JP2575610B2 - Color-AGC circuit in VTR - Google Patents

Description

The present invention relates to a color AGC for keeping a level of a chroma signal separated from a luminance signal substantially constant in a color video signal recording system of a VTR (video tape recorder). Circuit (also referred to as an ACC circuit).

[Conventional technology]

Since the relative speed between the head and the tape is slow in a generally used small home-use VTR, it is difficult to directly record a 3.58 MHz chroma signal (color signal) included in a standard color video signal.

Therefore, the color video signal is once separated into a luminance signal and a chroma signal, the luminance signal is subjected to low carrier FM modulation, and the chroma signal is converted from a 3.58 MHz carrier to a low frequency (around 700 KHz). Either the signals are recorded on different tracks of the magnetic tape, or both signals are mixed again and recorded on the same track.

Then, the chroma signal separated from the luminance signal is sent to the frequency conversion circuit through the color AGC circuit so that the chroma signal can be recorded with good S / N with optimum efficiency.

Such conventional chroma signal processing circuits are generally
It is configured as shown in the figure.

That is, only a chroma signal in an input color video signal is extracted by a band-pass filter (hereinafter abbreviated as “BPF”) 1 that passes only a signal near 3.58 MHz, the level is adjusted through a color AGC circuit 2, and the frequency is adjusted. Conversion circuit 3
To convert to low frequency. The chroma signal is mixed with the luminance signal again and recorded on the magnetic tape.

The chroma signal input to the color AGC circuit 2 is gain-adjusted by a voltage-controlled variable gain amplifier (hereinafter abbreviated as “VCA”) 4, and a part of the output is input to a clamp circuit 5 and the center of the amplitude is adjusted. The level is clamped to a constant value, and is input to the burst gate circuit 6 to output a burst extraction pulse
As a result, only the burst signal is extracted. Detection circuit 7
Then, the peak signal or average value of the burst signal is detected by detecting the burst signal, the magnitude relationship is inverted and the level is adjusted by the inverting amplifier 8, and the signal is fed back to the VCA4 as the AGC control signal CS.

Thus, when the burst signal is large, the gain of VCA4 is decreased, and when the burst signal is small, the gain of VCA4 is increased, so that the output level of the chroma signal is kept substantially constant.

[Problems to be solved by the invention]

However, in such a conventional color AGC circuit, a peak value or an average value (burst level) of only a burst signal in a chroma signal is detected and converted to a DC voltage, and the VCA is controlled using the DC voltage as a control signal. Because the gain for the entire chroma signal was controlled, the chroma signal recording current of the VTR deviated from the optimum recording current value during recording, depending on the state of the chroma signal component in the color video signal for recording. When the chroma signal component is insufficient,
There was a problem that S / N (C / N) deteriorated.

To elaborate on this issue, VTR
The method of setting the optimum recording current of the chroma signal at the time of recording is generally based on the color bar signal. As shown in FIG. 11, assuming that the maximum amplitude of the video (color bar) portion is a and the amplitude of the burst portion is b, a = 0.63V and b = 0.3V as shown in FIG. In other words, the amplitude of the chroma signal component of the video portion is larger than that of the burst portion (about twice).

The color bar signal is used to set the optimum recording current for the chroma signal, so the color bar signal is frequency-converted by applying color AGC according to the burst level as described above, and then recorded. A chroma signal with a good level of S / N can be obtained.

However, a general color video signal actually transmitted from a broadcasting station or a chroma signal of a color video signal from a TV camera has a different ratio between the burst level of the color bar signal and the chroma signal level. As shown, the amplitude a of the video portion is often smaller than the amplitude b of the burst portion.

FIG. 13 is an electromagnetic conversion characteristic curve diagram showing the relationship between the recording current of the chroma signal and the head reproduction output, but at the optimum recording current I ₁ set using the color bar signal, the head reproduction output is large at V ₁ ; However, in the case of an actual chroma signal as shown in FIG. 12, if the recording is performed at the same amplitude, the recording current I ₂ becomes smaller than I ₁ and the head reproduction output V ₂
Becomes smaller than V _1, the reproduction chroma signal of S / N is deteriorated correspondingly.

Therefore, as shown in, for example, Japanese Utility Model Application Laid-Open No. 51-71922, a peak value of a chroma signal (color signal) separated from a color video signal is detected by a bandpass filter, and the color is determined according to the detected level. There has also been developed one that controls a gain control circuit so that the peak value of a chroma signal becomes constant.

