JPS5837141Y2 - automatic gain control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control circuit for keeping the level of a television video signal constant.

In the transmission of television signals, level changes may occur due to changes in the characteristics of equipment included in the transmission path, or due to adjustment variations between systems when switching systems.

For this reason, conventional automatic gain control circuits have been used that detect the video signal, detect the video signal level, and use this signal as a control signal to automatically maintain the video signal level at a constant level.

However, in ordinary television signals, in addition to level changes due to changes in equipment characteristics, the level may be artificially varied for the purpose of creating a program.

That is, there are operations such as fade-out, which gradually reduces the level of the video signal, fade-in, which gradually increases the level from a fade-out state, and cut-in, which instantly switches the video signal to a specified level.

In these cases, if the automatic gain control operation is continuously applied, the response characteristics of the automatic gain control operation may impair the effectiveness of the manual operation.

For example, when going from a fade-out state to a cut-in state, the response of the automatic gain control circuit may cause a transient response that increases the output level more than necessary, or in a fade-out state, only noise is amplified, causing S/N deterioration. It turns out.

To improve this, conventional technology has been used to immediately disconnect the feedback loop of the automatic gain control circuit and switch the gain of the gain control circuit to a constant (OdB) when the input level becomes smaller than a certain value. It was getting worse.

However, this method has the drawback that the level changes at the time of switching are large.

The purpose of the present invention is to eliminate this drawback and provide a stable automatic gain control circuit.

The present invention will be described in detail below with reference to the drawings and compared with a conventional example.

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention. In the figure, a video signal from a terminal 1 is controlled in level by a gain control circuit 2 and outputted from a terminal 3.

On the other hand, the signal that controls the gain control circuit 2 is obtained by branching the output of the gain control circuit 2, converting it into a DC voltage in the detection circuit 4, amplifying it in the DC amplifier circuit 5, and smoothing it in the filter circuit 8 to control the gain. Supplied to circuit 2.

The detection circuit 6 also operates to close the switch 7 when the signal level falls below a certain value.

When switch 7 is closed, control voltage 9 will be fixed at ground potential.

Next, to explain the operation with reference to Fig. 2, when the input signal level changes to, for example, -6 dB in a long time compared to the response of the automatic gain control circuit 2, as shown by the solid line in Fig. 2 a, the output is shown by the broken line. As shown, it is kept almost constant due to the automatic gain control effect.

However, when the human power level changes in a time comparable to the response time of the automatic gain control circuit, as in the aforementioned fade-out, the change follows the input change, as shown by the broken line in FIG. 2.

However, as shown in the figure, even when the signal is a human input signal, if automatic gain control is applied continuously, noise etc. will be amplified and output.

For this reason, the present invention is provided with a switching circuit that fixes the gain of the automatic gain control circuit to O dB when the input level falls below the switching point shown in FIG. 2C.

When switching to this fixed gain, if the gain of the gain control circuit instantly becomes QdB (fixed gain), the response of the voltage that controls the gain control circuit will change as shown by the broken line in Figure 2 d, and the output will change. The change in level results in a response shown by the broken line S1 in FIG. 2C, resulting in a very unnatural response.

Therefore, in the circuit of the present invention, a switch for determining a fixed gain is provided in the front stage of the filter 8 as shown in FIG. 1.

Therefore, when the switch closes, the change in control voltage changes with the time constant of filter 8 (shown by the solid line in Figure 2 d), and the output response changes as shown by the broken line S2 in Figure 2 C, making it extremely smooth. We have received positive responses.

In the above description, a state in which the input signal gradually decreases (fade out) was described, but the same effect can be obtained even if the input signal is input from a fade out state.

As described above, according to the present invention, it is possible to obtain an automatic gain control circuit that responds very smoothly even when a human input signal is faded out or faded in.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 ab, C,
d is a diagram showing response characteristics. In the figure, 2...- gain control circuit, 4...
...Detection circuit, 5...DC amplifier, 6...
...Detection circuit, 8...Filter

Claims (1)

[Scope of utility model registration request] a gain control circuit; a gain control voltage generation circuit that receives the output of the gain control circuit and generates a gain control voltage for making the level of the output signal of the gain control circuit constant; and a gain control voltage generation circuit that receives the gain control voltage and receives the input; a detection circuit that generates a detection signal when the signal falls below a predetermined level; a fixed bias means for fixing the gain of the gain control circuit; and a path for supplying a gain control voltage to the gain control circuit. an automatic gain control circuit comprising: a time constant circuit; and switching means for switching the gain control voltage of the gain control circuit to the voltage from the fixed bias means according to the detection signal and sending the voltage to the time constant circuit. .