JPH0351158B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic color signal level control circuit (hereinafter referred to as an ACC circuit) for adjusting the level of a carrier color signal forming a color video signal to a predetermined value. Suitable for use in systems to adjust the level of reproduced color signals.
Regarding the ACC circuit.

When a color television signal is recorded on a magnetic tape by a home video tape recorder, the carrier color signal and luminance signal that form the color video signal are separated, and the carrier color signal is frequency-converted to a lower frequency band. After converting the luminance signal into a frequency modulation signal on the high frequency band side,
The mainstream is a recording format in which both are mixed and recording tracks are arranged obliquely with respect to the running direction of the magnetic tape.

In such a recording format, a recording track on a magnetic tape on which a color video signal, that is, a carrier color signal that has been converted to a low frequency band, that is, a color signal that has been converted to a low frequency band, and a luminance signal that has been frequency modulated, is capable of recording both of these signals. The magnetic tape is formed by two rotating magnetic heads supplied with the magnetic tape alternately scanning the magnetic tape at an angle with respect to the direction in which the magnetic tape travels;
In order to increase the recording density and enable long-term recording, it has been proposed to leave no gaps, or so-called guard bands, between adjacent inclined tracks, and instead allow the recording tracks to be adjacent to each other. In this case, crosstalk between adjacent recording tracks during playback becomes a problem, but by making the azimuth angles of the two rotating magnetic heads different from each other, it is possible to eliminate crosstalk between adjacent tracks related to frequency-modulated luminance signals during playback. The azimuth loss for the crosstalk component is increased, and for the low frequency conversion color signal where the azimuth loss effect is small, for example, the phase is made continuous in every other recording track. At the same time, the remaining every other recording track is inverted and recorded every horizontal period, and the reproduced carrier color signal is subjected to a predetermined phase restoration and passed through a comb-shaped characteristic filter, so that the signal from the adjacent track is inverted. The crosstalk problem is solved by making it possible to remove the crosstalk component.

When a video tape recorder reproduces a color video signal from a magnetic tape on which such recording has been performed, a luminance signal, which is a frequency modulated signal, and a low frequency converted color signal are separated from the reproduced signal from the magnetic tape. The former is demodulated by a frequency demodulator, and the latter is frequency-converted to become the original carrier color signal. At this time, since the reproduced low-pass converted color signal has frequency fluctuations due to changes in the running speed of the magnetic tape, etc., the reproduction system of the video tape recorder does not perform frequency conversion when converting the frequency of the reproduced low-pass converted color signal. In order to make the obtained carrier color signal stable with a regular frequency, the oscillation output of the local oscillator supplied to the frequency converter is changed in frequency according to the frequency change of the reproduced low-frequency conversion color signal. It is necessary to do so. Therefore, the burst signal is extracted from the carrier color signal obtained at the output side of the frequency converter, and the phase is compared with a reference oscillation output having an accurate subcarrier frequency. Based on the comparison output, the carrier color signal is An automatic phase control circuit (hereinafter referred to as an APC circuit) is provided to control the oscillation frequency of the local oscillator so that the phase of the burst signal extracted from the output matches the phase of the reference oscillation output. this
Due to the function of the APC circuit, the frequency of the oscillation output of the local oscillator changes in accordance with the frequency change of the reproduced low-pass conversion color signal, and the carrier color having a stable subcarrier frequency is output from the frequency converter. You will get a signal.

By the way, when the color television signal is based on the PAL system, that is, when it is a PAL color television signal, the burst signal in the carrier color signal has a phase of 90 degrees per horizontal period. It is possible to alternately switch between one and the other of two phases having a phase difference. Therefore, if the color video signal reproduced from a magnetic tape recorded as described above is a PAL color television signal, the reproduced low frequency converted color signal is frequency-converted to the original carrier color signal. In the APC circuit provided in connection with the frequency converter that obtains
Since the phase of the burst signal that is compared with the reference oscillation output changes by 90 degrees every horizontal period,
There is a risk that the APC circuit will not operate stably. Therefore, when a PAL color television signal is recorded on a magnetic tape, a pilot burst signal having a subcarrier frequency and a predetermined constant phase is inserted into the color video signal during recording. In the circuit, this pilot burst signal is extracted instead of the burst signal and compared in phase with the reference oscillation output.

The pilot burst signal is inserted into the color video signal, for example, at a position as shown in FIG. In Figure 1, S _v is the video signal and S _H
is a horizontal synchronizing signal, t _a is a horizontal blanking period, t _b is a front porch period, t _c is a back porch period, and _th is a horizontal synchronizing signal period. Furthermore, a burst signal S _B is present during the back porch period t _c . In such a situation, the pilot burst signal S _PB is activated during each horizontal synchronization signal period _th
The level is higher than the burst signal S _B , for example, the reference level of the burst signal.
The allowable range is 1.2 to 1.8 times.

