JPS6360956B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION With the implementation of television audio multiplex broadcasting, attention has been paid to the sound quality of television audio, which had been apt to be ignored in the past.

The present invention aims to improve the sound quality of this television sound.

The intercarrier system and the split carrier system are known as circuit systems for receiving and demodulating audio signals in television signals.

Figure 1 is a system diagram showing an overview of the intercarrier system. The signal received by antenna 1 is supplied to ET tuner 2 controlled by tuning circuit 2', and from this the signal of f _p = 58.75MHz is transmitted. A video carrier wave component and an audio carrier wave component of f _s = 54.25 MHz are obtained, and these carrier wave components are supplied to a video intermediate frequency amplifier 3 and also to a filter 4 that extracts only both carrier wave components, and the output thereof is passed through an amplifier 5. The signal is then supplied to the audio detection circuit 6. In this case, the tuning circuit 2'
The tuning voltage V _C , band switching signals V _L , V _H and U
is supplied to the ET tuner 21. Here V _C is
The signal to switch to the low channel of the VHF band, _VH is
The signal to switch to the Haitian channel in the VHF band, U is
This is the signal to switch to the UHF band. This detection circuit 6
Thus, an FM audio signal is obtained as a 4.5 MHz beat signal having a frequency that is the difference between the two carrier waves, which is supplied to a frequency discrimination circuit 7 for frequency demodulation, and its output is supplied to a decoder 8 for audio multiplexed signals. Then, from this decoder 8, when the TV audio is not a multiplexed audio, normal monaural audio signals are obtained as output signals S _A and S _B , and when the TV audio is multiplexed audio, for example, it is stereo, one output signal S The left channel audio signal is _A ,
The audio signal of the right channel is obtained as the other output S _B. Note that a part of the signal from the audio detection circuit 6 is fed back to the amplifier 5.
AGC is applied.

On the other hand, Figure 2 is a system diagram showing an overview of the split carrier system, which is controlled by the tuning circuit 2'.
The output signal of the ET tuner 2 is supplied to a surface wave filter 9 with bandpass characteristics, and from this the signal is 54.25MHz.
Only the audio carrier component f _s of is obtained. This audio carrier wave component is supplied to a mixing circuit 11 through an amplifier 10. On the other hand, a local oscillation circuit 12 is provided,
From this, the 64.95MHz oscillation signal is supplied to the mixing circuit 11, and both signals are multiplied. Therefore, the frequency of the difference between the two is 10.7M than the mixing circuit 11.
A Hz signal is obtained, which is passed through ceramic filter 1.
3 to the frequency discriminator circuit 14 for demodulation, and its output is supplied to the audio multiplex signal decoder 8 to produce output signals S A and S _A similar to the case of FIG.
S _B is obtained.

Note that the output of the frequency discrimination circuit 14 is supplied to the local oscillation circuit 12 to control the local oscillation frequency.
AFC loop is formed.

The intercarrier method described above uses the 4.5MHz frequency component that is the difference between the video carrier wave and the audio carrier wave, so it inherently has the disadvantage that the video signal component is easily mixed into the audio channel, causing buzz. .

However, unless audio multiplex broadcasting is taken into account, this buzz is not so noticeable because the high-frequency components are reduced considerably in the de-emphasis circuit of the audio FM detector output. Since it is necessary to extract this sub-channel signal from before the de-emphasis circuit, this buzz interference has a great effect on the sound quality.

On the other hand, in the split carrier method, the audio carrier wave component and the video carrier wave component are extracted separately, so that a high quality audio signal can be obtained without being affected by the video signal component.

However, in the split carrier system, the audio carrier wave is processed independently, regardless of the video carrier wave, so when preset tuning is performed with the preset tuner or fine tuning is performed, the local oscillation If the frequency shifts and AFC goes off, an unnatural situation will occur in which there is no sound but the image is still visible. It would be fine if the TV receiver and the decoder for demodulating the multiplexed audio signal were separate units, but if they were integrated, the average receiver would experience images being displayed but no sound. It is incomprehensible, inconvenient, and complicated to handle.

