentioned conventional circuit has a disadvantage, in that the user's selection is required on the basis of the- types of the multiplex broadcasting systems, resulting in inconvenience in use. Furthermore, the requirement of the two hardwares results in complexity in the construction of the circuit.

SUMMARY OF THE INVENTION Therefore, the present invention has been made in view of the above problem, and it is an object of the present invention to provide a circuit for automatically tuning audio signals of multiplex broadcasting systems which is capable of automatically demodulating a digital audio signal of a multiplex broadcasting signal on the basis of the type of the multiplex broadcasting system to which the digital audio signal belongs, by discriminating the type of the multiplex broadcasting system through detection of a FM carrier of the multiplex broadcasting signal.

In accordance with the present invention, the above object can be accomplished by a provision of a circuit for automatically tuning audio signals of multiplex broadcasting systems, comprising: FM audio detecting means for detecting a FM audio signal of a received broadcasting signal of the multiplex broadcasting system; broadcasting system type discriminating means for discriminating the type of the multiplex broadcasting system of the broadcasting signal on the basis of the detected FM audio signal from said FM audio detecting means and generating a control signal in accordance with the discriminated result; PCM clock generating means for generating PCM clock signals of different oscillating frequencies in response to the control signal from said broadcasting system type discriminating means; digital audio demodulating means for detecting a digital audio signal of the broadcasting signal in response to the control signal from said broadcasting system type discriminating means and performing PCM detection from the detected digital audio signal in response to the PCM clock signal from said PCM clock generating means to demodulate a digital audio signal; and audio output means for converting the demodulated digital audio signal from said digital audio demodulating means into an analog signal and outputting the analog signal as analog audio signals of left and right channels.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Figs. 1A and 1B are views illustrating base band spectra of a NICAM multiplex broadcasting signal according to PAL-B/G and PAL-I systems, respectively; and Fig. 2 is a circuit diagram of a circuit for automatically tuning audio signals of multiplex broadcasting systems in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 2, there is shown a circuit diagram of a circuit for automatically tuning audio signals of multiplex broadcasting systems in accordance with the present invention.

As shown in this figure, the circuit of the present invention comprises a tuner 20 for tuning a broadcasting signal received through an antenna 10, an intermediate frequency detector 30 for detecting a video intermediate frequency signal and an audio intermediate frequency signal from the broadcasting signal tuned by the tuner 20, an audio trap circuit 40 for removing the audio intermediate frequency signal from an output signal from the intermediate frequency detector 30 to pass only the video intermediate frequency signal, a FM audio detecting circuit 50 for filtering the audio intermediate frequency signal from the intermediate frequency detector 30 to pass on two paths FM audio carriers which are different on the basis of PAL-B/G and PAL-I systems, unifying the passed carriers as single carrier and detecting a FM audio signal from the single carrier, a broadcasting system type discriminating circuit 60 for detecting the presence of the carrier on a particular one of the two paths in the FM audio detecting circuit 50 and outputting a control signal in accordance with the detected result, a PCM clock generating circuit 70 for generating PCM clock signals of different oscillating frequencies in response to the control signal from the broadcasting system type discriminating circuit 60, a digital audio demodulating circuit 80 for removing the FM audio carrier from the audio intermediate frequency signal from the intermediate frequency detector 30, filtering the FM audio carrier removed signal to pass a digital audio carrier on the basis of the discriminated type of the broadcasting system in response to the control signal from the broadcasting system type discriminating circuit 60 and performing PCM detection from the digital audio carrier in response to the PCM clock signal from the PCM clock generating circuit 70 to demodulate a digital audio signal, and an audio output circuit 90 for converting the demodulated digital audio signal from the digital audio demodulating circuit 80 into an analog signal and outputting the analog signal as analog audio signals of left and right channels.

The FM audio detecting circuit 50 includes first and second band pass filters 51 and 52 for detecting the FM carriers of 5.5MHz and 6.0MHz from the output signal from the intermediate frequency detector 30, respectively, on the two paths, a mixer 53 for mixing an output signal from the first band pass filter 51 or from the second band pass filter 52 with an output signal from a local oscillator 54 to unify the output signals from the first and second band pass filters 51 and 52 as the single carrier of common frequency band and to output the FM audio signal of the single carrier, a third band pass filter 55 for filtering an output signal from the mixer 53 to pass the single carrier, and a FM audio detector 56 for detecting the FM audio signal from an output signal from the third band pass filter 55.