However, even when the level is controlled so that the peak value of the chroma signal separated from the color video signal is simply constant, the burst signal is included in the maroma signal, so that the level of the burst signal is low. If the signal level of the video part is lower than that of the chrominance signal, the level of the burst signal is controlled to be constant, and the video part of the chroma signal cannot be recorded at a sufficiently high level. There was a problem that the improvement of S / N became insufficient.

An object of the present invention is to solve such a problem.

[Means for solving the problem]

The color AGC circuit according to the present invention is a VTR that separates a color video signal into a luminance signal and a chroma signal, and separately performs signal processing to record the signal. A burst cut circuit for removing a burst signal from an output signal, and a detection circuit for detecting a peak value or an average value of a chroma signal from which the burst signal has been removed by the circuit are provided. The output of the detection circuit, that is, a video period is provided. The gain of the VCA is controlled in accordance with the peak value or average value of the middle chroma signal component, so that the output level of the chroma signal is kept substantially constant.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and the same parts as those in FIG. 10 are denoted by the same reference numerals.

The difference between the color AGC circuit 10 of this embodiment and the conventional example shown in FIG. 10 is that the burst gate circuit 6 is replaced with a burst signal for removing a burst signal from a chroma signal and extracting only a chroma signal during a video period. A cut circuit 11 is provided, and a circuit of a peak detection circuit 12 and a diode 13 is newly provided. These details will be described below together with the operation.

A chroma signal separated from a color video signal by a BPF (bandpass filter) 1 is level-adjusted by a VCA (voltage-controlled variable gain amplifier) 4 and sent to a frequency conversion circuit 3 as in the conventional example. A part of the chroma signal output from the VCA 4 is input to the clamp circuit 5 to clamp the center level of the amplitude to a constant value C as shown in FIG. The same is true.

This is, for example, while inputting the chroma signal to the base of the transistor of the gemitsu follower by capacitor coupling,
This can be easily realized by applying a DC voltage C to the base via another transistor and clamping the base potential to the voltage C.

Next, a burst sampling pulse GP which becomes "H" only in a burst period Tb as shown in FIG. 2 (b) is supplied to the burst cut circuit 11 to which the chroma signal clamped by the DC voltage C is inputted. Accordingly, the burst cut circuit 11 alternately short-circuits the supply circuit of the chroma signal to the next stage only during the burst period.

Therefore, from the burst cut circuit 11, the second
As shown in FIG. 3C, only the chroma signal during the video period from which the burst signal has been removed is output.

Then, a peak value (peak value) or an average value of the chroma signal is detected by the detection circuit 7, and a DC voltage corresponding to the detected level is output.

For example, as shown in FIG. 3 as the detection circuit 7 to charge the capacitor C ₁ directly poles in a short time by the detection current of the diode D _1, a relatively long time discharging the electric charge of the capacitor C ₁ by a resistor R ₁ When a peak detection circuit is used,
As shown in FIG. 4, each of the chroma signal waveforms during the video period clamped by the DC voltage C is represented by a voltage d when indicated by a solid line and a voltage e when indicated by a broken line. A DC voltage close to the peak value is output.

Further, as shown in Figure 5 as a detection circuit 7, the resistor R ₂
The input signal is detected by the diode D ₁ through the capacitor C ₂
Which charges with a time constant in the, in the case of adopting the average value detection circuit which is adapted to increase the time constant of the discharge by the resistor R ₃ is discharged as compared with the time constant at the time of charging, the sixth
As shown by d 'and e' in the figure, a voltage close to the average value of the chroma signal waveform during the video period is output.

However, the same applies to the peak value detection and the average value detection in that a DC voltage corresponding to the amplitude of the AC component of the input chroma signal is detected.

The DC voltage detected by the detection circuit 7 is inverted in magnitude and level adjusted by the polarity inversion amplifier 8 and fed back to the control input terminal of the VCA 4 as the AGC control signal CS.

Accordingly, when the chroma signal level of the video portion is small, the output of the detection circuit 7 becomes small, so that the AGC control signal CS, which is the output of the inverting amplifier 8, becomes large, and the gain of VCA4 is increased to increase the input chroma signal. When the chroma signal level of the video part is amplified, the gain of VCA4 is reduced to reduce the amplification of the input chroma signal, and the output level of the chroma signal is controlled to a constant level so that the recording current value is always optimal. I do.

For example, as shown in (a) of each of FIGS. 7 to 9, when the amplitude a of the video portion of the input chroma signal is too small (FIG. 7), the amplitude b of the burst signal is large. If too much (Figure 8) or if it changes (Figure 9)
As shown in (b) of each figure, the chroma signal output from the color AGC circuit 10 is controlled so that the amplitude a 'of the video portion becomes a substantially constant optimum value, and the b 'also changes accordingly.