On the other hand, since the intercarrier method uses the frequency difference between the video carrier wave and the audio carrier wave, there is no possibility that the image and sound will not match.

Also, it is true that the split carrier system
In the case of VHF reception, as mentioned above, there is no buzz or buzzbeat, and the performance is perfect in terms of improved sound quality, but in the case of UHF reception, improvement in sound quality cannot necessarily be expected, and in some cases It was found that the performance of the intercarrier method was sometimes inferior to that of the intercarrier method.

That is, in Japan, the local oscillation frequency in the tuner is 58.75 MHz higher than the received signal. When the level of the received signal and therefore the input signal level is low, the local oscillator oscillates stably at a predetermined local oscillation frequency, but when the input signal level increases, the frequency determining element such as the varicap of the local oscillator A so-called local pull-in phenomenon occurs in which the local oscillation frequency fluctuates under the influence of the image carrier wave.

By the way, the tuner has AGC (automatic gain control)
It is very stiff, and the S/V of video for both VHF and UHF is
In order to ensure N, AGC is activated from the point where the input signal level exceeds 65 dBμ.

In VHF, this AGC suppresses the above-mentioned pull-in phenomenon to a negligible level.

However, in UHF, since the frequency is high, the sensitivity of frequency determining elements such as the variable cap of the local oscillator is about four times higher than in the case of VHF.
Therefore, in the case of UHF, the above-mentioned pull-in phenomenon occurs at an input signal level of about 50 to 60 dBμ at which AGC does not work, and the local oscillation frequency varies according to the input signal level. For television receivers,
Since the input signal is an AM modulated signal, the local oscillation frequency will vary according to the video carrier.

In the split carrier system, this variation in the local oscillation frequency is directly separated and extracted from the audio signal that has undergone the variation, so this variation appears as a sound, resulting in a buzz sound. The degree of deterioration in sound quality due to this buzz sound is rapid in the region where AGC starts to apply in UHF, and after the buzz sound is detected, by increasing the input level by about 5 dB, the sound quality becomes the same as that of the intercarrier method. It gets much worse in comparison.

To prevent this phenomenon, use a UHF tuner.
It would be better if the AGC could be heard from 50 to 60 dBμ,
As mentioned above, this would degrade the S/N of the video, making it impractical. Therefore, it is conceivable to insert a buffer amplifier between the local oscillator of the UHF tuner and the mixing circuit, but it is difficult to realize a buffer amplifier that covers the entire UHF band due to cost and design aspects.

Also, although this is a problem on the transmitter side, if there is AM-PM conversion on the transmitter side, in the case of satellite multi-stage relay, not only the video carrier wave but also the audio carrier wave will undergo phase modulation, so in the intercarrier system, the phase modulation are canceled out and no buzz is generated due to this.However, in the case of the split carrier method, the audio carrier component that has undergone phase modulation is separated as is, extracted, and demodulated, so the phase modulation component cannot be removed. , a buzz occurs.

As described above, the split carrier system is not perfect in terms of improving the sound quality of television audio, and has various drawbacks. In view of this, the split carrier method is usually used to demodulate the audio signal, and when the signal is detuned, the intercarrier method can be demodulated automatically or, if necessary, forcibly, as shown in Figure 3. A receiver has been proposed.

That is, in the receiver shown in FIG. 3, the output signal of the tuner 2 controlled by the tuning circuit 2' is passed through the surface wave filter 1 having video intermediate frequency selection characteristics.
5 to an amplifier 16, and its output is supplied to a video detection circuit 18 through an audio trap circuit 17, from which a video detection output is taken out. The signal from the amplifier 16 is then supplied to an AFT circuit 19 to detect the level of the video carrier wave, and the detected output controls the local oscillation frequency of the tuner 2 to apply AFT (auto fine tuning).

