JPS63194480A - Agc circuit - Google Patents

Abstract

PURPOSE:To require no adjustment of a circuit and to easily obtain an IC formation by clamping the end of the synchronizing signal of a composite video signal, forming an AC voltage corresponding to the average value of the color burst signal part of the composite video signal and removing an AC part. CONSTITUTION:The end of the synchronizing signal of the composite video signal 100 outputted from an AGC amplifier 5 is clamped by a synchronizing chip clamp circuit 6 and inputted to a comparison circuit 7. The AC voltage corresponding to the average level of the color burst signal present in the pedestal part P of the composite video signal 200 is outputted to a capacitor 8 from the collector of a transistor 83 and the collector of a transistor 81 constituting a current mirror circuit. The capacitor 8 removes the AC component from the inputted AC voltage to form a DC and outputs this DC voltage to a direct current amplifier 9. The AGC amplifier 5 changes a gain according to the DC voltage outputted from the direct current amplifier 9 to have the constantly prescribed level of the output composite video signal 100.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides an AGC that maintains a composite video signal at a predetermined level.
Regarding circuits.

(Prior Art) FIG. 4 is a block diagram showing an example of a conventional AGC circuit of this type. Figure 5 (A
) The composite video signal 100 as shown in FIG. The signal is output to the detection circuit 3 and the recording luminance signal processing system 5. In the AGC detection circuit 3, the first
After clamping the leading edge of the synchronizing signal of the input video signal at a reference potential, the pedestal level of the signal is compared with a second reference potential, and an AC voltage corresponding to the pedestal level is output to the loop control filter 4. The loop control filter 4 converts the input AC voltage into a DC voltage corresponding to the pedestal level of the input video signal through an integrating circuit, etc., and outputs this to the AGC amplifier 1. The gain of AGC amplifier 1 is changed depending on the input DC voltage, and eventually A
Feedback control is performed so that the level of the video signal input to the GC detection circuit 3 is always at a predetermined level.

However, in the conventional AGC circuit described above, the low-pass filter 2
Since the color signal component A of the composite video signal 100 is removed by this method, and the video signal after the color signal component removal is used as the luminance signal for the recording system, the low-pass filter circuit becomes complicated, and the AGC circuit needs to be integrated into an IC. It was becoming a bottleneck for me to do so. Further, since there are variations in the amount of attenuation of the low-pass filter 2, there is a drawback that the level of the video signal input to the AGC detection circuit 3 needs to be adjusted to within a predetermined value.

(Problems to be Solved by the Invention) In the conventional AGC circuit, the color signal component of the input composite video signal must be removed by a low-pass filter, which makes it difficult to integrate the circuit into an IC, and it is difficult to reduce the amount of attenuation of the low-pass filter. There was a drawback that circuit adjustment was required to correct it. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an AGC circuit which eliminates the above-mentioned drawbacks, does not require circuit adjustment, and can be easily integrated into an IC.

[Structure of the Invention] (Means for Solving the Problems) The AGC circuit of the present invention includes a clamp circuit that clamps the leading edge of a synchronizing signal of a composite video signal at a first reference potential, and a clamp circuit that clamps the leading edge of a synchronizing signal of a composite video signal, and a color burst signal amplitude of the composite video signal. a comparison circuit that compares a color burst signal portion of a composite video signal having an input dynamic range equal to or higher than a second reference potential with a second reference potential, and generates an AC voltage corresponding to the average value thereof; a filter circuit that removes an AC portion of an AC voltage to convert it into a DC voltage; and a filter circuit that changes the gain according to the DC voltage to adjust the level of the input composite video signal, and the composite video signal before clamping the synchronization signal. The AGC amplifier is configured to include an AGC amplifier.

(Function) In the AGC circuit of the present invention, the clamp circuit clamps the synchronization signal tip of the composite video signal at the first reference potential,
This is output to the comparison circuit. The comparator circuit compares the composite video signal with the leading end of the synchronization signal clamped with a second reference potential to generate an AC voltage corresponding to the average value of the color burst signal portion, and outputs this to the filter circuit. The filter circuit removes the AC part of the input AC voltage and converts it into a DC voltage.
This is supplied to the AGC amplifier. The AGC amplifier is the D
The level of the output composite video signal is set to a predetermined value by the C voltage, and this is supplied to the clamp circuit.

(Example) An example of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram showing an embodiment of the AGC circuit of the present invention. 5 is an AGC amplifier that always amplifies the input composite video signal to a predetermined level; 6 is a sync tip clamp circuit that clamps the synchronization signal tip of the composite video signal 100;
7 is a comparator circuit that generates an AC voltage corresponding to the pedestal portion of the video signal with the tip of the synchronization signal clamped; It consists of a transistor 85 that turns on and off the amplification section. 8 is a capacitor that converts the AC voltage output from the comparison circuit 7 into DC; 9 is a DC amplifier that amplifies the DC voltage output from the capacitor 8 and outputs it to the AGC amplifier 5; 1
0 is the reference voltage ref1 is the sink chip clamp circuit 6
Reference voltage source 11 is a reference voltage source that supplies reference voltage V ref2 to comparator circuit 8 . The input dynamic range of the comparison circuit 7 is greater than the color burst signal amplitude of the input composite video signal.