Therefore, even when the level of the video portion of the input chroma signal is low, recording / reproduction with a good S / N can be performed.

By the way, when the amplitude a of the video portion of the input chroma signal becomes smaller than that shown in FIG. 7A, the amplitude is amplified by the VCA 4 so that the amplitude becomes optimum, and the burst in the output signal is increased. In some cases, the signal amplitude b 'becomes too large, and the recording current becomes excessive.

In order to prevent this, a circuit of a peak detection circuit 12 and a diode 13 is provided, and a chroma signal (including a burst signal as shown in FIG. 2A) passing through the clamp circuit 6 is provided. When the output voltage exceeds the output voltage of the detection circuit 7 by a certain value or more, the diode 13 conducts to prevent the voltage input to the polarity inversion amplifier 8 from decreasing, and the gain of the VCA 4 is further increased. This prevents over recording of the burst signal.

In addition, at the time of reproduction, the reproduced chroma signal is subjected to color AGC, and then the frequency is converted back to the original 3.58 MHz signal.The color AGC circuit at this time uses the conventional circuit shown in FIG. The level of the burst signal is detected and the gain of the VCA is controlled so that the level of the burst signal in the chroma signal becomes substantially constant.

Accordingly, for example, each of FIGS. 7 to 9 (b)
As shown in (a), the signal recorded by applying the color AGC so that the chroma signal level of the video portion becomes substantially constant is returned to the original level where the burst signal level is substantially constant as shown in (a). Will be.

In that case, the color AGC circuit for recording and playback is common, and during recording, the burst extraction pulse is inverted and input to the burst gate circuit that extracts only the burst signal from the chroma signal during playback, and only the burst signal from the chroma signal is input. May be removed.

In the embodiment shown in FIG. 1, the circuit of the peak detection circuit 12 and the diode 13 may be omitted, or the clamp circuit 5 may be omitted if the chroma signal does not include a fluctuating DC component. If a VCA is used in which the relationship between the magnitude of the AGC control signal CS and the increase or decrease of the gain of the VCA 4 is used, the inverting amplifier 8 can also be omitted.

〔The invention's effect〕

As described above, the color AGC circuit in the VTR according to the present invention detects the level of the video portion in the chroma signal and controls the gain of the VCA that amplifies the chroma signal. Thus, the fluctuation of the recording current caused by the change in the level can be reduced, and the chroma signal can always be recorded with the optimum recording current with good recording efficiency.

As a result, even when the amplitude of the video portion in the chroma signal at the time of recording is small, the reproduction output can be increased, so that the S / N of the chroma signal is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention. FIGS. 2 (a), 2 (b) and 2 (c) are signal waveform diagrams for explaining the operation of the embodiment of FIG. 1, respectively. 3 is a circuit diagram showing an example of a peak value detection circuit, FIG. 4 is an explanatory diagram of a detection output by the peak value detection circuit, FIG. 5 is a circuit diagram showing an example of an average value detection circuit, FIG. 6 is an explanatory diagram of a detection output by the average value detection circuit. FIGS. 7, 8, and 9 are input signal (a) and output signal (b) of the color AGC circuit in the embodiment of FIG. FIG. 10 is a block diagram showing an example of a chroma signal processing circuit at the time of recording in a conventional color VTR, FIG. 11 is a waveform diagram showing a chroma signal of a color bar signal, and FIG. The figure shows a waveform diagram showing an example of a chroma signal of an actual color video signal. FIG. 13 shows the recording current and head of a chroma signal. An electromagnetic conversion characteristic curve diagram showing the relationship between the raw output. 1 band pass filter (BPF) 3 frequency conversion circuit 4 voltage controlled variable gain amplifier (VCA) 5 clamp circuit 7 detection circuit 8 inverting amplifier 10 color AGC circuit 11 Burst cut circuit 12 Peak detector circuit

Claims (1)

(57) [Claims] 1. A VTR for separating a color video signal into a luminance signal and a chroma signal, and separately performing signal processing for recording, a voltage-controlled variable gain amplifier for amplifying the chroma signal, and an output signal of the amplifier. And a detector for detecting the peak value or average value of the chroma signal from which the burst signal has been removed by the burst cut circuit, according to the output of the detector. A color AGC circuit in a VTR, wherein the gain of the voltage controlled variable gain amplifier is controlled to keep the output level of a chroma signal substantially constant.