20 constitutes a split carrier type audio demodulation circuit, in which the output of the tuner 2 is supplied to a surface wave filter 21 to extract a 54.25MHz audio carrier wave component, which is supplied to a mixing circuit 23 through an amplifier 22. be done. On the other hand, a local oscillation signal of 64.95 MHz is supplied from the local oscillation circuit 24 to the mixing circuit 23. As the output of this mixing circuit 23, a signal with the frequency of the difference and the sum of both signals is obtained, and the output is passed through a ceramic filter 25, and an audio carrier wave component of 10.7 MHz, which is the frequency of the difference, is extracted, and this is sent to the frequency discrimination circuit 26. The demodulated output is supplied to the audio multiplex signal decoder 8 through the amplifier 27. The oscillation frequency of the local oscillation circuit 24 is controlled by the output of the frequency discrimination circuit 26.
AFC is done.

Further, 30 indicates an intercarrier type audio demodulation circuit, in which the signal from the amplifier 16 is supplied to the audio detection circuit 31 to extract a 4.5MHz audio carrier wave component, which is supplied to the frequency discrimination circuit 32 and demodulated. The output thereof is supplied to the audio multiplex signal decoder 8 via the amplifier 33.

And split carrier type demodulation circuit 2
The output of the 0 frequency discrimination circuit 26 is supplied to the comparator circuit 41, and when the local oscillation frequency of the tuner 2 changes by, for example, 250 kHz or more due to fine tuning or the like, the comparator circuit 41 outputs a detection signal that rises to, for example, "1". is obtained, which is supplied to the amplifiers 27 and 33 through the OR gate 42. When the detection signal cannot be obtained from the comparison circuit 41, the output signal of the amplifier 33 is muted so that it is not supplied to the decoder 8, and the split carrier type demodulation circuit 20 is activated, so that the detection signal is not supplied to the decoder 8. When the signal is obtained, the output signal of the amplifier 27 is muted so that it is not supplied to the decoder 8, and the intercarrier demodulation circuit 30 is activated. In this way, the amplifiers 27 and 33 constitute a selection means.

Therefore, under normal good reception conditions, no detection signal is obtained from the comparator circuit 41, so the output demodulated by the split carrier demodulation circuit 20 is supplied to the decoder 8 and is buzz-free. Good TV sound can be obtained.

When the tuner 2 performs fine tuning or the like, if the local oscillation frequency deviates by more than a predetermined value, a detection signal is obtained from the comparison circuit 41, and the intercarrier demodulation circuit 30 demodulates the local oscillation frequency. The output is supplied to the decoder 8. Therefore, there is no longer a situation where no sound is output even though an image appears on the screen.

For example, the receiver is provided with a terminal 44 to which a switching signal for switching to intercarrier reproduction is supplied when receiving UHF, and an output demodulated by the intercarrier demodulation circuit 30 is supplied to the decoder 8 when receiving UHF. In addition, if necessary, a forced switch to the intercarrier method is provided, and if there is noticeable buzz in the output audio,
When this switch is turned on, a forced intercarrier signal similar to the detection signal of the comparison circuit 41 is supplied through the terminal 43, and this is supplied to the amplifiers 27 and 33 through the OR gate 42, so that the output signal of the amplifier 27 is muted. , the demodulation circuit 30 of the intercarrier system is forcibly switched. Therefore, the drawbacks of only the split carrier method are improved. In FIG. 3, the deviation in the oscillation frequency of the local oscillation circuit 24 is calculated by
1, but as shown by the broken line in Figure 3.
The output of the AFT circuit 19 may be divided by resistors 46 and 47 and detected by the comparison circuit 48.

However, in the circuit shown in FIG. 3, the frequency discrimination circuit 26 and the comparison circuit 41 (or the AFT circuit 19 and the comparison circuit 48)
It was found that because the response speed of the motor was not fast enough, a fairly loud shock noise was generated transiently. Furthermore, it has been found that when presetting a channel and adjusting the preset volume by operating the channel selection preset operator, a fairly loud shock sound is similarly generated in a transient manner.

In view of this point, the present invention proposes a receiver capable of preventing such a shock sound from occurring during preset channel selection.

An embodiment of a television audio signal receiver according to the present invention will be described below with reference to FIGS. 4 and 5. In FIG. 4, parts corresponding to those in FIG. 3 are designated by the same reference numerals, and some redundant explanations will be omitted.

