JPH022359B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a home video tape recorder (VTR).
This relates to signal processing.

In home VCRs with additional functions, in addition to the standard playback mode in which the tape is run at a speed specified by the standard and reproduced images are obtained, there is also a standard playback mode in which the tape is run in the forward or reverse direction at an integral multiple of the speed. There is a variable speed playback mode that allows you to obtain a playback image. However, since the tape speed changes rapidly when converting from the standard playback mode to the variable speed playback mode or from the variable speed playback mode to the standard playback mode, the time axis fluctuation of the playback signal is very large. For this reason, the automatic phase control (APC) and automatic frequency control (AFC) circuits that are conventionally used to remove the time axis fluctuations of reproduced color signals cannot absorb the large time axis fluctuations mentioned above, and when switching the reproduction mode, the automatic phase control (APC) and automatic frequency control (AFC) circuits There was a drawback that the color disappeared within seconds.

An object of the present invention is to shorten or eliminate the time for color fading during mode switching as described above.

To achieve the above object, the present invention improves the response and pull-in characteristics of the APC circuit by setting the loop gain of the APC circuit to a value suitable for each mode using a synchronized control signal when switching modes. It is characterized by taking the above-mentioned measures against color fading.

Hereinafter, the present invention will be explained in detail with reference to Examples.

The figure shows a block diagram of an embodiment in which the present invention is applied to a VTR that handles PAL video signals. In home VTRs, color subcarriers are used to reduce the effects of time axis fluctuations caused by uneven cylinder rotation, etc.
f _sc is recorded as a low frequency converted color subcarrier signal f _LSC with a low frequency of 40 f _H +1/8 f _H (f _H : horizontal synchronization frequency). Therefore, the low frequency conversion color subcarrier signal (f _LSC +
Δf)1 is the same time axis fluctuation component Δ in balanced modulator 2
Frequency conversion carrier wave with f (f _SC + f _LSC + Δf)
3 and is converted into a color subcarrier f _SC in which the time axis fluctuations of the difference frequency are canceled by a bandpass filter 4. After the level difference between channels caused by head sensitivity difference is corrected by the ACC circuit 5, the color subcarrier f _SC is output to the 2H delay line 6, which removes crosstalk components from adjacent tracks, and in the case of black and white signals. It passes through the killer circuit 7 which does not output any output, and is mixed with the luminance signal to form a PAL color composite signal. The output signals of 2H delay line 6 are each ACC
Guided by a detector 8, a phase detector 9, a killer detector 10, an ACC circuit 5, a 160f _H voltage controlled oscillator 11,
This serves as a comparison signal to obtain a control signal for controlling the killer circuit 7. In order to obtain the frequency conversion carrier wave 3 having the same time axis fluctuation component as above, the following is described.
APC circuit is configured.

First, an error signal corresponding to the phase difference between the comparison signal and the output signal of the subcarrier oscillator 12 is obtained from the phase detector 9, and the error signal is filtered by the loop filter 13 to eliminate unnecessary harmonic components and noise. After being removed, it is applied to the voltage control terminal of the 160f _H voltage controlled oscillator 11. The output of the oscillator 11 is passed through the 1/4 frequency divider 1.
4 and the horizontal synchronizing signal 15 as a comparison signal, and is branched to a decoder 16 which applies an output to a loop filter 13 when the time axis fluctuation is larger than a specified value. 1/4
The output of the frequency divider 14 is mixed by the balanced modulator 17 with the output signal of the crystal oscillator 18 having the oscillation frequency (f _SC +1/8f _H ), and is mixed with the output signal of the crystal oscillator 18 having the oscillation frequency (f SC +1/8f H ), and then by the bandpass filter 19 as the carrier wave 3 for frequency conversion.
is converted to That is, since the frequency conversion carrier wave 3 is fed back so that the 160f _H voltage controlled oscillator 11 oscillates so that it always has the same time axis fluctuation as the low frequency conversion color carrier signal 1, the time axis fluctuation is A color subcarrier f SC with no color subcarrier f _SC will be obtained.
When transitioning from the standard playback mode to the variable speed playback mode, the tape speed changes abruptly, resulting in large time axis fluctuations. However, since the time constant of the loop filter is selected so that a reproduced image with a high signal-to-noise ratio can be obtained in the standard reproduction mode, it is not possible to respond quickly to sudden large fluctuations in the time axis, and the color changes for several seconds. This is a problem because it disappears. Therefore,
In order to avoid this color fading, the loop filter 1
The output of 3 is amplified to an appropriate level by a phase inversion amplifier 20, and then passed through a switch circuit 21 to 160f _H
It is applied to the reference oscillation frequency control terminal of the voltage controlled oscillator 11. As a result, the reference oscillation frequency control terminal, which should be stable in nature, is swayed in the opposite direction to the error signal, so that an equivalently large level error signal is applied to the _160fH voltage controlled oscillator 11. The switch circuit 21 is controlled to be off in the standard playback mode and turned on in the still image playback mode by a muting control signal 22 that controls the period during which audio is erased. That is, since the muting control signal 22 is applied to the switch circuit 21 at the moment when the variable speed playback mode is set,
Since frequency control is performed quickly, the above-mentioned time for color fading is significantly reduced. In particular, the method of applying control to the reference oscillation frequency control terminal is suitable as a method of increasing the loop gain because the reference frequency changes in response to time axis fluctuations, and the frequency control sensitivity of the oscillator 11 increases significantly.

According to the present invention, the color fading phenomenon that occurs for several seconds when transitioning from standard playback mode to variable speed playback mode or variable speed playback mode in the conventional circuit configuration can be improved to only momentary or no color fading. .

[Brief explanation of the drawing]

The figure is a block diagram showing one embodiment of the present invention. 2, 17... Balanced modulator, 4, 19... Band filter, 5... ACC circuit, 6... 2H delay line, 7... Killer circuit, 8... ACC detector,
9... Phase detector, 10... Killer detector, 11
...160f _H voltage controlled oscillator, 12... subcarrier oscillator, 13... loop filter, 14... 1/4 frequency divider, 16... decoder, 18... (f _SC + 1/8f _H )
Oscillator, 20... Phase inversion amplifier, 21... Switch circuit.

Claims (1)

[Claims] 1. In an automatic phase control device for eliminating time axis fluctuations in reproduced color signals of a video tape recorder, an output signal of a loop filter is passed through a voltage control terminal of a voltage controlled oscillator, a phase inverting amplifier, and a switch circuit. A signal processing device for a video tape recorder, characterized in that the voltage is applied to a reference oscillation frequency control terminal of the voltage controlled oscillator, and the switch circuit is controlled by a signal synchronized with tape speed switching.