JPS6243625B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a two-channel audio broadcast discrimination circuit suitable for application to a television audio multiplex broadcast or stereo radio broadcast receiver. First, with reference to FIGS. 1 and 2, we will discuss the broadcast format of a West German TV audio multiplex broadcast as an example of a TV audio multiplex broadcast that is received by a suitable TV audio multiplex broadcast receiver to which the present invention is applied. explain. First, the contents of the modulation signal that modulates the main carrier signal (AM modulation or FM modulation) will be explained.
This modulated signal consists of a video signal, a first FM modulated audio signal (with a frequency deviation of ±30 kHz) located above the band, and a second FM modulated audio signal _S1 located above the band.
FM modulated audio signal (frequency deviation is ±30kHz)
S ₂ and FM/AM modulated pilot signal (occurs only in stereo and bilingual mode) Sp
It consists of a composite signal of The frequency _f 1 of the first subcarrier signal Sc ₁ of the first FM modulated audio signal S ₁ with respect to the video carrier signal Spc is
5.5MHz (see Figure 1). Further, let the first modulation signal be _Sm1 . _The frequency f 2 of the second subcarrier signal Sc ₂ of the second FM modulated audio signal S ₂ with respect to the video carrier signal Spc
is 5.742MHz (see Figure 1). Further, let the second modulation signal be Sm ₂ (see FIG. 2). The FM/AM modulated pilot signal Sp is the frequency
fp (=54.6875kHz) carrier wave signal Scp (see Figure 2) is one of two types of pilot modulation signals.
This AM modulated pilot signal
Sp' is a signal obtained by FM modulating the second subcarrier signal _Sc2 described above. Further, the two types of pilot modulation signals are Smp ₁ (single frequency signal with a frequency of 117.5 Hz) and Smp ₂ (single frequency signal with a frequency of 274.1 Hz). Next, each modulation signal Sm ₁ in each mode,
Contents of Sm ₂ and pilot modulation signals Smp ₁ and Smp ₂
The table shows the different types. Note that L and R indicate two-channel stereo audio signals, that is, left and right audio signals.
M ₁ and M ₂ are the first and second languages (German and
Indicates a monaural audio signal (English, etc.).

