JPH1188901A - Automatic gain controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce quantization noise by giving a color signal with an optimum level to an A/D converter means at all times with respect to the automatic gain controller for the color signal. SOLUTION: An analog gain adjustment means 11, an A/D converter means 12, a maximum value detection means 16, a maximum value control means 17, and a D/A converter means 18 form a feedback loop, and the maximum value control means 17 changes a gain of the analog gain adjustment means 11 so that a maximum value of a digital color signal detected by the maximum value detection means 16 reaches a prescribed level. Thus, the level of the analog color signal is automatically adjusted so that the dynamic range of the analog color signal is always in matching with the dynamic range of the A/D converter means 12. On the other hand, a burst level control means 14 controls a gain of a digital gain adjustment means 15 so that the amplitude of the burst detected by a burst level detection means 13 is constant and the digital color signal with the constant burst amplitude is outputted from a terminal 110.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control device used for adjusting a gain of a color signal in a video tape recorder (hereinafter, VTR) or the like.

[0002]

2. Description of the Related Art Conventionally, an automatic gain control device is disclosed in
What is described in the 6987 gazette is known.

FIG. 4 shows a block diagram of a conventional automatic gain control device. In FIG. 4, reference numeral 11 denotes analog gain adjusting means, 12 denotes AD converting means, 13 denotes burst level detecting means, 14 denotes burst level controlling means, 15 denotes digital gain adjusting means, 18 denotes DA converting means, and 19 denotes analog color signal. An input terminal 110 is a digital color signal output terminal. Hereinafter, a conventional automatic gain control device will be described with reference to FIG.

An analog color signal input from an input terminal 19 is adjusted to an optimum gain by an analog gain adjusting means 11 and is converted from analog to digital by an AD converting means 12 to become a digital color signal. Next, the gain of the digital chrominance signal is adjusted again by the digital gain adjusting means 15, and
Output from the output terminal 110. Thereafter, the digital color signal is subjected to digital signal processing such as encoding and decoding.

On the other hand, the burst level of the burst of the digital color signal is detected by a burst level detecting means 13. The burst level control means controls the analog gain adjustment means 11 and the digital gain adjustment means 15 to vary the respective gains so that the detected burst amplitude becomes a predetermined value. That is, first, the gain of the analog gain adjusting means 11 is varied so as to make the burst amplitude such that the digital color signal completely falls within the dynamic range of the AD converting means 12. At this time, since the output of the burst level control means 14 is digitized, the digital-to-analog conversion is performed by the DA conversion means 18 and converted into an analog signal, and then the gain of the analog gain adjustment means 11 is controlled. In the digital gain adjusting means 15, the output terminal 110
The gain is adjusted by the burst level control means 14 so that the amplitude of the burst of the digital color signal output from is constant.

[0006]

However, in the above-described conventional configuration, when the amplitude of the analog color signal becomes large due to tracking deviation during VTR reproduction or crosstalk noise from an adjacent track during long-time mode reproduction. Also, since it is necessary to keep the analog color signal within the dynamic range of the AD converter 12, the gain of the analog gain adjuster 11 must be set smaller. For this reason, in a normal reproduction state where there is no crosstalk noise, a sufficient dynamic range cannot be obtained, and the AD conversion means 1
2 has a problem that the quantization noise increases.

The present invention solves the above-mentioned conventional problems, and always keeps the level of an analog color signal at an optimum level.
It is an object to provide an automatic gain control device that reduces quantization noise.

[0008]

SUMMARY OF THE INVENTION In order to solve this problem, an automatic gain control device according to the present invention varies the gain of an analog color signal according to an analog gain control signal and outputs an analog color signal whose gain has been adjusted. Analog gain adjusting means for
AD conversion means for converting an analog color signal whose gain has been adjusted into a digital color signal, burst level detection means for detecting the burst amplitude of the digital color signal, and burst level for maintaining the burst amplitude at a first level Burst level control means for outputting a control signal; digital gain adjustment means for varying the gain of the digital chrominance signal according to the burst level control signal and outputting a digital chrominance signal whose burst amplitude has been adjusted to a first level; Maximum value detection means for detecting the maximum value of the digital color signal output from the AD conversion means, maximum value control means for outputting a digital gain control signal for maintaining the maximum value at the second level, and digital gain control signal And DA conversion means for converting the signal into an analog gain control signal.