The broadcasting system type discriminating circuit 60 is constructed to discriminate the type of the broadcasting system in response to the output signal from the first band pass filter 51 or from the second band pass filter 52 in the FM audio detecting circuit 50 and output the control signal in accordance with the discriminated result to the PCM clock generating circuit 70 and the digital audio demodulating circuit 80.

Namely, in the broadcasting system type discriminating circuit 60, the carrier on the particular one of the two paths in the FM audio detecting circuit 50 is connected to a base of a transistor TR1 through a connection point of resistors R1 and R2 which are connected in series to each other between a voltage source terminal and a ground terminal. A collector of the transistor TR1 is connected in common to one side of a condenser C1, the other side of which is connected to the voltage source terminal, a coil L1 and a cathode of a diode D1, an anode of which is connected to the ground terminal.

The common connection point is connected through a condenser C2 to one side of a resistor R4, the other side of which is connected to the ground terminal, and a base of a transistor TR2, a collector of which is connected to the voltage source terminal and an emitter of which is connected to a ground terminal through a resistor R5. In result, the control signal is outputted from the emitter of the transistor TR2.

The PCM clock generating circuit 70 includes an oscillator 71 for oscillating a desired frequency, a frequency controller 72 for controlling the oscillating frequency from the oscillator 71 in response to the control signal from the broadcasting system type discriminating circuit 60, and a PCM clock generator 73 for dividing the oscillating frequency from the oscillator 71 by a predetermined number to generate the PCM clock signal. The frequency controller 72 is provided with a transistor TR3 having its base for inputting the control signal from the broadcasting system type discriminating circuit 60, its collector connected to the voltage source terminal through a resistor R6 and its emitter connected to the ground terminal. In the frequency controller 72, a frequency control signal is outputted through a variable condenser C3 from the collector of the transistor TR3.

The digital audio demodulating circuit 80 includes PAL B/G and PAL-I FM trap circuits 81 and 82 for in sequence removing the corresponding FM audio carrier signals from the output signal from the intermediate frequency detector 30, a switch 83 for selecting a path of an output signal from the FM trap circuit 82 in response to the control signal from the broadcasting system type discriminating circuit 60, PAL-B/G and PAL-I band pass filters 84 and 85 for filtering the output signal from the FM trap circuit 82 on the path selected by the switch 83 to pass the corresponding digital audio carriers of 5.85MHz and 6.552MHz, a quadrature phase shift keying (QPSK) detector 86 for performing QPSK detection from an output signal from the band pass filter 84 or from the band pass filter 85 to demodulate a quadrature modulated phase of the digital audio signal, a PCM detector 87 for performing the PCM detection from an output signal from the QPSK detector 86 in response to the PCM clock signal from the PCM clock generating circuit 70 to demodulate the digital audio signal, and a SRAM 88 for storing PCM detection data from the PCM detector 87 in the PCM detection operation of the PCM detector 87.

The audio output circuit 90 includes a D/A converter 91 for converting the demodulated digital audio signal from the digital audio demodulating circuit 80 into the analog signal, and first and second low pass filters 92 and 93 for filtering an output signal from the D/A converter 91 to output the analog audio signals of left and right channels, respectively.

The operation of the circuit with the above-mentioned construction in accordance with the present invention will hereinafter be described in detail.

Upon receiving the broadcasting signal of the PAL-B/G or PAL-I system through the antenna 10 and then through the tuner 20, the intermediate frequency detector 30 detects the video intermediate frequency signal and the audio intermediate frequency signal from the received broadcasting signal. The output signal from the intermediate frequency detector 30 is applied to the audio trap circuit 40, which removes the audio intermediate frequency signal from the output signal from the intermediate frequency detector 30 and passes only the video intermediate frequency signal.

On the other hand, the audio intermediate frequency signal from the intermediate frequency detector 30 is filtered by the first band pass filter 51 which passes the 5.5MHz carrier in the PAL-B/G system or by the second band pass filter 52 which passes the 6.0MHz carrier in the PAL-I system.