Next, the operation of this embodiment will be explained. AGC amplifier 5
The composite video signal 100 outputted from the sync tip clamp circuit 6 clamps the leading edge of the sync signal and inputs it to the comparator circuit 7.

However, the sync tip clamp circuit 6 is connected to the reference voltage source 1.
The synchronization signal leading edge of the input composite video signal 100 is clamped to a reference voltage V refl supplied from V refl.

A composite video signal @200 as shown in FIG. 2 output from the sync tip clamp circuit 6 is input to a differential amplifier section constituted by transistors 81 and 82 of the comparator circuit 7. However, the input dynamic range of the comparator circuit 7 is the range shown in D in FIG. That is, the video signal 2 whose synchronizing signal tip is clamped to the reference voltage Vref1
00 is applied to the base of transistor 81, while reference voltage ?00 is applied to the base of transistor 82. Reference voltage V r of l110
Since ef2 is applied, the reference voltage V ref
AC!2 according to the average level of the color burst signal present in the pedestal portion P of the composite video signal 200, with reference to AC! The voltage is output to the capacitor 8 from the collector of the transistor 83 and the collector of the transistor 81 that constitute the current mirror circuit. However, in the comparison circuit 7, the emitters of the transistors 81 and 82 are connected to the transistor 85.
Since it is grounded through the transistor 85, the third
It operates only during the period T during which the key pulse 300 shown in the figure is applied. Therefore, the AC voltage output to the capacitor 8 is proportional to the deviation of the average value of the pedestal portion P of the composite video signal shown in FIG. 2 from the reference voltage V ref2. Capacitor 8 removes the AC component from the input AC voltage, converts it to DC, and outputs this DC voltage to DC amplifier 9. The AC component is due to the color burst signal component A of the input composite video signal 200 shown in FIG.

The AGC amplifier 5 changes its gain according to the DC voltage output from the DC amplifier 9, and outputs a composite video signal 10.
The level of 0 is always the predetermined level. In addition, in this example, A
A composite video signal 100 outputted from the GC amplifier 5 is passed through a low-pass filter (not shown) to remove its color signal components, and then supplied to a recording signal processing system.

According to this embodiment, the composite video signal 200 containing the color burst signal component is directly inputted to the comparator circuit 7 whose input dynamic range is larger than the amplitude value of the color signal component included in this composite video signal, and the pedestal portion thereof is The AC voltage corresponding to the average value of the color burst signal level existing in
By removing the C portion and converting it to DC voltage, and changing the gain of the AGC amplifier 5 using this DC voltage,
Since the low-pass filter provided at the front stage of the low-pass filter is omitted and the function of the conventional low-pass filter is performed by the capacitor 8 which is the loop sub-part filter, the AGC circuit can be easily integrated into an IC, and the function of the low-pass filter is Circuit adjustment due to variations in attenuation rate can be omitted.

[Advantageous Effects of the Invention 1] As described above, the AGC circuit of the present invention has the advantage that no circuit adjustment is required and it can be easily integrated into an IC.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the AGC circuit of the present invention, FIG. 2 is a waveform diagram of the input composite video signal of the comparator circuit 8 shown in FIG. 1, and FIG. 3 is the waveform diagram shown in FIG. 1. comparison circuit 7
Figure 4 is a block diagram showing an example of a conventional AGC circuit, and Figure 5 (A) is an example of the input video signal to the low-pass filter shown in Figure 4. FIG. 5B is a waveform diagram showing an example of an input signal to the AGC detection circuit 3 shown in FIG. 4. 5... AGC amplifier 6... Sync chip clamp circuit 7... Comparison circuit 8... Capacitor 9... DC amplifier representative Patent attorney Nori Chika Ken Yudo Hiroshi Uji Figure 1 Figure 2 Figure 3 Figure 4 (A) (B) Figure 5

Claims (1)

[Claims] a clamp circuit that clamps a synchronization signal leading edge of a composite video signal at a first reference potential; and a color of the composite video signal having an input dynamic range equal to or higher than a color burst signal amplitude level of the composite video signal and having the synchronization signal leading edge clamped. a comparison circuit that compares a burst signal portion with a second reference potential and generates an AC voltage corresponding to the average value; a filter circuit that removes the AC portion of the AC voltage and converts it into a DC voltage; An AGC circuit comprising: an AGC amplifier that adjusts the level of an input composite video signal by changing the input composite video signal to obtain the composite video signal before clamping a synchronization signal.