[0009]

According to the present invention, the analog gain adjusting means changes the gain of the analog color signal in accordance with the analog gain control signal, and outputs the analog color signal whose gain has been adjusted. The analog color signal whose gain has been adjusted is converted into a digital color signal by AD conversion means. Burst level detecting means detects the amplitude of the burst of the digital color signal, and burst level controlling means controls the gain of the digital gain adjusting means so that the amplitude of the burst becomes constant. The maximum value detection means detects the maximum value of the digital color signal, and outputs a digital gain control signal such that the maximum value becomes a predetermined level by the maximum value control means. The DA converter converts the digital gain control signal into an analog gain control signal, and the analog gain adjuster varies the gain of the analog color signal in accordance with the analog gain control signal.

An embodiment of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a block diagram showing an automatic gain control device according to an embodiment of the present invention. In FIG. 1, reference numeral 11 denotes analog gain adjustment means, 12 denotes AD conversion means, 13 denotes burst level detection means, 14 denotes burst level control means, 15 denotes digital gain adjustment means, 16 denotes maximum value detection means, and 17 denotes maximum value control. Means, 18 DA conversion means, 19 an analog color signal input terminal, 110 a digital color signal output terminal. Components that perform the same operations as in FIG. 4 described in the section of the prior art are denoted by the same reference numerals. Hereinafter, the automatic gain control device according to the present embodiment, particularly the maximum value detecting means 16 and the maximum value controlling means 17, which are characteristic parts of the present invention, will be described with reference to FIG.

Maximum value detecting means 16, Maximum value controlling means 17
FIG. 2 shows a specific example of the configuration. In FIG. 2, 21 is an absolute value circuit, 22 and 29 are comparators, 23, 24, 26,
210 and 212 are selectors, 25, 27 and 213 are flip-flops, 28 and 214 are counters, 211 is an adder, 215 is a digital color signal input terminal, and 216 is a digital gain control signal output terminal.

The maximum value detecting means 16 detects and outputs the maximum value of the digital color signal within a predetermined period. As an example, a method for detecting the maximum value within one horizontal period will be described.

The digital chrominance signal input from the terminal 215 is, for example, an 8-bit signal whose DC level is 0 in the range of -128 to 127. In order to determine the maximum value of the amplitude, the absolute value of the maximum value is determined by the absolute value circuit 21 to obtain a signal in the range of 0 to 128. The comparator 22 compares the output of the flip-flop 25 with the output of the flip-flop 25. If the absolute value of the digital color signal is larger, the output of the comparator 22 becomes 1, and the selector 23 selects the output of the absolute value circuit 21. This output is held in the loop 25.
By repeating these operations, the maximum value of the absolute value of the digital color signal is output from the flip-flop 25.

The counter 28 outputs 1 once in one horizontal period. For example, if the clock frequency is 14.31818 MH
If z, 1 is output once every 910 clocks. Then, the selector 24 outputs 0 once in one horizontal period.
The flip-flop 25 holds 0 corresponding to the minimum value of the absolute value of the digital color signal, and the flip-flop 25 is initialized.

By the above operation, the value of the flip-flop 25 is always updated to the maximum value of the digital color signal. Finally, the maximum value within one horizontal cycle determined by the counter 28 is output from the flip-flop 25, and this is
And is held in the flip-flop 27.

By the above operation, the maximum value of the absolute value of the digital color signal in one horizontal cycle, that is, the maximum amplitude is output from the flip-flop 27 every one horizontal cycle.

Next, the maximum value control means 17 controls the gain of the analog gain adjustment means 11 so that the maximum value of the digital color signal detected by the maximum value detection means 16 becomes a predetermined level Th.

