JPH01236828A - Receiving circuit for television signal - Google Patents

Abstract

PURPOSE:To automatically return to a normal receiving condition even after the reception of a TV input signal is stopped by providing an intermediate frequency level control means to control the level of an intermediate frequency signal in accordance with the value of a direct current voltage outputted from an automatic fine tuning circuit. CONSTITUTION:An automatic fine tuning circuit(AFT)9 detects an inputted intermediate frequency(IF) signal and even when the oscillation frequency of a local oscillator 2 is dislocated from a regular local oscillation frequency due to some factor, the action is executed so as to always correct the dislocation of a local oscillation frequency. At the extent to the output voltage due to an ordinary AFT action, the balance of the bias of an AGC amplifier 8 is not demolished and an ordinary amplifying action is not damaged, according to the negative feedback due to the connection of a resistor 11. As this result, the dislocation of a video IF signal is always automatically corrected. Thus, even when the reception of the TV input signal is interrupted and the TV input signal arrives again, the return is automatically executed to a normal receiving condition.

Description

[Detailed Description of the Invention] [Industrial Application Field J] This invention is applicable to television receivers, CATV (battling line television)
The present invention also relates to a television signal receiving circuit used in VTRs and the like.

BACKGROUND ART FIG. 3 is a block diagram showing a schematic configuration of a receiving circuit in a conventional television receiver.

In the figure, symbol l indicates a tuner, and this tuner l is composed of a local oscillator 2 and a mixing circuit 3. Further, 4 indicates a surface acoustic wave filter (hereinafter referred to as a SAW filter), and this SAW filter 4 is a bandpass filter (band bass filter) that passes only signals in a certain range of frequencies and blocks both higher and lower frequencies. filter) is (fourth filter).
(see figure). 5 is a resistor, and 6 is a capacitor for cutting DC components. Further, 7 indicates a demodulation integrated circuit (hereinafter referred to as 10 demodulation circuits), and this demodulation IC circuit 7 includes an automatic gain control amplifier (hereinafter referred to as AGC amplifier) 8, an automatic fine tuning circuit (hereinafter referred to as A (referred to as I”T)9
and a demodulation circuit 10.

In the above conventional configuration, a television input signal (hereinafter referred to as a TV input signal) input to the tuner I is frequency-converted into an intermediate frequency signal (hereinafter referred to as an F signal) by a local oscillator 2 and a mixing circuit 3. Ru. Next, the signal is passed through the IP multiplied signal SAW filter 4 to remove unnecessary signals outside the band. The IP multiplied signal after passing through the SAW filter 4 is the video IP multiplied signal lr (45.75M in the United States).
Hz) and audio IF signal S, (in the US, 41.2
5MHz) is included. This IF multiplied signal, SA
After passing through the W filter 4, the signal is amplified to a certain level in the AGC amplifier 8, and then supplied to the AFT 9. At this time, under normal conditions, the video IP times signal 1. is supplied as a higher signal level than the audio IF signal sir. For example, in CATV, the video IF signal ptr is about +15 dB higher than the audio IP signal if. The AFT 9 detects the frequency of the video IF signal P1r and the audio IF signal Str, which has a higher signal level, and the detected frequency is the center frequency f of the figure-8 curve of the internal frequency discriminator. Degree of departure from (degree of deviation)
Generates a positive or negative DC voltage depending on the AFT9
applies the generated DC voltage to the local oscillator 2. The local oscillator 2 changes the local oscillation frequency according to the applied DC voltage and corrects the deviation (that is, adjusts the local oscillation frequency to the center frequency f.). Note that the above-mentioned center frequency f. is the regular video IF signal Par (in the US, 45
．． 75MHz). Fifth
The figure shows a regular video IF signal Ptt and an audio IF signal S.
The relationship between rt and the band characteristics of the SAW filter 4 is shown.

In this way, the regular IF' frequency is always supplied to the demodulation circuit 10, and outputted from the demodulation circuit 10 as a television demodulation signal (TV demodulation signal).