In addition, the recording system and the reproducing system of the video tape recorder that records the color television signal on the magnetic tape as described above and reproduces it from the magnetic tape has a function for adjusting the level of the conveying color signal to a predetermined value. An ACC circuit is provided. this house,
The ACC circuit provided in the reproduction system is provided with a gain control circuit for the reproduced low-frequency converted color signal, and the level fluctuation of the carrier color signal obtained by frequency-converting the low-frequency converted color signal on the output side is detected. The detection output is configured to control the gain in the gain control circuit.

Figure 2 shows a conventional regeneration system.
The ACC circuit is shown. In this conventional ACC circuit,
A reproduced low-pass converted color signal is supplied to an input terminal 1 , and this reproduced low-pass converted color signal is supplied to a frequency converter 3 via a gain control circuit 2 . The carrier color signal having the original subcarrier frequency from the frequency converter 3 is then led to the output terminal 5 via the comb-shaped characteristic filter 4 for removing adjacent crosstalk components. It will be destroyed. Further, the carrier color signal from this comb-shaped characteristic filter 4 is supplied to a level detector 6. The level detector 6 has its control terminal supplied with a burst flag pulse that arrives at the terminal 7 corresponding to the burst signal in the carrier color signal, and performs level detection only when the burst flag pulse arrives. . Therefore, the level detector 6 detects only the level of the burst signal in the carrier color signal from the comb-shaped characteristic filter 4 and supplies its detection output to the gain control circuit 2. The level of the burst signal in the carrier color signal is relatively strictly defined, and the level detector 6 detects the level of the carrier color signal using the level of the burst signal. The gain control circuit 2 is
According to the detection output of the level detector 6, the output terminal 5
Gain control is performed so that the level of the burst signal in the carrier color signal guided by the carrier color signal takes a predetermined value. As a result, the level of the entire carrier color signal guided to the output terminal 5 is controlled to a predetermined value.

However, in this case, the burst signal in the carrier color signal obtained by frequency converting the reproduced low-pass converted color signal has a relatively small relative amplitude, and the signal-to-noise ratio (SN ratio) may not be good. As a result, the level of the burst signal may be inaccurately detected, and the level control of the carrier color signal may not be performed stably, leading to the risk that the carrier color signal may remain subject to level fluctuations.

When the color video signal reproduced from the magnetic tape is a PAL color television signal, the above-mentioned burst signal Although a pilot burst signal with a high level is inserted, the level of this pilot burst signal is not strictly regulated like the burst signal level, and is assumed to have a relatively wide variation tolerance range. ing. Therefore, even if the level of the pilot burst signal is used, it is not possible to accurately detect the level of the carrier color signal, and the pilot burst signal cannot be used in place of the burst signal in the ACC circuit. Ta.

In view of the above, the present invention provides a control method for adjusting the level of a carrier color signal having a burst signal and a pilot burst signal of a higher level, such as a carrier color signal of a PAL color television signal, to a predetermined value. , using both the burst signal level and the pilot burst signal level,
The present invention provides an ACC circuit which is suitable for use in a playback system of a video tape recorder and is capable of stable operation. Examples of the present invention will be described below.

FIG. 3 shows an example of an ACC circuit according to the present invention.
In FIG. 3, parts corresponding to those shown in FIG. 1 are given the same reference numerals as those in FIG. 1, and detailed explanation thereof will be omitted. This example is applied to the playback system of a video tape recorder, in which a carrier color signal that is played back from a magnetic tape and includes a burst signal and the aforementioned pilot burst signal is low-frequency converted. The low frequency converted color signal obtained by Then, a comb-shaped characteristic filter 4
The input terminal of the peak detection circuit 8 is connected to the output terminal of , and the cutoff frequency is set to the output terminal, for example.
A 100Hz low pass filter 9 is connected. Also,
A control end of the peak detection circuit 8 is connected to a terminal 7 to which a burst flag pulse is supplied. The peak detection circuit 8 and the low-pass filter 9 constitute a first detection circuit 10.
The output terminal of this first detection circuit 10, that is, the output terminal of the low-pass filter 9, is connected to one input terminal of an adder 11, and the output terminal of this adder 11 is connected to the control terminal of the gain control circuit 2. connected to. Furthermore, a peak detection circuit 1 is connected to the output end of the comb-shaped characteristic filter 4.
The input terminal of 2 is also connected, and this peak detection circuit 12
The cutoff frequency at the output end is, for example, 100Hz
A high pass filter 13 is connected. Further, the control terminal of the peak detection circuit 12 is connected to a terminal 14 to which a pilot burst flag pulse that arrives corresponding to the pilot burst signal in the carrier color signal obtained at the output terminal 5 is supplied. . The peak detection circuit 12 and the high-pass filter 13 constitute a second detection circuit 15. The output terminal of the second detection circuit 15, that is, the output terminal of the high-pass filter 13 is connected to the adder 1.
1 is connected to the other input terminal of 1.