First, the comparator 29 compares the output of the flip-flop 27 with a predetermined level Th. When a digital color signal is input in 8 bits and the maximum output of the flip-flop 27 is 128, Th is selected to be, for example, about 100. The comparator 29 outputs 1 when the output of the flip-flop 27 is larger than Th, and outputs 0 when the output is smaller than Th. For example, when the output of the flip-flop 27 is larger than Th, the output of the selector 210 becomes −1, the adder 211 subtracts 1 from the output of the flip-flop 213, and the digital gain control signal output from the flip-flop 213 is The value is smaller by 1 and output from the terminal 216. The counter 214 measures one horizontal period similarly to the counter 28, and outputs, for example, 1 every 910 clocks.
The digital gain control signal changes once in one horizontal period.
When this is converted into an analog gain control signal by the DA converter 18 and input to the analog gain adjuster 11, the gain becomes smaller than before and the amplitude of the analog color signal becomes smaller. The analog color signal is converted into a digital color signal by the AD converter 12, and the maximum value is detected by the maximum value detector 16. The maximum value detected here becomes smaller than the previous one, that is, approaches a predetermined level Th.

Conversely, when the maximum value output from the flip-flop 27 is smaller than a predetermined level Th, the selector 210
Is selected, and the digital gain control signal output from the flip-flop 213 is increased by one. Therefore, the analog gain control signal converted by the DA converter 18 increases,
The gain of the analog gain adjusting means 11 becomes larger than before, and the amplitude of the analog color signal increases. AD conversion means 1
Since the amplitude of the digital color signal converted by 2 increases, the maximum value detected by the maximum value detecting means 16 becomes larger than before, and the output of the flip-flop 27 approaches the predetermined level Th.

By the above feedback loop, the maximum value detected by the maximum value detecting means 16 gradually approaches a predetermined level Th. Therefore, the analog gain adjusting means 11, A
An analog color signal is converted into an analog signal by the feedback loop constituted by the D conversion means 12, the maximum value detection means 16, the maximum value control means 17, and the DA conversion means 18.
The gain is automatically adjusted to fall within the dynamic range of. Further, by setting the predetermined level Th to an optimum value, the dynamic range of the AD converter 12 can be utilized to the maximum.

The amplitude of a burst of a digital color signal whose gain has been adjusted varies depending on the degree of saturation. For example,
For a color signal in which the amplitude of the video portion is larger than the amplitude of the burst, the maximum value of the video portion becomes the predetermined level Th. On the other hand, for a color signal whose video portion is smaller than the amplitude of the burst, the maximum value of the burst becomes a predetermined level Th, and a signal whose amplitude is larger than that of the color signal whose amplitude of the video portion is larger is input. That is, the amplitude of the burst of the digital chrominance signal varies depending on the video signal. For this reason, the burst amplitude is detected by the burst level detecting means 13 and the gain of the digital gain adjusting means 15 is automatically adjusted by the burst level control means 14 so that the burst amplitude becomes constant.

As described above, according to the present invention, the gain is automatically adjusted so that the analog color signal falls within the dynamic range of the AD converter 12 and the dynamic range can be utilized to the maximum. Can be.
Further, for a color signal whose video portion is smaller than the burst amplitude, the input amplitude to the AD converter 12 can be increased, and the quantization noise can be reduced.

In the above description, the maximum value detecting means 1
6. Although the operation of the maximum value control means 17 is performed once in one horizontal period, the same operation can be performed with an arbitrary length. For example, in order to set the counter once in one frame, the counters 28 and 214 are set to once in one frame, that is, 910 × 525 = 4777.
What is necessary is just to set it as the structure which outputs 1 every 50 times.

In reproducing a VTR, a signal recorded on a magnetic tape is reproduced by a different head for each field, so that the amplitude of a reproduced analog color signal varies for each field. FIG. 3 shows a configuration example of the maximum value detection means 16 and the maximum value control means 17 for absorbing such fluctuations.

In FIG. 3, 31 is an even field maximum value detection circuit, 32 is an odd field maximum value detection circuit,
3 is an even field maximum value control circuit, 34 is an odd field maximum value control circuit, 35 is a selector, and 36 is a head switch input terminal. The configuration of the even field maximum value detection circuit 31 and the odd field maximum value detection circuit 32 is shown in FIG.
The configuration is the same as that of the maximum value detecting means 16 indicated by. The configuration of the even field maximum value control circuit 33 and the odd field maximum value control circuit 34 is the same as that of the maximum value control means 17 shown in FIG.