The output signal from the first or second band pass filter 51 or 52 is applied to the mixer 53. In the mixer 53, the PAL B/G carrier of 5.5MHz from the first band pass filter 51 or the PAL-I carrier of 6.0MHz from the second band pass filter 52 is mixed in an arithmetic progression manner, such as addition, subtraction, multiplication and etc., with the oscillating frequency signal (5.5MHz + 6.0MHz or 6+0MHz 5.5MHz) from the local oscillator 54, so that the PAL-B/G carrier of 5.5MHz and the PAL-I carrier of 6.0MHz are unified as the single carrier.Namely, the mixer 53 mixes the 5.5MHz carrier and the 6.0MHz carrier with the oscillating frequency signal from the local oscillator 54 in the respective cases and, thus, outputs the FM audio signal containing the resulting single carrier of common frequency band regardless of the type of the broadcasting system. Since the single carrier of common frequency band is present in the output signal from the mixer 53 regardless of the type of the broadcasting system, a passing band of the band pass filter 55 can be determined optionally for the filtering of the output signal from the mixer 53. For example, the passing band of the band pass filter 55 may be 5.5MHz or 6.0MHz.

For instance, in the case of the PAL-B/G broadcasting system, the mixer 53 mixes the 5.6MHz carrier from the first band pass filter 51 with the oscillating frequency of 500KHz from the local oscillator 54, thereby to output the single carrier of 6.0MHZ. Also, in the case of the PAL-I broadcasting system, the mixer passes directly the 6.OMHz carrier from the second band pass filter 52, thereby to output the single carrier of 6.0MHz. That is, in the mixer 53, the 5.5MHz carrier and the 6.0MHz carrier are outputted respectively as the single carrier of 6.0MHz in the respective cases, due to the mixing with the oscillating frequency of 5QOKHz. Then, the band pass filter 55 filters the output signal from the mixer 53 to pass the single carrier of 6.0MHz irrespective of the type of the broadcasting system. In result, the FM audio detector 56 detects the FM audio signal from the output signal or single carrier from the band pass filter 55.

On the other hand, the broadcasting system type discriminating circuit 60 generates the control signal according to the detection of the output signal from the PAL B/G band pass filter 51 in the FM audio detecting circuit 50.

Namely, for example, in the case of the reception of the broadcasting signal of the PAL-B/G broadcasting system, the 5.5MHz carrier is outputted from the band pass filter 51 in the FM audio detecting circuit 50, thereby causing the broadcasting system type discriminating circuit 60 to output the control signal of high level. In other words, in the broadcasting system type discriminating circuit 60, the 5.5MHz carrier from the band pass filter 51 in the FM audio detecting circuit 50 is applied to the base of the transistor TR1, thereby causing the transistor TR1 to be turned on. The turning-on of the transistor TR1 results in a resonance operation of the condenser C1 and the coil L1. As a result, the collector voltage of the transistor TR1 becomes the maximum.The maximum collector voltage of the transistor TR1 is applied to the base of the transistor TR2 through the condenser C2, thereby causing the transistor TR2 to be turned on. In result, the control signal of high level is outputted from the emitter of the transistor TR2.

On the contrary, in the case of the reception of the broadcasting signal of the PAL-I broadcasting system, no signal is outputted from the band pass filter 51 in the FM audio detecting circuit 50, thereby causing the broadcasting system type discriminating circuit 60 to output the control signal of low level.

The control signal from the broadcasting system type discriminating circuit 60 is applied to the base of the transistor TR3 of the frequency controller 72 in the PCM clock generating circuit 70. In response to the control signal from the broadcasting system type discriminating circuit 60, the transistor TR3 in the frequency controller 72 controls the oscillating frequency from the oscillator 71 such that the PCM clock generator 73 can generate the PCM clock signal corresponding to the digital audio carrier (5.85MHz) of the PAL-B/G broadcasting system or the digital audio carrier (6.552MHz) of the PAL-I broadcasting system. The control signal from the broadcasting system type discriminating circuit 60 is also applied as a select signal to the switch 83 in the digital audio demodulating circuit 80.In response to the control signal from the broadcasting system type discriminating circuit 60, the switch 83 selects the path of the FM audio carrier removed signal such that the digital audio carrier (5.85MHz) of the PAL-B/G broadcasting system or the digital audio carrier (6.552MHz) of the PAL-I broadcasting system can be detected from that signal.

In the PCM clock generating circuit 70, the transistor TR3 in the frequency controller 72 is turned on by the control signal from the broadcasting system type discriminating circuit 60, thereby causing the ground voltage to be applied to one side of the condenser C3 at the collector of the transistor TRS. As a result, the capacitance of the condenser C3 is additionally applied to the oscillator 71. In the oscillator 71, the oscillating frequencies are different according to the presence of the capacitance of the condenser C3. For this reason, the oscillating frequencies from the oscillator 71 are different according to the types of the broadcasting systems.The PCM clock generator 73 divides the oscillating frequency from the oscillator 71 by a predetermined number to generate the PCM clock signal for the PCM detection, which is applied to the PCM detector 87 in the digitaT audio demodulating circuit 80.

In the digital audio demodulating circuit 80, the audio intermediate frequency signal from the intermediate frequency detector 30 is applied through a resistor R8 to the PAL-B/G and PAL-I FM trap circuits 81 and 82, which remove in sequence the corresponding FM audio carriers from the received signal and output the digital audio intermediate frequency signal to the switch 83. The switch 83 switches the FM audio carrier removed signal from the FM trap circuit 82 to the PAL-B/G band pass filter 84 or to the PAL-I band pass filter 85 in response to the control signal from the broadcasting system type discriminating circuit 60.

For instance, in the case of the reception of the broadcasting signal of the PAL-B/G broadcasting system, the control signal of high level from the broadcasting system type discriminating circuit 60 is applied to the switch 83, thereby causing the switch 83 to switch the output signal from the FM trap circuit 82 to the band pass filter 84 which passes the digital audio carrier (5.85MHz) of the PAL-B/G broadcasting system. On the contrary, in the case of the reception of the broadcasting signal of the PAL-I broadcasting system, the control signal of low level from the broadcasting system type discriminating circuit 60 is applied to the switch 83, thereby causing the switch 83 to switch the output signal from the FM trap circuit 82 to the band pass filter 85 which passes the digital audio carrier (6.552MHz) of the PAL-I broadcasting system.

The QPSK detector 86 performs the QPSK detection from the output signal from the PAL-B/G band pass filter 84 or from the PAL-I band pass filter 85 to demodulate the digital audio signal of the phase quadrature-modulated upon the transmission thereof and the PCM detector 87 performs the PCM detection from the output signal from the QPSK detector 86 in response to the PCM clock signal from the PCM clock generating circuit 70 to demodulate the original digital audio signal.At this time, in response to the control signal from the broadcasting system type discriminating circuit 60, the PCM clock generating circuit 70 oscillates the frequency corresponding to the PAL-B/G digital audio carrier (5.85MHz) or to the PAL-I digital audio carrier (6.552MHz) and divides the oscillating frequency by a predetermined number to generate the PCM clock signal, which is applied as a reference clock signal to the PCM detector 87. As a result, synchronously with the applied PCM clock signal based on the broadcasting system type, the PCM detector 87 restores the original digital audio signal, with the PCM detection data being stored in the SRAM 88.

The digital audio signal demodulated in this manner on the basis of the broadcasting system type is applied to the audio output circuit 90 for the analog process thereof. In the audio output circuit 90, the digital/analog converter 91 converts the digital audio signal into the analog signal and outputs the analog signal to the first and second low pass filters 92 and 93, which filter the received analog signal to output the analog audio signals of left and right channels to left and right output terminals L and R, respectively.

Therefore, in accordance with the present invention, the broadcasting system type discriminating circuit 60 discriminates whether the broadcasting system type of the received broadcasting signal is PAL-B/G or PAL-I and outputs the control signal in accordance with the discrlminated result. In response to the control signal from the broadcasting system type discriminating circuit 60, the PCM clock generating circuit 70 generates the PCM clock signals of different oscillating frequencies and the digital audio demodulating circuit 80 performs the filtering of the band of the corresponding broadcasting system, thereby to demodulate the digital audio signal.In other words, the broadcasting system type can automatically be discriminated without the user's selection or switching, thereby enabling the digital audio signal to be automatically demodulated on the basis of the corresponding broadcasting system.

As hereinbefore described, according to the present invention, in the case of the reception of the broadcasting signal of the PAL-B/G system or the PAL-I system in the NICAM system which is the digital sound system, the broadcasting system type discriminating circuit discriminates the system type of the received broadcasting signal and the digital audio signal is demodulated on the basis of the corresponding broadcasting system in accordance with the discriminated result. Therefore, the digital audio signal of the broadcasting signal of any type of the PAL-B/G and PAL-I broadcasting systems can automatically be tuned in one receiver.