[Problems to be Solved by the Invention] By the way, the following drawbacks have been pointed out in the above-mentioned conventional receiving circuit.

For example, after a temporary interruption of broadcasting due to nighttime broadcast suspension, antenna construction, cable reconnection, etc., there was a problem in which when a TV signal came back to an ON TV receiver, normal reception could no longer be performed. .

In other words, when the TV input signal is interrupted, the AGC amplifier 8
has a high gain and amplifies the noise input via the antenna and the noise generated inside the tuner l and outputs it to the subsequent stage. At this time, the noise signal output from the AGC amplifier 8 passes through the SAW filter 4 having the maximum gain characteristic at point A, as shown in FIG. , the frequency fn of the maximum level noise signal is the center frequency f of AFT9.

It will occur at a slightly lower location. For this reason,
The AFT 9 receives the noise signal amplified by the AGC amplifier 8 and has its center frequency f. A slightly lower frequency f. Detect a noise signal with this frequency f. AF
Center frequency f of T9. The local oscillator 2 is operated to match the . Even if the AFT 9 starts the above operation, the noise signal having the maximum level at the frequency fn is always present in the SAW filter 4 and the AGO amplifier 8.
Since it is supplied to AFT9 via
T operation continues. Under such conditions,'r
When the v input signal is restored, at that moment, as shown in FIG. The output deviates significantly from the output. Therefore,
The AFT 9 operates to match the video IP multiplied signal lf with fo, and as a result, the frequency of the video IP multiplied signal ir decreases (see arrow Y in FIG. 6). During this operation process, the audio IF signal Stf enters the band of the SAW filter 4 earlier than the video IP multiplied signal, t, and after entering the band, the slope characteristic of the SAW filter 4 (see Figure 6) ), the level of the audio IP multiplied signal lt becomes higher earlier than that of the video IF signal Ptr. Therefore, the frequency discriminator of APT9 detects the frequency of the audio IF' signal Slf, and the frequency of the detected audio IP multiplied signal st is AF'T9.
The center frequency f. It operates in a manner consistent with the . As a result, a deviation occurs in the local oscillation frequency generated by the local oscillator 2, making it impossible for the television receiver to perform normal tuning operation.

Note that the response characteristic of the SAW filter 4 (or AGC amplifier 8) is not the one shown in FIG. 4, and for example, the point (frequency) showing the maximum gain characteristic is the center frequency f of the AFT 9. If it exists higher than
The operation of the AFT 9 due to the noise signal is in the opposite direction, and in this case, when the TV input signal returns, it malfunctions so that it is drawn into the adjacent channel. In addition, in the above case, S
A point showing the maximum gain characteristic of the AW filter 4 and the center frequency f of the AFT 9. If the values are made to match, the above problem will not occur, but in reality, some deviation occurs due to variations in circuits and components.

This invention was made in view of the above problems, and
To provide a television signal receiving circuit capable of automatically returning to a normal reception state when a TV human input signal arrives again even after reception of an input signal is interrupted.

[Means for Solving the Problems] The present invention solves the above problems by providing intermediate frequency level control means for controlling the level of an intermediate frequency signal according to the value of the DC voltage output from the automatic fine tuning circuit. It's resolved.

[Even if the AFT outputs a large DC voltage due to operational noise, the intermediate frequency level control means will suppress the level of the intermediate frequency signal, and eventually it will be suppressed to below the APT's impressive level.

Therefore, even after the reception of the TV input signal is interrupted, when the TV input signal arrives again, the normal reception state can be automatically restored.

[Embodiments] Hereinafter, embodiments of the receiving circuit of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of the receiving circuit of the present invention, and in this figure, the same components as those of the conventional example are given the same reference numerals to simplify the explanation. In the receiving circuit of this example, the output terminal of AFT9 is connected to, for example, 2
The point connected to one of the two input terminals (for example, b terminal) of the AGC amplifier 8 configured by the differential amplifier through the resistor 11 of 00Ω is the AFT of the above conventional example. This is the difference.