As shown in FIG. 5, the channel selection preset operator (connected to the preset volume, but not shown) is accommodated in a housing section (accommodation recess, housing box section, etc.), for example, on the front side of the television receiver. A switch element 50 is provided which operates in conjunction with the opening and closing of a lid 53 (52) provided on a panel, side panel, etc. The switch element 50 has an actuator 50a, and when the lid body 53a is closed, a protrusion 53a provided on the back side presses the actuator 50a to turn off, and when it is opened, the pressure on the actuator 50a is released. It is configured to be on. In addition, 54 is a plate body to which the switch element 50 is attached, and 55 is a plate body 5.
4 is attached to the wall body 55, and the lid body 5 is attached to this wall body 55.
3 is attached via a hinge 56 so as to be openable and closable.

As shown in FIG. 4, this switch element 50 is connected between the positive side of a power source 51 whose negative side is grounded and the input end of the OR circuit 42.

Thus, by opening the lid 53 of the housing section 50,
When adjusting the preset volume by operating the tuning preset operator, the switch element 50 is turned on, and the signal "1" is supplied to the amplifiers 27 and 33 through the OR gate 42, and the output signal of the amplifier 27 is muted. Then, the demodulation circuit 30 is forcibly switched to the intercarrier type demodulation circuit 30. Therefore, it is possible to eliminate the shock noise that occurs when the response speed of the frequency discrimination circuit 26 and the comparison circuit 41 is not fast enough in a transient state when adjusting the preset volume. Since it is possible to check the channel selection operation while listening to the sound.

In this example, a mute pulse is further generated in the channel selection circuit 2' when switching channels, and the mute pulse generated from the channel selection circuit 2' is transmitted to a monostable multivibrator 49 having a time constant of 1 second, for example. Supplied to the trigger terminal. In this case, the time constant of the monostable multivibrator 49 is set to be a sufficient time corresponding to the response speed of the frequency discrimination circuit 26 and the comparison circuit 41. The output signal of this monostable multivibrator 49 is input to the OR gate 42.
so that it is supplied to the input side of the

Therefore, at the time of channel switching, the output side of the monostable multivibrator 49 becomes a high level signal "1" for one second, which is supplied to the amplifiers 27 and 33 through the OR gate 42, and the amplifier 27
The output signal is muted and forcibly switched to the intercarrier demodulation circuit 30.

Therefore, it is possible to eliminate the shock noise that occurs when the response speed of the frequency discrimination circuit 26 and the comparison circuit 41 is not fast enough in a transient state when selecting a channel.

The rest of FIG. 4 is constructed in the same manner as FIG. 3.

As described above, according to the present invention, the disadvantages of only the split carrier system as shown in FIG. You can get TV sound.

[Brief explanation of drawings]

Figures 1, 2 and 3 are block diagrams showing examples of conventional television audio signal receivers, respectively.
The figure is a block diagram showing one embodiment of the television audio signal receiver of the present invention, and FIG. 5 is a sectional view showing the specific configuration of a part thereof. 2 is a tuner, 2' is a tuning circuit, 8 is a decoder,
20 is a split carrier type demodulation circuit, 27
and 33 is an amplifier, 42 is an OR gate, 49 is a monostable multivibrator, 50 is a switch element,
51 is a power source, 52 is a housing section, and 53 is a lid body.

Claims (1)

[Claims] 1. It has an intercarrier type audio demodulation circuit, a split carrier type audio demodulation circuit, and a selection means for selecting the demodulated outputs of both demodulation circuits, and usually the demodulated output of the split carrier type audio demodulation circuit is selected. is selected, and when the deviation of the local oscillation frequency of the tuner is equal to or greater than a predetermined value, the demodulated output of the intercarrier audio demodulation circuit is selected. A switch element is provided that operates in conjunction with the opening and closing of the lid of the housing section in which the operator is housed, and even when the lid is opened, the demodulated output of the intercarrier audio demodulation circuit is selected by the operation of the switch element. A receiver for television audio signals, characterized in that: