JPH02260810A - Mute circuit - Google Patents

Abstract

PURPOSE:To prevent production of noise at channel selection by providing an attenuator having a prescribed attenuation constant, a 1st switching element energized when a mute control signal is supplied thereto, a mute control signal delay means and a 2nd switching element energized when a mute control signal is supplied thereto. CONSTITUTION:When channel selection switching is activated, a mute control signal is fed from a control circuit to a switching element 22 and a delay circuit 23. Upon the receipt of the mute control signal, the switching element 22 is immediately turned on, a level of one terminal goes to a ground level, an audio signal is attenuated by an attenuation element 21 and a multiple control signal is lost. On the other hand, when a charging voltage across a capacitor C reaches a prescribed voltage by the control signal supplied to the delay circuit 23, a switching element 24 is turned on, and a voice signal path 20 is grounded. Thus, the attenuated voice signal is not outputted to the modulation circuit. Thus, production of noise at channel selection is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a mute circuit suitable for use particularly when switching channels in cable television broadcasting.

"Prior Art" Output methods for CATV (cable television) terminals have conventionally been divided into frequency conversion output methods and baseband output methods.

FIG. 3 is a professional diagram showing the configuration of a CATV terminal using the frequency conversion output method. In this figure, a multiplex broadcast signal sl from a broadcast station is supplied to a channel selection circuit 1. In this channel selection circuit 1, a predetermined channel is selected by a control circuit 2, and a broadcast signal S2 of the same channel is output.

The broadcast signal S2 of the selected channel is supplied to the frequency conversion circuit 3. In the frequency conversion circuit 3, the broadcast signal S2 is converted to the frequency of an empty television broadcast channel (channel 1 or channel 2) and output to the RF output terminal. A CATV terminal based on this method is used, for example, in the configuration shown in FIG. In Figure 7,
The multiplex broadcast signal sl is supplied to the CATV terminal 10, and after the broadcast signal of the channel selected by the CATV terminal IO is converted into a predetermined frequency, it is supplied to the antenna terminal of the television receiver 11.

By the way, when switching the channel selection of a CATV terminal, the channel selection state may be unstable until the channel is switched from the previous channel to a new channel. Therefore, during this period, a broadcast signal with unstable frequency and signal strength is output to the RF output terminal of the CATV & i terminal, and unpleasant sounds are emitted from the speakers of the television receiver 11 to which this broadcast signal is supplied. The problem arises.

In view of this problem, some conventional CATV terminals use a mute circuit to cut off audio signals when selecting a channel.

FIG. 4 is a block diagram showing the configuration of a C ATV terminal using the baseband output method and equipped with the above-mentioned mute circuit. In this figure, first, a multiplex broadcast signal S1 from a broadcast station is supplied to a channel selection circuit 1. Then, a specific channel is selected by the channel selection circuit 1, and the broadcast signal S2 of the selected channel is supplied to the demodulation circuit 4. This demodulation circuit 4 demodulates the supplied broadcast signal S2 into a video signal and an audio signal. The video signal is then supplied to a modulation circuit 5, and the audio signal is supplied to an audio multiplexing/demodulation circuit 6 and a mute circuit 7. In the audio multiplex demodulation circuit 6, when the audio signal is stereo broadcasting or bilingual broadcasting, the audio signal is separated into a right channel (sub audio) and a left channel (main audio), and the mute circuit 8a
, 3b.

The mute circuits 3a and 8b are composed of switching elements 15 shown in FIG.
The sub audio signal and main audio signal supplied from the sub audio signal and the main audio signal are output as audio signals (R) and (L), respectively. These audio signals (R), audio signals (L), and video signals output from the demodulation circuit 4 are received via an AV (audio-visual) terminal of a television receiver 11 shown in FIG.

This CATV terminal also has a frequency conversion output method,
Did I mention it above? The audio signal supplied from the M-tone circuit 4 to the mute circuit 7 is supplied to the modulation circuit 5.

The mute circuit 7 is constituted by a switching element 15 shown in FIG. 6, like the mute circuits 8a and 8b. In the modulation circuit 5, the supplied video signal and audio signal are mixed, re-modulated to the frequency of an empty television broadcast channel, and outputted to the RF output terminal. This re-modulated broadcast signal is received via the antenna terminal of the television receiver 11 shown in FIG.

