JPS62135083A - Gain adjusting circuit for video signal - Google Patents

Abstract

PURPOSE:To easily fix a preset state of a gain adjusting circuit whose gain is kept constant by adding an output from a fixed gain circuit with a control input for gain adjusting, comparing the output of the said addition with a feedback signal, and controlling the gain of a variable gain circuit by means of the output of the said comparison. CONSTITUTION:The synchronization level L of an input video signal from an input terminal 1 is detected by a synchronization-level detector 15, and multiplied G0 by the fixed gain circuit 16, and goes to be G0.L. Since the input video signal is multiplied by G by the variable gain circuit 3, an output of G.L is obtained from a synchronization-level detector 11. Assuming a gain-control input from an input terminal 5 to be (x), the output of an adder 17 will be G0.L+(x), which is transferred to a comparator circuit 12 to be compared with the output of G.L. The gain G of the circuit 3 is controlled so as to make the difference of the said comparison zero. When the preset state is desired, the gain control input (x) is made zero so that the gain G of the circuit 3 comes to be G0, and the level is fixed at the constant preset gain G0 regardless of the synchronization level L.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video signal gain adjustment circuit, and more particularly to a video signal gain adjustment circuit whose gain can be fixed to a preset gain.

[Disclosure of the invention]

The present invention provides a video signal gain adjustment circuit having a feedback loop that detects a reference level of an output signal from a variable gain circuit whose gain can be adjusted by a control input and controls the gain of the variable gain circuit. detecting a reference level of an input video signal to the circuit, sending it to a fixed gain circuit fixed to a predetermined precent gain, and adding the output from the fixed gain circuit and a control input for gain adjustment; By controlling the gain of the variable gain circuit using the comparison output obtained by comparing this addition output with the signal to be fed back, the gain of the variable gain circuit is not dependent on fluctuations in the input signal level while taking advantage of the feedback loop. This allows fixing to a predetermined preset gain easily and with high precision.

[Conventional technology]

As a gain adjustment circuit used to adjust an input video signal to a desired gain and output it, circuits such as those shown in FIGS. 5 and 6, for example, are conventionally known.

First, in FIG. 5, a video signal whose gain is to be adjusted is supplied to an input terminal 41, and this human video signal is sent to a variable gain circuit 43 via a buffer circuit 42 with a gain of 1. The gain circuit 43 is generally formed using a multiplier, a variable resistance element, or the like, and its gain is controlled in accordance with a gain control input from the input terminal 45. The output signal from the variable gain circuit 43 is taken out via the output terminal 44. FIG. 6 shows an example with a feedback loop, in which the variable gain circuit 4
A synchronization level detector 46 detects the reference level of the output video signal from the input terminal 3, for example, the level of a synchronization signal, and a comparator 47 using a differential amplifier or the like detects the synchronization signal level and the gain from the input terminal 45. A comparison output is sent to the gain control terminal of the variable gain circuit 43. Note that the video signal whose gain is to be adjusted is sent to the variable gain circuit 43 via the input terminal 41 and the buffer circuit 42, and is taken out from the output terminal 44.

[Problem that the invention seeks to solve]

By the way, when a multiplier is used in the variable gain circuit 43 in the configuration shown in FIG. 5, the circuit configuration tends to become complicated for reasons such as preventing the characteristics of the video signal from deteriorating. Therefore, a variable resistance element is used in the variable gain circuit 43, but this variable resistance element has a feedback loop as shown in FIG. 6 due to variations in temperature characteristics and resistance value with respect to voltage. In other words, since the synchronization signal level is generally standardized in video signals, this synchronization signal level is used as feedback information and the variable gain circuit 43 is controlled so that the level is equal to the gain control input. By doing so, temperature characteristics etc. are compensated.

However, in actual video signals, the synchronization signal level that should serve as the reference may also fluctuate.
A type of AGC operation occurs in which the gain of the variable gain circuit 43 is controlled according to fluctuations in the level of the synchronizing signal. In such an AGC operation, the gain of the level adjustment device is controlled at a predetermined fixed preset. This becomes an obstacle when attempting to adjust the gain, and a preset state of constant gain cannot be obtained because the gain fluctuates depending on the synchronization signal level.

The present invention has been made in view of the above circumstances, and it is possible to use a variable resistance element in a variable gain circuit, and to obtain a preset state of constant gain while having a feedback loop for temperature characteristic compensation, etc. The purpose of the present invention is to provide a video signal gain adjustment circuit that is capable of adjusting the video signal.

