JPS6019871B2 - dropout compensator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dropout compensation device, which uses a circuit that can delay an input composite video signal over a wide band for a predetermined period synchronized with a horizontal synchronization signal in the input composite video signal, and It is an object of the present invention to provide a dropout compensation device configured to automatically adjust gain to a predetermined value.

Conventionally, an IH delay circuit that delays an input composite video signal for one horizontal scanning period (IH) has been used in a dropout compensation device that compensates for partial loss of a composite video signal reproduced from a recording medium.

However, as a delay circuit used in this conventional dropout compensator, an ultrasonic delay line using glass as a medium (hereinafter referred to as "glass delay line") was generally used, but the delay amount is constant, Additionally, the signal belt, which could cause delays, was narrow, resulting in the following problems. That is,
In a conventional dropout compensator using a glass delay line, the reproduced input composite video signal has time axis fluctuations due to, for example, tape speed deviation. In addition, since the band width is narrow, the entire input composite video signal is not delayed for a predetermined period of time, and in particular, if the reduction is less than H2 in 3, it will be significantly attenuated and will not be output. Also, the low-frequency sidebands did not pass through the glass delay line, causing resolution degradation and inversion. Furthermore, since only the input signal and the frequency modulated wave can be used, phase discontinuity occurs when it is replaced with the output of the glass delay line when a dropout occurs, and this is demodulated by the frequency demodulator, resulting in noise. Occasionally this occurred. In addition, in a dropout compensator with a feedback loop, it is necessary to prevent a level difference between the input and output signals of the delay circuit, and to prevent the signal from going through the feedback loop of the delay circuit many times when the dropout period is long. It is conceivable to adjust the loop gain of the feedback loop of the delay circuit to 1 so that the delayed signal does not gradually become saturated or become attenuated to zero.

However, it is actually extremely difficult to precisely adjust the loop gain to 1. The present invention eliminates the above-mentioned drawbacks, and an embodiment thereof will be described below with reference to the drawings.

FIG. 1 shows a block diagram of a delay circuit 28, which will be described later, and which constitutes a main part of the dropout compensator according to the present invention.
In the figure, reference numeral 1 denotes an input terminal, through which a composite video signal, for example, is supplied to a horizontal synchronization signal separation circuit 6 in a clock pulse generation circuit 2, which will be described later, and adjusted to an appropriate level by a level adjuster 3. After being adjusted, it passes through a reduction filter 4 and becomes an N-bit charge coupled device (hereinafter referred to as "N").
bit CCD) 5. The role of the low castle filter 4 is that Katsuragi of the input composite video signal causes a beat with the clock pulse frequency No. The purpose is to remove frequency components above No.fH so as not to impair image quality. Therefore, in the input composite video signal, the above N
o. If high frequency components higher than fH are not included (generally not included in simple magnetic recording/reproducing devices for household use), this reduction filter 4 may be unnecessary. On the other hand, the frequency (f
The H horizontal synchronizing signal is phase.

The signal is supplied to a clock relay loop (PLL) 7, where a clock pulse having a repetition frequency of No.fH is generated in synchronization with this horizontal synchronizing signal. This clock pulse is applied to the N-bit CCD 5 via a clock driver 8 to drive it. As a result, a composite video signal delayed by the time 7 (=N/(No·fH)) is extracted from the N-bit CCD 5, and after the clock frequency component is removed by the reduction filter 9, it is output from the output terminal 10. Here, if N=No, the delay time 7 due to the N-bit CCD 5 becomes 1/fH (=TH), so the first
The circuit shown in the figure is an IH delay circuit. Since the above clock pulse is generated by the PLL 7, it is synchronized with the phase of the horizontal synchronization signal in the input composite video signal, and even if there is a time axis variation like the reproduced composite video signal mentioned above, the N-bit CCD
The delay time 7 due to 5 follows this, so that the input composite video signal is correctly IH delayed. "As the above N-bit CCD 5, for example, by driving two columns of 455 bits with anti-phase clocks and using them in parallel, the cutoff frequency according to the sampling theorem can be Since the carrier wave frequency is twice as high as $, this delay circuit can operate from DC to 8 Hz.
It is possible to pass signals up to about 1000 yen with almost no attenuation. That is, the delay circuit constituting the present invention can also delay signals in an extremely wide band compared to the conventional ones. FIG. 2 shows a block diagram of an embodiment of the dropout compensator according to the present invention. Reference numeral 19 denotes a demodulated video signal input terminal obtained by demodulating a reproduced frequency modulated wave (the modulated signal is a video signal) from, for example, a magnetic tape, a magnetic sheet, or a disk, and has a time axis fluctuation that comes from this. The demodulated video signal taken out from the AGC circuit 29 is input to the other input terminal b' of the switching circuit 20.