Here, the configuration of the television receiver 11 is shown in FIG. In this figure, the broadcast signal supplied to the above-mentioned antenna terminal is supplied to a video demodulation circuit 13a and an audio demodulation circuit 1.3b of a demodulation circuit 13 via a channel selection circuit 12. In this case, the reception channel is set in the channel selection circuit 12 so that the frequency converted or remodulated by the CATV terminal 10 can be received. In the video demodulation circuit 13a described above, a video signal is extracted from the broadcast signal and output.

Further, the audio demodulation circuit 13b extracts an audio signal from the broadcast signal, and this audio signal is transferred to the audio multiplexing demodulation circuit 14.
Subcarrier demodulation circuit 1 via frequency discrimination circuit 14a of
4b and ma) are supplied to the IJ control circuit 14c. In this frequency discrimination circuit 14a, a main audio signal is extracted from the audio signal, and this main audio signal is supplied to a matrix circuit 14d. The matrix control circuit 14c extracts a multiplexed control signal, and this multiplexed control signal is supplied to the subcarrier demodulation circuit 14b and the matrix circuit 14d. Furthermore, when the subcarrier demodulation circuit 14b is supplied with the multiplex control signal, the subaudio signal is extracted from the audio signal and supplied to the matrix circuit 14d. Then, in the matrix circuit 14d, the main audio signal and the sub audio signal are calculated based on the multiplex control signal, and a predetermined audio signal (R) is generated.
, (L).

Next, CAT using the above-mentioned baseband output method
The channel selection operation at the end of V Oi will be explained. First, the control circuit 2 shown in FIG. 4 switches the reception channel of the channel selection circuit 1 from the previous channel to a new channel (see FIG. 9(a)). Then, until the tuning state becomes stable, the control circuit 2 mutes the mute circuit 7.
, 8a and 8b (see FIG. 9(b)). In each of the mute circuits 7.3a and 3b to which the mute and control signals are supplied, the audio signal is cut off as shown in FIG. 9(c). Then, at a time when the channel selection operation is finished and the audio signal becomes stable, the mute control signal from the control circuit 2 is cut off, and each mute circuit 7
．． Audio signals are output from 8a and 3b.

The audio signal from the mute circuit 7 is mixed with the video signal in the modulation circuit 5 and re-modulated into a predetermined broadcast signal, and then received via the antenna terminal of the television receiver 11.

In addition, audio signals (R) from mute circuits ga and 3b,
The audio signal (L) and the video signal output from the demodulation circuit 4 are received via the AV terminal of the television receiver 11.

[Problems to be Solved by the Invention] By the way, in the above-mentioned baseband output type CATV terminal equipped with the frequency conversion output method, there is a usage pattern in which a remodulated audio signal is supplied from the RF output terminal to the television receiver. In this case, when the audio signal supplied to the modulation circuit is interrupted, even the multiplex control signal included in the audio signal is instantaneously interrupted. When this multiplex control signal is no longer input to the television receiver 11, the matrix control circuit stops the operations of the subcarrier demodulation circuit and the matrix circuit. In this case, there is a very short delay from when the multiplex control signal is cut off until the subcarrier demodulation circuit and matrix circuit stop operating. Therefore, although the subcarrier demodulation circuit 14b is still in an operating state, it outputs noise because it is no longer supplied with an input signal. This noise is a loud Hm sound peculiar to an FM demodulator, and this causes a problem in that it is output as an unpleasant sound from the speakers of the television receiver via the matrix circuit.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a mute circuit that prevents noise from occurring during a channel selection operation.

"Means for Solving the Problem" In order to solve these problems, the present invention provides an attenuator having one end connected to the audio signal path and having a predetermined attenuation constant, and one end connected to the other end of the attenuator. a first switching element that is connected to the switch, the other end of which is grounded, and becomes conductive when a mute control signal is supplied; a delay means that creates a signal that delays the mute control signal by a predetermined time; The second switching element is connected to a line, the other end is grounded, and becomes conductive when the delay signal is supplied from the delay means.

"Operation" When the mute control signal is supplied, the first switching element immediately becomes conductive. Next, the delay means outputs a delayed signal delayed by a predetermined time with respect to the mute control signal. Then, by this delayed signal, the second
The switching element becomes conductive.