[Means for solving problems]

That is, the video signal gain adjustment circuit of the present invention has a variable gain circuit whose gain is controlled according to a gain control input, outputs an input video signal through this variable gain circuit, and outputs an input video signal through this variable gain circuit. In the video signal gain adjustment circuit that detects the reference level of the output signal from the input video signal, for example, the synchronization signal level, and supplies it to the gain control terminal of the variable gain circuit, the reference level of the input video signal (synchronization signal level ); a fixed gain circuit having a preset gain to which the output from the level detector is supplied; and an adder for adding the output signal from the fixed gain circuit to the gain control input. and the reference level of the output from this adder and the above output signal (
synchronous signal level) and a comparison circuit that supplies the signal to the control terminal.

[For production]

It has a feedback loop that detects the reference level (sync signal level) of the output video signal and sends it to the gain control terminal of the variable gain circuit, so even if a variable resistance element is used in the variable gain circuit, temperature characteristics compensation and It is possible to compensate for variations in resistance values, etc., and when the gain control input is 0, the reference level (synchronization signal level) of the input video signal is amplified (or attenuated) by a constant preset gain and the reference level of the output video signal is The variable gain circuit is controlled so that the (synchronizing signal level) becomes equal, and the gain of this variable gain circuit is fixed to the preset gain, so that it is possible to easily obtain a preset state in which the gain is constant and unchanging.

〔Example〕

FIG. 1 is a block circuit diagram showing a video signal gain adjustment circuit according to an embodiment of the present invention.

In FIG. 1, an input terminal 1 is supplied with a video signal whose gain is to be adjusted. This input video signal is sent to a variable gain circuit 3 via a buffer circuit 2 with a gain of 1, and the gain-adjusted video signal from the variable gain circuit 3 is taken out from an output terminal 4. A gain control input corresponding to the gain to be adjusted is supplied to the input terminal 5.

The variable gain circuit 3 may be configured by a multiplier or the like, but since the circuit configuration is generally complicated, it is preferably configured using a variable resistance element. For example, in Figure 2,
A specific example of a variable gain circuit 3 using a so-called photocoupler 20 is shown, in which a light emitting element, for example, a light emitting diode 21, and a light receiving element, for example, a phototransistor 22 are coupled through light. There is. The input signal (video signal) from the input terminal 23 of the variable gain circuit 3 in FIG. A phototransistor 22 as a variable resistance element is inserted and connected in series with one resistor, for example 25, of this voltage dividing circuit. Further, the control signal from the gain control terminal 27 of the variable gain circuit 3 is transmitted to the light emitting diode 21.
is supplied to. Therefore, the light emission intensity or amount of light from the light emitting diode 21 is controlled according to this control signal, and the resistance value of the phototransistor 22 changes, thereby changing the voltage division ratio, that is, the gain. Note that the gain is 1
If more than that is desired, a fixed gain amplifier may be inserted and connected in the front stage or the rear stage of the voltage rotation circuit.

In the case of a variable gain circuit 3 using such a variable resistance element, a feedback loop as shown in Fig. 1 is required to compensate for temperature characteristics and variations in resistance value with respect to voltage. . That is, the reference level of the output signal from the variable gain circuit 3, for example, the synchronization signal level, is detected by the synchronization level detector 11, and the level of the output signal from the variable gain circuit 3 is detected by the synchronization level detector 11. The signal is sent to the gain control terminal 21.

Further, as a main configuration of the embodiment of the present invention, a reference level of the input signal to the variable gain circuit 3 (output signal from the buffer circuit 2), for example, a synchronization signal level, is detected by a synchronization level detector 15, This detected synchronization level is supplied to an adder 17 via a fixed gain circuit 16 with a constant gain Go required at the time of presetting. This adder 1
At 7, the output from the fixed gain circuit 16 is summed with the gain control input from the input terminal 5, and this summed output is supplied to the other input terminal of the comparator circuit 12.