At this time, the other input terminal a of the AGC amplifier 8 is connected to the capacitor 6 as in the conventional example.

In such a configuration, the channel selection button t'- of the television receiver
Therefore, the selected TV input signal follows the same process as in the conventional example described above, and is converted into an IP multiplied signal from the AGC amplifier 8 and sent to A.
It is supplied to F'T9. The AFT 9 detects the input IF multiplied signal and always operates to correct the deviation of the local oscillation frequency even if the oscillation frequency of the local oscillator 2 deviates from the normal local oscillation frequency due to some factor. In addition, at the output voltage level due to normal AFT operation,
The negative feedback caused by the connection of the resistor 11 does not disrupt the bias balance of the AGC amplifier 8, and therefore the normal amplification operation is not impaired. As a result, the video ■F times signal shift is always automatically corrected.

Next, while the TV receiver is in the ON state, for example, when broadcasting is temporarily interrupted due to night broadcasting suspension, antenna construction, cable reconnection, etc., the above-mentioned noise signal is output from the AGC amplifier 8. be done. And A.P.
The output voltage of T9 is about to rise significantly. At this time, the output voltage is negatively fed back to the input terminal of the AGC amplifier 8 via the resistor II, but the capacitor 6
Since it is provided in front of the input terminal a of the C amplifier 8, it is not applied to the input terminal a of the C amplifier 8, but is applied only to the input terminal S. Thus, the output voltage applied to the input terminal acts to unbalance the bias of the AGC amplifier 8. As a result, the amplification degree of the AGC amplifier 8 decreases, lowering the level of the noise signal. And AFT9
The operation continues until the noise signal level drops below the impression level, and a stable state is reached below the impression level.

This stable state is maintained until the TV input signal arrives again, so the moment the TV input signal returns is at the center frequency f of the video IP multiplied signal AFT9, as shown in FIG. However, the state in which the level of the video IF signal is higher than that of the audio IP signal is always maintained. That is, a situation does not occur in which the signal level of the video IP multiplied signal and the audio IF signal is reversed and the signal level of the audio ■F signal becomes higher.

Thus, following the above-described process, normal image receiving operation is resumed.

According to the above configuration, even after reception of the TV input signal is interrupted due to temporary suspension of TV broadcasting, the
When a TV human signal arrives, the user of the receiver can automatically return to a normal reception state without having to perform a channel selection operation again.

In the above example, a resistor 2 of 200Ω is used, but the resistor 2 is not limited to this, and any resistance value that does not impair the normal operation of the AGC amplifier 8 may be used.

Further, the present invention provides AF' for CATV (cable television).
It can also be applied to T.

Furthermore, it can be applied to VTRs with tuners, so that even if nighttime TV broadcasts are temporarily stopped while the VTR (video tape recorder) is away, normal broadcasting will resume the next morning. Reception is possible and failures are eliminated.

[Effect of the invention] As explained above, according to the APT of this invention, TV
Even after the reception of the input signal is interrupted, when the TV input signal arrives again, the normal reception state is automatically restored without the user of the receiver inputting the channel selection signal again. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a block diagram according to an embodiment of the present invention, and FIG.
The figure is an explanatory diagram for explaining the operation of the same embodiment.
The figure is a block diagram showing a schematic configuration of a conventional television receiver, and FIGS. 4 to 6 are explanatory diagrams for explaining the operation of the conventional example. 1...Tuner, 2...Local oscillator,
3...Mixing circuit, 4...SAW filter, 8...Automatic gain control amplifier (AGC amplifier), 9...Automatic fine tuning circuit (AFT) ), 11
······Resistor.

Claims (1)

[Claims] Receiving a television signal in which a DC voltage output from an automatic fine tuning circuit is applied to a local oscillator of the frequency conversion circuit in accordance with a frequency deviation of an intermediate frequency signal from the frequency conversion circuit. A television signal receiving circuit comprising: intermediate frequency level control means for controlling the level of the intermediate frequency signal according to the value of the DC voltage output from the automatic fine tuning circuit.