"Embodiments" Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention.

The mute circuit shown in this figure is similar to the conventional CA shown in FIG.
This corresponds to the mute circuit 7 of the TV terminal 10.

Further, the other circuit configurations of this embodiment are the same as those of the CATVitJ terminal 10 shown in FIG. 4, and the explanation thereof will be omitted.

In FIG. 1, 20 is an audio signal path, the input end of which is connected to the output end of the demodulation circuit 4 shown in FIG. It is connected. Reference numeral 21 denotes an attenuation element made of a resistor or a filter, one end of which is connected to the audio signal path 20, and the other end of which is grounded via the switching element 22. The attenuation constant of this attenuation element 21 is such that the multiplex control signal included in the multiplex audio signal is transmitted to the television receiver 1 as shown in FIG. 2(b).
1, the value is set to such a value that it cannot be received. This switching element 22 includes a control circuit 2 shown in FIG.
A mute control signal is supplied from. Further, this mute control signal is supplied to the switching element 24 via a delay circuit 23 made up of a resistor ER and a capacitor C.

This switching element 24 has one end connected to the audio signal path 20 and the other end grounded.

Next, the operation of this embodiment with the above configuration will be explained.

First, when a channel selection switching operation is performed, a mute control signal is supplied from the control circuit 2 to the switching element 22 and the delay circuit 23. When the switching element 22 is supplied with the mute control signal, it immediately turns on, and the potential at one end becomes the ground level. Therefore, the audio signal shown in FIG. 2(a) supplied to the audio signal path 20 is attenuated by the attenuation element 21 as shown in FIG. 2(b), and the multiple control signal is eliminated. The attenuated audio signal is sent to the modulation circuit 5
The signal is mixed with a video signal, modulated, and output as a predetermined broadcast signal to the television receiver 11 shown in FIG. The television receiver 11 receives the broadcast signal,
The audio signal is separated into audio components by the audio demodulation circuit 13b shown in FIG. This audio signal is supplied to the matrix control circuit 14C via the frequency discrimination circuit 14a. In this case, since the audio signal does not have a multiplex control signal as shown in FIG. 2(a), the operations of the subcarrier demodulation circuit 14b and the multiplex circuit 14d are stopped by the matrix control circuit 14b. Therefore, the audio output of the television receiver 11 becomes monaural audio.

On the other hand, the control signal supplied to the delay circuit 23 is
The capacitor C is charged according to the time constant τ caused by the capacitor C. When the charging voltage of the capacitor C reaches a predetermined voltage, the switching element 24 is turned on and the audio signal path 20 is grounded. Therefore, the attenuated audio signal flows to the ground side and is no longer output to the modulation circuit 5. In this case, since the subcarrier demodulation circuit 14b of the television receiver 11 is no longer in operation, no noise is generated.

"Effects of the Invention" As explained above, according to the present invention, it is possible to prevent the generation of noise during the channel selection operation, and there is an advantage that no unpleasant sound is generated.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the configuration of an embodiment of the present invention, and Figure 2 (
a) and (b) are waveform diagrams for explaining the operation of the same embodiment, FIG. 3 is a block diagram showing the configuration of a CATV terminal using the conventional frequency conversion output method, and FIG. 4 is the conventional baseband output method. 5 is a circuit diagram showing the structure of a conventional mute circuit. FIGS. 6 and 7 are block diagrams showing the structure of one usage form of the CATV terminal. The figure is a block diagram showing the configuration of a general television receiver, and FIG. 9 is a waveform diagram for explaining the operation of a conventional mute circuit. 21... Attenuation element (attenuator), 22...
- Switching element (first switching element), 23
...Delay circuit (delay means), 24...Switching element (second switching element), R...
...Resistance, C...Capacitor. Figure 5 Figure 6

Claims (1)

[Claims] an attenuator having a predetermined attenuation constant, one end of which is connected to the audio signal path, and a first end of which is connected to the other end of the attenuator, the other end of which is grounded, and which conducts when a mute control signal is supplied. A switching element, a delay means for creating a signal obtained by delaying the mute control signal by a predetermined time, one end of which is connected to the audio signal path, the other end of which is grounded, and conduction occurs when a delayed signal is supplied from the delay means. A mute circuit comprising a second switching element.