Here, the synchronization level detector 11.15 may be configured as shown in FIG. 3, for example. That is, the video signal supplied to the input terminal 31 in FIG. 3 is sent to the synchronization separation/pulse generation circuit 32, where the synchronization signal is separated, and a sampling pulse with a fixed timing is generated for this synchronization signal. be done. These sampling pulses include a pulse at timing a for sampling the sync chip portion of the synchronizing pulse PI (in the video signal shown in FIG. 4) and a pulse at timing bl for sampling the pedestal portion.
Alternatively, two types of pulses, b2 and b2, are required. Further, the video signal from the input terminal 31 is sent to switching means 33a and j3b for the sampler, respectively, and the switching means 33a receives the pulse p8 at the timing a, and the pulse [[] at the timing b1 or b2.
)b, the switching means 33b is controlled to switch, whereby the voltages of the video signal at each of the timings mentioned above are sampled. The sampling outputs from these switching means 33a, 33b are connected to a hold capacitor 34a.

34b, each voltage is held, and the buffer circuit 3
5a, 35b to the adder 36, the amplitude level of the synchronizing pulse portion PI (that is, the synchronizing level L) is detected and taken out from the output terminal 37. Here, the buffer circuits 35a, 35b and the adder Vessel 3
6 is configured to subtract the sync step level held by the capacitor 34a from the pedestal level held by the capacitor 34b. For example, the gain of the buffer circuit 35a is set to -1.
It is said that

In the above configuration, the synchronization level L of the input video signal from the input terminal 1 is detected by the synchronization level detector 15 and multiplied by 00 by the fixed gain circuit 16, thereby increasing the synchronization level L of the input video signal from the input terminal 1.
It becomes O-L. Also, the input video signal is input to the variable gain circuit 3.
Since it is multiplied by 0, the synchronization level detector 11 outputs G
-L output will be obtained. Here, input terminal 5
When the gain control input of is set to X, the output from the adder 17 becomes GO・L+X, which is sent to the comparison circuit 12 and compared with the above GL, and the variable gain circuit 3 is set so that the difference between them becomes 0. Since the gain G of is controlled, GL-(Go-Lx)20G=Go+-...Song...Song ■. At this time, if we consider the gain control human power X to be GX-L, then G -Go 1 Gx
......■, and the gain G of the variable gain circuit 3 is
It can be seen that the gain controller can be adjusted by 4x (=Gx-L).

Next, when it is desired to obtain a preset state, by setting the gain control manual power X to O, the gain G of the variable gain circuit 3 is
is the above Go, and is fixed to a constant preset gain Go regardless of the synchronization level L. This is also clear from the fact that the term X/L in the above equation 0, which varies depending on the level L, becomes 0.

〔Effect of the invention〕

Because it has a feedback loop, even if the circuit configuration is simplified by using an inexpensive variable resistance element in the variable gain circuit, it can not only compensate for variations in resistance value with respect to temperature characteristics and voltage, but also compensate for variations in resistance value in the video signal. Even if the reference level such as the synchronization level of the gain fluctuates, it is possible to easily obtain a preset state in which the gain remains constant.

[Brief explanation of drawings]

Fig. 1 is a block circuit diagram showing a video signal gain adjustment circuit according to an embodiment of the present invention, Fig. 2 is a circuit diagram showing a specific example of a variable gain circuit, and Fig. 3 is an example of a synchronous level detector. A block circuit diagram showing a specific example, FIG. 4 is a graph for explaining sampling timing for detecting the synchronization level of a video signal, and FIGS. 5 and 6 are block circuit diagrams showing different conventional examples. . 1......Video signal input terminal 3.
......Variable gain circuit 4...
・・・・・・ Video signal output terminal 5 ・・・・・・・・・
...Input terminal i1 for gain control input. is...
Synchronous level detector 12... Comparison circuit 16... Fixed gain circuit 17... Adder 27...・Gain control terminal 1 i'lq level output history first column A column 3 synchronous level line stop R menu S;
Surplus 1 of the detailed songs! ! Ichigu 1 (Fig. 5) of the 1st edition of the circle (Illustration 6)

Claims (1)

[Claims] It has a variable gain circuit whose gain is controlled according to a gain control input, and outputs an input video signal through this variable gain circuit, and also outputs an input video signal through this variable gain circuit and uses it as a reference for an output signal from this variable gain circuit. A gain adjustment circuit for a video signal that detects a level that becomes a reference level and supplies it to a gain control terminal of the variable gain circuit includes a level detector that detects a reference level of the input video signal; a fixed gain circuit having a preset preset gain; an adder for adding an output signal from the fixed gain circuit to the gain control input; and a reference level for the output from the adder and the output signal. and a comparison circuit that compares and supplies a comparison output to a gain control terminal of the variable gain circuit.