JPH0736617B2 - Automatic gain control device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an automatic gain control device used in a television receiver.

(Structure of Conventional Example and its Problems) Generally, in a television receiver, as shown in FIG. 1, a channel 1 is selected by a tuner 1 and an image intermediate frequency signal passes through an image intermediate frequency amplifier 2 and is detected by a detector 3. To be done. On the other hand, the detected signal causes the AGC circuit 4 to generate a voltage for automatic gain control, which is fed back to the video intermediate frequency amplifier 2 for stable gain control.

FIG. 2 shows a part of a conventional example of such an AGC circuit. In general AGC circuit is built in the IC, the transistor T _R 1~T _R 4 in the IC, with the capacitor 5 is connected to the outside of the IC through the terminal A of the IC. Second
The circuit shown in the figure is a charging / discharging circuit that corresponds to the final stage of the AGC circuit, and is a part that generates a voltage for gain control. T _R 1
Is a transistor for charging the capacitor 5, T _R 2 and T _R 3
Of the transistors form a differential pair, and the AGC-detected signal is input to the terminal 6 connected to the base of the transistor T _R 2. T _R 3 is a discharging transistor for capacitor 5, T _R 4
Is a constant current source transistor connected to the emitters of transistors T _R 2 and T _R 3.

Now, the input to the terminal 6, and go up to the high transistor T _R
3 is turned off, and the capacitor 5 is charged through the transistor T _R 1. On the other hand, when the input to the terminal 6 becomes low, the transistor T _R 3 becomes conductive and the capacitor 5 is discharged.
The AGC voltage thus obtained is supplied from the point 7 to the gain control terminal of the intermediate frequency amplifier circuit to perform automatic gain control.

The problems in such a conventional example will be described below. FIG. 3 (1) shows the vertical synchronizing signal portion of the video signal, and the AGC voltage in such a signal portion is as shown in FIG. 3 (2). That is, discharging is performed at the tip of the sync signal and charging is performed at the recess (scanning period). Then, in the vertical synchronizing signal portion, the pulse period of the synchronizing signal is inverted from that of the horizontal synchronizing signal portion, and the discharge period becomes longer, so that the waveform becomes as shown in the figure and so-called sag occurs. Horizontal like this,
It is necessary to increase the AGC time constant to reduce the sag in the vertical part.

On the other hand, the audio signal is also sent to the TV signal by the inter-carrier method, and the audio beat of 4.5 MHz is taken out between the video carrier and FM detection to obtain the audio signal. Therefore, the audio signal is also affected by the video carrier. FIG. 3 (3) shows the waveform of the FM detection output when an unmodulated voice carrier is input. Since there is no modulation, the detection output has only a noise component like the waveform shown in the figure. , The sloped portion of the waveform in Fig. 3 (2) (AGC
At the point where the voltage returns to a steady value), large noises like a and b in the figure occur. This is because the fluctuation of the AGC voltage due to long-term sag changes the phase characteristic of the video intermediate frequency amplifier, and therefore the audio beat also changes the phase, and the change appears as noise in the FM detection output. Causes the S / N in the audio output to deteriorate. That is, in the conventional case, the AGC time constant must be increased in order to prevent the deterioration of the audio S / N due to the sag.On the other hand, if the AGC time constant is increased, the phase of the carrier wave in the amplifier circuit of the video carrier wave Due to the fluctuation, a noise is generated when the audio signal is FM-detected, which causes a change in the audio S / N. In particular, since S / N becomes a problem in the sub-channel in recent audio multiplex broadcasting, the S / N deterioration caused by the influence of AGC is a big problem.

(Object of the Invention) An object of the present invention is to provide a means for simultaneously solving the sag in such an AGC circuit and the S / N deterioration in the audio output.

(Structure of the Invention) In order to achieve the above object, an automatic gain control device of the present invention controls the gain of a tuner for receiving a television signal, a video intermediate frequency amplifier, a video detector, and a video intermediate frequency amplifier. Has an AGC (automatic gain control) circuit for reducing the fluctuation of the AGC voltage generated in the vertical sync signal part by superimposing a pulse generated in the vertical sync signal part on the charge / discharge time constant circuit of the AGC circuit. This is characterized by the fact that the S /
N deterioration can be eliminated at the same time.

(Description of Embodiments) Hereinafter, one embodiment of the present invention will be described with reference to the drawings.
FIG. 4 is a block diagram of an automatic gain control device showing an embodiment of the present invention. In the figure, 11 is an IC including the AGC circuit of FIG. 2, and a charging / discharging capacitor 5 is connected to a terminal A. Reference numeral 12 denotes an IC for generating a vertical synchronizing pulse of the television, and at the terminal B, a vertical synchronizing signal is integrated and a shaped pulse is output. This output is connected to the AGC terminal A through the resistor 13. FIG. 3 (4) shows a pulse waveform output to the terminal B in FIG. 4, and the waveform at the terminal A when this pulse is superimposed on the terminal A in FIG. 4 is shown in FIG. 3 (5).
Shown in. Comparing the waveforms in Fig. 3 (2) and (5), AGC
The period (5) from the fall of voltage to the restoration is shorter. That is, the vertical sync pulse raises the AGC voltage to accelerate the recovery. If the recovery is fast in this way, the phase fluctuation period in the intermediate frequency amplifier is shortened, so that the noise level after FM detection is suppressed as shown in FIG. 3 (6) and hardly occurs.

(Effects of the Invention) As described above, in the past, due to the contradictory conditions, it is necessary to lengthen the time constant to prevent sag and to shorten the time constant to improve the voice S / N ratio. Although both performances could not be satisfied, the vertical sync pulse easily obtained in the receiver as in the present invention was used to superimpose it on the AGC circuit to increase the time constant and improve the sag. It is possible to improve the voice S / N in the state of being kept. Especially in the sub-channel signal output of audio multiplex broadcasting
It is possible to improve S / N deterioration due to AGC by about 6 dB.

[Brief description of drawings]

FIG. 1 is a block diagram of a circuit relating to the present invention, and FIG. 2 is
FIG. 3 is a diagram showing a conventional example of an AGC circuit, FIG. 3 is a waveform diagram in each part for explaining the conventional example and the present invention, and FIG. 4 is a block diagram showing an embodiment of the present invention. 5 ... Charging / discharging capacitor, 11 ... I including AGC circuit
C, 12 ... I for generating vertical sync pulse for TV
C, 13 ... resistance.

Claims (2)

[Claims] 1. A tuner for receiving a television signal, a video intermediate frequency amplifier, a video detector, and an AGC (automatic gain control) circuit for controlling the gain of the video intermediate frequency amplifier, and the AGC circuit. An automatic gain control device characterized by superposing a pulse generated in the vertical synchronizing signal portion on the charging / discharging time constant circuit of to reduce fluctuations in the AGC voltage generated in the vertical synchronizing signal portion. 2. The automatic gain control device according to claim 1, wherein a pulse whose waveform is shaped by integrating a vertical synchronizing signal is used as the superimposed pulse.