[Table] In a conventional receiver that receives such television audio multiplex broadcasting, the first and second modulated signals Sm ₁ and Sm ₂ are supplied to the multiplex demodulation circuit, and the presence or absence of the pilot modulated signals Smp ₁ and Smp ₂ is determined. A discriminator determines the difference, and a multiplex demodulation circuit is controlled based on the discriminatory output. By the way, in such conventional receivers, even though monaural broadcasting is being received, the discriminator may malfunction and the monaural signal is demodulated into stereo in the multiplex demodulation circuit, making the audio difficult to hear. Ta. This is due to the following reason. In the case of stereo broadcast reception, the left audio signal L is obtained by performing, for example, the following calculation in the multiplex demodulation circuit, and the right audio signal R is left unchanged, and these left and right audio signals L and R are respectively and is switched and supplied to the right speaker. (L+R/2)×2-R=L However, if the multiplex demodulation circuit operates in the same way as when receiving a stereo broadcast when receiving a monaural broadcast, the following will occur. In this case, the first modulation signal Sm ₁ is Sm ₁ =M ₁ , but the second modulation signal Sm ₂
becomes Sm ₂ =N (however, N indicates a noise component). Therefore, for the left and right speakers,
Since the audio signals 2M ₁ -N and N are supplied, respectively, noisy audio will be emitted from the left and right speakers. In view of this, the present invention proposes a discrimination circuit that can reliably discriminate whether or not a received audio broadcast is a two-channel audio broadcast. An embodiment in which the present invention is applied to an audio multiplex demodulation circuit in a West German television audio multiplex broadcast receiver mentioned above will be described below with reference to FIG. The received high frequency signal is subjected to heterodyne detection, and the obtained intermediate frequency signal is subjected to AM demodulation or FM demodulation. The demodulated output (combined signal) is passed through the input terminal 1 to the left and right bandpass filters 2L and 2.
R, and the first and second FM modulated audio signals S ₁ and S ₂ and the FM and AM modulated pilot signals Sp are separated and extracted. The first FM modulated audio signal S ₁ is supplied to the left FM demodulator 3L, where the first modulated signal Sm ₁ is demodulated and supplied to the multiple demodulator 6 (including a stereo demodulator and a switching circuit). still,
6L and 6R are auxiliary output terminals. Second FM
The modulated audio signal S ₂ and the FM/AM modulated pilot signal Sp are supplied to the right FM demodulator 3R, where the second modulated signal Sm ₂ and the AM modulated pilot signal Sp' are demodulated, and then passed through the low-pass filter 4 and the band. The signal is supplied to the pass filter 5. The second modulated signal Sm ₂ is extracted from the low-pass filter 4 and sent to the multiplex demodulator 6.
is supplied to One-channel or two-channel audio signals from the multiplex demodulator 6 are supplied to left and right output amplifiers 7L and 7R, and their respective outputs are supplied to left and right speakers 8L and 8R, respectively. The AM modulated pilot signal Sp' is extracted from the bandpass filter 5 and is supplied to the AM detector 9, where the pilot modulated signal Smp ₁ (Smp ₂ ) is detected. The pilot discriminator 10 discriminates whether the pilot modulation signals Smp ₁ and Smp ₂ are present or not, and the multiplex demodulator 6 is controlled by the output of the discrimination. Next, the discrimination circuit 11 for two-channel audio broadcasting will be explained. Reference numeral 12 denotes a detection circuit for detecting noise components within the frequency band between the sub-channel signal (second modulated signal Sm ₂ in this example) and the pilot signal (AM modulated pilot signal Sp' in this example). Then, as shown in FIG. 2, the frequency band between the second modulated signal Sm ₂ and the AM modulated pilot signal Sp' is a bandpass filter that extracts a noise component N in a predetermined narrow band with a band center frequency of 30 kHz. (In this example, an active filter using an operational amplifier) 13 is provided, the demodulated output of the right FM demodulator 3R is supplied to this filter 13 via a coupling capacitor, and the output is supplied to the rectifier circuit 14. A detection circuit 12 is configured. The output of this rectifier circuit 14 is supplied to a discriminator 15. This discriminator 15 is configured as follows.
The emitter of the switching transistor (PNP type) 16 is connected to the power supply +B (12V), its base is connected to the output terminal of the rectifier circuit 14, and its collector is grounded through the resistor 17.
It is connected to an output terminal 20 through a resistor 18 .
Output terminal 20 is grounded through capacitor 19. In this discrimination circuit 11, when the level of the noise component N extracted by the bandpass filter 13 exceeds a predetermined level, the transistor 16 of the discriminator 15 is turned on, and the discrimination output of the output terminal 20 becomes high level. Therefore, the television audio broadcast is determined to be monaural broadcast. When the level of the noise component N becomes less than a predetermined level, the transistor 16 is turned off, the determination output of the output terminal 20 becomes low level, and the television audio broadcast is determined to be a two-channel broadcast. In the multiplex demodulation circuit 6, the discrimination output of the discriminator 15 is the discriminator 10.
is accepted in preference to the discrimination output of . Next, the operation of the audio multiplex demodulation circuit shown in FIG. 3 will be explained. First, the case of monaural broadcast reception will be explained. At this time, the second FM modulated audio signal
Both _S2 and the FM/AM modulated pilot signal Sp do not exist. Therefore, a discriminating signal for monaural broadcast reception is supplied from the discriminator 10 to the multiplex demodulator 6, and a discriminating signal for monaural broadcast reception is also supplied from the discriminator 15 to the multiplex demodulator 6. At this time, the first modulation signal Sm ₁ is output from the left FM demodulator 3L.
A monaural audio signal M ₁ is output as a signal, and this audio signal M ₁ is switched in the switching circuit of the multiplex demodulator 6 so that it is commonly supplied to the output amplifiers 7L and 7R. Next, the case of two-channel multiplex broadcast reception will be explained. At this time, the discriminator 10 supplies a discrimination signal for stereo broadcasting or bilingual broadcast reception to the multiplex demodulator 6, and the discriminator 15 supplies a discrimination signal for two-channel multiplex broadcast reception to the multiplex demodulator 6. Ru. For stereo broadcast reception, left FM demodulator 3L
The audio signal L+R/2 is output as the first modulation signal Sm ₁ from the low-pass filter 4, and the second modulation signal Sm ₂ is output from the low-pass filter 4.
A right audio signal R is obtained as follows and is supplied to the multiplex demodulator 6. In the stereo demodulator in the multiplex demodulator 6, the calculation L+R/2×2-R=L is performed to obtain the left audio signal. Then, in the switching circuit of the multiplex demodulator 6, the left and right audio signals L and R are switched to be supplied to the output amplifiers 7L and 7R, respectively. In the case of bilingual broadcast reception, the first and second modulated signals Sm ₁ and Sm ₂ become bilingual monaural audio signals M ₁ and M ₂ , respectively, which are supplied to the multiplex demodulator 6. Then, in the switching circuit of the multiplex demodulator 6, one of the audio signals M ₁ and M ₂ is selected and commonly supplied to the output amplifiers 7L and 7R, and the other is commonly supplied to the auxiliary output terminals 6L and 6R. switched to be supplied. Incidentally, the present invention can be applied to a multiplex demodulation circuit of a receiver for television audio multiplex broadcasting in Japan and a stereo demodulation circuit of a receiver for FM stereo broadcasting in Japan. According to the present invention described above, the apparatus includes a detection circuit that detects a noise component within the frequency band between the sub-channel signal and the pilot signal, and a discriminator that determines the presence or absence of the sub-channel signal based on the detection output of this detection circuit. Therefore, it is possible to reliably determine whether the received audio broadcast is a two-channel audio broadcast. When the discrimination circuit according to the present invention is applied to a stereo demodulation circuit (multiple demodulation circuit), even though a monaural audio broadcast is being received, the stereo demodulation is performed and the left and right speakers (stereo headphone is also possible) are used. This prevents noisy audio from being played.

[Brief explanation of the drawing]

1 and 2 are frequency spectrum diagrams, and FIG. 3 is a block diagram showing one embodiment of the present invention. 6 is a multiplex demodulator, 8 is a bandpass filter, 9
is an AM detector, 10 is a discriminator, 11 is a discriminator circuit,
12 is a detection circuit, 13 is a bandpass filter, 1
4 is a rectifier circuit, and 15 is a discriminator.

Claims (1)

[Claims] 1 a detection circuit that detects noise components within the frequency band between the sub-channel signal and the pilot signal;
A discriminating circuit for two-channel audio broadcasting, comprising a discriminator for discriminating the presence or absence of the sub-channel signal based on the detection output of the detection circuit.