Although the preferred embodiments of the present invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

WHAT IS CLAIMED IS:

1. A circuit for automatically tuning audio signals of multiplex broadcasting systems, comprising: means for discriminating the type of the multiplex broadcasting system of a received broadcasting signal on the basis of a FM audio signal of the received broadcasting signal and generating a control signal in accordance with the discriminated result; and means for detecting a digital audio signal of the broadcasting signal in response to the control signal from said broadcasting system type discriminating means and demodulating the digital audio signal in response to a PCM clock signal which is controlled by the control signal from said broadcasting system type discriminating means.

2. A circuit for automatically tuning audio signals of multiplex broadcasting systems, comprising: FM audio detecting means for detecting a FM audio signal of a received broadcasting signal of the multiplex broadcasting system; broadcasting system type discriminating means for discriminating the type of the multiplex broadcasting system of the broadcasting signal on the basis of the detected FM audio signal from said FM audio detecting means and generating a control signal in accordance with the discriminated result; PCM clock generating means for generating PCM clock signals of different oscillating frequencies in response to the control signal from said broadcasting system type discriminating means;; digital audio demodulating means for detecting a digital audio signal of the broadcasting signal in response to the control signal from said broadcasting system type discriminating means and performing PCM detection from the detected digital audio signal in response to the PCM clock signal from said PCM clock generating means to demodulate a digital audio signal; and audio output means for converting the demodulated digital audio signal from said digital audio demodulating means into an analog signal and outputting the analog signal as analog audio signals of left and right channels.

3. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 2, wherein said FM audio detecting means includes: first and second band pass filters for detecting the FM audio signals of the corresponding broadcasting systems, respectively; a local oscillator for oscillating a desired frequency; a mixer for mixing an output. signal from said first band pass filter or from said second band pass filter with an output signal from said local oscillator to unify the output signals from said first and second band pass filters as a single carrier of common frequency band and to output the FM audio signal of the single carrier;; a third band pass filter for filtering an output signal from said mixer to pass the FM audio signal of the single carrier5 and a FM audio detector for detecting the FM audio signal from an output signal from said third band pass filter.

4. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 3, wherein said broadcasting system type discriminating means discriminates the type of the broadcasting system of the received broadcasting signal in response to the output signal from said first band pass filter in said FM audio detecting means and outputs the control signal in accordance with the discriminated result to said PCM clock generating means and said digital audio demodulating means.

5. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 3, wherein said broadcasting system type discriminating means discriminates the type of the broadcasting system of the received broadcasting signal in response to the output signal from said second band pass filter in said FM audio detecting means and outputs the control signal in accordance with the discriminated result to said PCM clock generating means and said digital audio demodulating means.

6. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 2, wherein said PCM clock generating means includes: an oscillator for oscillating a desired frequency; a frequency controller for controlling the oscillating frequency from said oscillator in response to the control signal from said broadcasting system type discriminating means; and a PCM clock generator for dividing the oscillating frequency from said oscillator by a predetermined number to generate the PCM clock signal.

7. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 2, wherein said digital audio demodulating means includes: first and second FM trap circuits for in sequence removing the corresponding FM signals of the received broadcasting signal; first and second band pass filters for filtering an output signal from said second FM trap circuit to pass the digital audio signals of the corresponding broadcasting systems; a switch for switching the output signal from said second FM trap circuit to said first band pass filter or said second band pass filter in response.to the control signal from said broadcasting system type discriminating means;; a QPSK detector for performing QPSK detection from an output signal from said first band pass filter or from said second band pass filter to demodulate a quadrature modulated phase of the digital audio signal; a PCM detector for performing the PC,M detection from an output signal from said QPSK detector in response to the PCM clock signal from said- PCM clock generating means to demodulate the digital audio signal; and a SRAM for storing PCM detection data from said PCM detector in the PCM detection operation of said PCM detector.

8. A circuit for automatically tuning audio signals of multiplex broadcasting systems, as set forth in Claim 2, wherein said audio output means includes: a D/A converter for converting the demodulated digital audio signal from said digital audio demodulating means into the analog signal; and first and second low pass ftl.ters for filtering an output signal from said D/A converter to output the analog audio signals of left and right channels, respectively.

9. A circuit for automatically tuning audio signals of multiplex broadcasting systems substantially as hereinbefore described with reference to and as shown in the accompanying drawings.