The even field maximum value detection circuit 31 detects the maximum value of the digital color signal in the even field, and the even field maximum value control circuit 33 outputs the even field digital control signal corresponding to this maximum value. The odd field maximum value detection circuit 32 detects the maximum value of the digital color signal in the odd field, and the odd field maximum value control circuit 34 outputs a digital gain control signal of the odd field according to the maximum value. The selector 35 switches the digital gain control signals of the even field and the odd field in accordance with the polarity of the head switch input from the terminal 36, and outputs an optimal digital gain control signal for each field.

As described above, the detection of the maximum value of the digital chrominance signal separately in the even field and the odd field,
By adjusting the gain of the analog color signal using the digital gain control signal, fluctuations in the amplitude of the reproduced color signal for each field due to variations in the reproducing head are absorbed, and the analog color signal having the optimum amplitude for the AD conversion means 12. Can be entered.

[0028]

As described above, according to the present invention, analog gain adjusting means for varying the gain of an analog color signal according to an analog gain control signal, and AD conversion means for converting an analog color signal into a digital color signal. Burst level detecting means for detecting the amplitude of a burst of a digital color signal;
Burst level control means for outputting a burst level control signal for keeping the burst amplitude constant; digital gain adjustment means for varying the gain of the digital color signal in accordance with the burst level control signal; Providing maximum value detection means for detecting, maximum value control means for outputting a digital gain control signal for keeping the maximum value at a predetermined level, and DA conversion means for converting the digital gain control signal into an analog gain control signal By
An analog color signal having an optimum amplitude can always be input to the AD conversion means, and the quantization noise can be reduced, which has a great practical effect.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an automatic gain adjustment device according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a specific configuration example of maximum value detection means and maximum value control means of the automatic gain control device.

FIG. 3 is a block diagram showing another specific configuration example of the maximum value detection means and the maximum value control means of the automatic gain control device.

FIG. 4 is a block diagram showing a configuration of a conventional automatic gain adjustment device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Analog gain adjustment means 12 AD conversion means 13 Burst level detection means 14 Burst level control means 15 Digital gain adjustment means 16 Maximum value detection means 17 Maximum value control means 18 DA conversion means

Claims (4)

[Claims] 1. An analog gain adjusting means for varying a gain of an analog color signal in accordance with an analog gain control signal and outputting an analog color signal whose gain has been adjusted, and converting the analog color signal whose gain has been adjusted into a digital color signal. A / D conversion means for converting the digital color signal into a burst signal, burst level detection means for detecting the amplitude of the burst of the digital color signal, and burst level control means for outputting a burst level control signal for keeping the amplitude of the burst at a first level. Digital gain adjusting means for varying the gain of the digital color signal in accordance with the burst level control signal and outputting a digital color signal in which the amplitude of the burst is adjusted to the first level; Maximum value detection means for detecting the maximum value of the obtained digital color signal; and maintaining the maximum value at a second level. An automatic gain control device comprising: a maximum value control unit that outputs a digital gain control signal for converting the digital gain control signal to a digital gain control signal; and a DA conversion unit that converts the digital gain control signal into the analog gain control signal. 2. The automatic gain control device according to claim 1, wherein the maximum value detecting means detects a maximum value of the digital color signal output from the A / D converting means within one horizontal period. 3. The automatic gain control device according to claim 1, wherein the maximum value detecting means detects a maximum value in one frame of the digital chrominance signal output from the AD converting means. 4. A maximum value detecting means for detecting a maximum value in an odd field and a maximum value in an even field of the digital color signal output from the A / D conversion means, and a maximum value control means for detecting the maximum value in the odd field. And a second digital gain control signal for keeping the maximum value in the even field at the second level by switching the digital gain control signal for each field. 2. The automatic gain control device according to claim 1, wherein the automatic gain control device outputs the control signal as a control signal.