Next, according to the present invention configured as described above,
The operation of the ACC circuit will be explained.

The reproduced low-pass converted color signal having a burst signal component and a pilot burst signal component, which is supplied to an input terminal 1, is supplied to a frequency converter 3 via a gain control circuit 2. And frequency converter 3
The carrier color signal containing the burst signal and the pilot burst signal, which is assumed to have the original subcarrier frequency from It is led to the output terminal 5. Further, the carrier color signal from the comb-shaped characteristic filter 4 is supplied to the input terminals of the first detection circuit 10 and the second detection circuit 15, respectively.

Then, the first detection circuit 10 detects the low frequency component of the level fluctuation of the burst signal included in the carrier color signal obtained at the output terminal 5. Here, the peak detection circuit 8 performs a peak detection operation only when a burst flag pulse arrives at the terminal 7. Therefore, the peak detection circuit 8 performs the peak detection operation only when a burst flag pulse arrives at the terminal 7. Peak detection of the burst signal in the signal is performed, thereby detecting level fluctuations in the burst signal. The output of this peak detection circuit 8 is supplied to a low pass filter 9, and the low frequency component of the level fluctuation of the burst signal is obtained at its output terminal. This is then supplied to one input terminal of the adder 11.

Further, the second detection circuit 15 detects the pilot color included in the carrier color signal obtained at the output terminal 5.
A high frequency component of the level fluctuation of the burst signal is detected. Here, the peak detection circuit 12 performs the peak detection operation only when the pilot burst flag pulse arrives at the terminal 14, and therefore the peak detection circuit 12 is supplied to the input terminal of the second detection circuit 15. Peak detection of the pilot burst signal in the carrier color signal is performed, thereby detecting level fluctuations in the pilot burst signal.
The output of the peak detection circuit 12 is supplied to a high-pass filter 13, and the high-frequency component of the level fluctuation of the pilot burst signal is obtained at its output terminal. This is then supplied to the other input terminal of the adder 11.

In this way, the low frequency component of the level fluctuation of the burst signal detected by the first detection circuit 10 and the high frequency component of the level fluctuation of the pilot burst signal detected by the second detection circuit 15 are added. Vessel 11
, and based on the output of the adder 11, the gain control circuit 2 is designed to eliminate the low frequency component of the level fluctuation of the burst signal in the carrier color signal guided to the output terminal 5, and , gain control is performed to eliminate high-frequency components of level fluctuations of the pilot burst signal. This results in
The reference level of the carrier color signal obtained at the output terminal 5 is controlled based on the level of the burst signal whose level is strictly regulated, and the variation from the reference level has a good signal-to-noise ratio. Therefore, the level fluctuations from the predetermined value are accurately detected and the level fluctuations are corrected. It becomes something that is controlled so that it disappears, and it becomes something that keeps the set level stable.

As explained above, according to the ACC circuit according to the present invention, the level of the carrier color signal can be set correctly based on the level of the burst signal whose level is strictly regulated, and the level of the carrier color signal can be set correctly. Level fluctuations from the set level can be detected accurately by using the large-level pilot burst signal obtained with This can be done extremely stably. Since the ACC circuit according to the present invention utilizes the burst signal and pilot burst signal in the carrier color signal, it is suitable for use in video tape recorders, particularly for recording and reproducing PAL color television signals. It is suitable for use in a playback system of a video tape recorder.

It should be noted that the ACC circuit according to the present invention is not limited to the scope of the above-described embodiments, and may be modified in various ways without departing from the gist of the invention, such as by removing the comb-shaped characteristic filter. Of course it's a good thing.

[Brief explanation of drawings]

Figure 1 shows the pilot signal inserted into the carrier color signal.
FIG. 2 is a waveform diagram for explaining the burst signal, FIG. 2 is a block connection diagram showing a conventional ACC circuit, and FIG. 3 is a block connection diagram showing an example of the ACC circuit according to the present invention. In the figure, 1 is an input terminal, 2 is a gain control circuit, 3 is a frequency converter, 5 is an output terminal, 8 and 12 are peak detection circuits, 9 is a low-pass filter, and 10 is a first
13 is a high-pass filter, and 15 is a second detection circuit.

Claims (1)

[Claims] 1. A gain control circuit for a carrier color signal having a burst signal and a pilot burst signal inserted at a higher level than the burst signal, and a burst included in the carrier color signal obtained at the output side of the gain control circuit. The first and second detection circuit sections detect low-frequency components of signal level fluctuations and high-frequency components of pilot burst signal level fluctuations, respectively. An automatic color signal level control circuit that performs gain control in the gain control circuit section based on the output.