JPS60832B2 - Control signal discriminator in audio multiplex broadcast receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control signal discriminator in an audio multiplex broadcast receiver.

Audio multiplexing systems are being put into practical use in general television broadcasting.

Figure 1 shows the frequency spectrum in this audio multiplexing system, where the main channel has a maximum frequency deviation of ±
The modulation frequency band ranges from 50 to 1500 plates,
The subchannel has a subcarrier center frequency of 31. The maximum frequency shift of the subcarrier wave is 20 kHz, and the maximum frequency shift of the main carrier wave due to the carrier wave is 20 kHz during stereo broadcasting.
In addition, a control signal of 55.12 Hz is provided to distinguish whether the sub channel is in stereo or dual audio mode, and 982 Hz during stereo broadcasting. When the signal is a double audio signal, the amplitude is modulated at 922.8 ce. 0 In the current system, in the stereo state, the sum signal of the left signal and right signal is placed on the main channel, and the difference signal between the left signal and right signal is placed on the sub channel.
In a dual audio state, for example, in dubbing broadcasts, the main channel is in Japanese and the secondary channel is in a foreign language.

FIG. 2 is a block diagram showing a conventional device for discriminating control signals, in which a composite signal a input from a composite signal input terminal 1 is passed through a bandpass filter 2 to extract a control signal b with a sphere ratio of 55.12. After being amplified by the tenth amplifier 3, it is detected by the amplitude detector 4.

This detected low frequency output c is 922. The same detected low frequency output c is connected to the multiplex switch 8 through the lead filter 5, the second amplifier 6 and the first rectifier circuit 7 of the group z, while the same detected low frequency output c is connected to the 982.8 flat lead filter 9, the third amplifier 1
The stereo switch 12 is connected through the zero and second rectifier circuits 11 to control the changeover switch 13. Multiplex switch 8 and stereo switch 12
If there is no input to either, the monaural switch 14 operates. The outputs of these changeover switches 13 are supplied to a signal matrix 15, and each signal d, e of a main channel demodulated signal d input terminal 16 and a sub-channel demodulated signal e input terminal 17 is
A left output signal f and a right output signal g corresponding to each case are obtained from the output terminals 18 and 19, respectively. However, this conventional device has drawbacks such as having a large number of components such as a lead filter, and that it is difficult to perform a squelch operation because the presence or absence of a modulated output of a control signal is determined based on a low signal level.

This invention has been made in view of the above points, and it detects the control signal itself, and when the detection level does not reach a predetermined level, automatically squelches) to operate as a monaural state. It is an object of the present invention to provide a device that is not affected by slight deviations in the tuning frequency of a filter circuit by utilizing the transmission phase characteristics of a filter circuit to detect the modulation frequency of a control signal.

FIG. 3 is a block diagram showing an embodiment of the present invention.

In the figure, 20 is a rectifier circuit that directly rectifies the control signal b amplified by the amplifier 3, 21 is a rectifier circuit that discriminates the output voltage level of the rectifier circuit 20, and when the level is higher than a predetermined value, the logic value is "1", and the predetermined value is The first logical value “0” in the following cases.
The first comparison switching circuit outputs a switching signal m. On the other hand, the detection output c of the amplitude detector 4 is 922. beat z
and 982. The filter circuit 22 is supplied to a filter circuit 22 that causes a change in the transmission phase characteristic between the porridge z and the porridge z. Filter circuit 22
Although the lead filler is shown as an example, the RC filler,
An LC filter or the like may also be used. The transfer phase characteristics of the lead filter (952. mother z) are shown in Figure 4.
-The transfer phase characteristics of the T-type RC filter are shown in FIG. In either case, 9 pine. The transmission phase is reversed at z and 982.8 ce. Therefore, in the embodiment shown in FIG. 3, the signals before and after the filter circuit 22 are shaped into waveforms through limiters 23 and 24, respectively, passed through a phase comparator 25, and then rectified by a rectifier 26, thereby providing a signal corresponding to the modulation frequency of the control signal. Rectified voltage can be obtained. By determining this rectified voltage through the comparator 27, the modulation frequency is determined to be 9
22. Porridge zka982. A second switching signal n is obtained which identifies the group z. In this example, 922. When group z, logical value is "1", Satoshi 2. It is assumed that the logical value is ``0'' when the mother z is present.The changeover switch circuit 13 has two
inverter circuits 28, 29 and two NOR circuits 30
, 31, and the first switching signal m is the inverter 28
A monaural operating signal h is output through the NOR circuit 30.
, 31.

The second switching signal n provides an input signal to the NOR circuit 31, is inverted by the inverter 29, and supplies a signal to the input of the NOR circuit 30. It will be easily understood that the operation of this changeover switch circuit 13 is as shown in the following table. That is, in addition to performing the desired control signal discrimination operation,
When the signal level of the control signal is low and does not reach a predetermined level, monaural operation is forced.

In the above example, 922. Although the modulated wave of z is detected, it is obvious that the modulated wave of 982.8 ce may also be detected. As detailed above, in this invention, first, the control signal is directly detected, and if the detection output does not reach a predetermined level, monaural operation is performed unconditionally and the squelch function is activated. Since the self-control signal detection output is at a predetermined level or higher, there is no misidentification of stereo or multiple audio operation, and reliable operation is possible. In addition, to detect the modulation frequency of the control signal, we used a filter that causes a change in the transfer phase characteristics at a frequency approximately midway between the two modulation frequencies, so even if there is a slight change in the filter characteristics, the frequency will change considerably. The two modulation frequencies that are far apart can be reliably identified, and the device can be constructed at low cost.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a frequency spectrum in an audio multiplexing system for television broadcasting, Fig. 2 is a block diagram showing a conventional diamond arrangement for discriminating control signals in this audio multiplexing system, and Fig. 3 is a diagram showing one example of the present invention. A block configuration diagram showing the embodiment, Fig. 4 is a transmission phase characteristic diagram of the lead filter used in this embodiment, and Fig. 5 is a Twm-T type R used in the same example.
It is a transfer characteristic diagram of a C filter. In the figure, 2 is a bandpass filter that passes the control signal;
2 rectifiers, 21 a comparator, 4 an amplitude detector, 22 a filter, 23 and 24 a limiter, 25 a phase comparator,
13 is a changeover switch, and 15 is a signal matrix. Note that the same reference numerals in the figures indicate the same or corresponding parts. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. A detection circuit that detects the signal level of an audio multiplex control signal of an audio multiplex television receiver, a comparator that detects whether the signal level is above a predetermined value, and an amplitude detection circuit for the audio multiplex control signal. a wave detector, a filter whose transmission phase characteristic changes at a frequency intermediate between two predetermined modulation frequencies used for the audio multiplexing control signal, and a filter that causes the output of the wave detector to pass through the filter and compares the phases before and after passing through the filter. A phase comparator is provided to detect whether the detector output is one of the predetermined modulation frequencies, and when the detection signal level by the detection circuit is equal to or higher than the predetermined value, it corresponds to the modulation frequency detected by the phase comparator. 1. A control signal discriminator in an audio multiplex broadcast receiver, characterized in that the control signal is controlled in such a manner that monaural operation is performed unconditionally when the detected signal level is below a predetermined value.