Reference numeral 22 is the input terminal for the reproduced frequency modulated wave before demodulation.The reproduced frequency modulated wave input from this input terminal is supplied to the dropout detector 23, where the presence or absence of dropout is detected.Drop When no out occurs, the common terminal of the switching circuit 20 is connected to the terminal a, and the demodulated video signal from the input terminal 19 passes through the switching circuit 20 as it is and is output from the input terminal 24.
On the other hand, when a dropout occurs, the level of the reproduced catastrophic wave number modulated wave is different from that when a dropout does not occur (during normal reproduction).
At this time, the dropout detector 23 detects the occurrence of dropout and outputs a dropout detection signal. This dropout detection signal is applied as a switching signal to the switching circuit 201, and the common terminal of the switching circuit 20 is connected to the terminal b side. As a result, when a dropout occurs, the IH-delayed demodulated video signal from the AGC circuit 20 passes through the switching circuit 20 and is output from the output terminal 24. On the other hand, the demodulated video signal from the input terminal 19 has its vedestal level clamped by a vedestal clamp circuit 25a, is sliced at a predetermined DC level by a slicing circuit 26a, and its peak value is further sampled and held by a sampling and hold circuit 27a. Ru.

When dropout does not occur, the above demodulated video signal is output from the output terminal 24, while being delayed by IH in the delay circuit 28 used in the present invention explained in FIG.
The signal is supplied to terminal b of the switching circuit 20 after its level is controlled by a control signal to be described later. A
The delayed demodulated video signal outputted from the GC circuit 29 is further clamped to a predetermined level by a vedestal clamp circuit 25b, and then sliced by a slicing circuit 26b at the same DC level as the slicing circuit 26a to extract a synchronizing signal. The peak value is sampled and held by the sampling and holding circuit 27b. Each output signal of the sampling hold circuits 27a and 27b is output to the differential amplifier 3.
01 is supplied and the levels are compared here.

The output signal of the differential amplifier 30 is supplied to the AGC circuit 29 as a gain control signal. On the other hand, from the output demodulated signal of the output terminal 24, the horizontal synchronizing signal is separated by a horizontal synchronizing signal separation circuit 31, the position is adjusted by a monostable multipipulator (hereinafter referred to as MM) 32, and the pulse width is further adjusted by the input demodulating signal by MM33. The signals are adjusted to near the back porch of the video signal and supplied to the vedestal clamp circuits 25a and 25b, respectively.

Further, the output horizontal synchronization signal (assumed to be positive polarity) of the horizontal synchronization signal separation circuit 31 is transmitted to the sampling hold circuit 27.
b is applied as a sampling pulse, while AND
It is input to one input terminal of the gate 34. This AND
The other input terminal of the gate 34 receives an H level signal from the dropout detector 23 when no dropout occurs. When a dropout occurs, an L level dropout detection signal is input. The output signal of the AND gate 34 is inputted to the sampling hold circuit 27a, and when it is at H level, a sampling operation is performed. As a result, even if a dropout occurs in the horizontal synchronizing signal portion, the output of the AND gate 34 remains at the L level, and the level of the horizontal synchronizing signal before IH from the slice circuit 26a is sampled and held by the sampling and holding circuit 27a. Since the hold is continued, the level comparison is performed normally in the differential amplifier 30. In this way, the gain of the AGC circuit 29 is set such that both input signal levels of the differential amplifier 30 are equal.
Automatically and precisely controlled.

Note that other charge transfer elements such as a bucket brigade device or a variable delay line may be used in place of the CCD.
As mentioned above, the dropout compensator of the present invention has the following features:
A pulse generation circuit generates a pulse synchronized with a horizontal synchronization signal in an input composite video signal to be delayed, and the amount of delay is controlled by the output pulse of the pulse generation circuit, and the input composite video signal is synchronized with the horizontal synchronization signal period. A variable delay element that delays a period of time that is an integral multiple of , and an AG that controls the level of the output delayed composite video signal of this variable delay element according to a control signal
C circuit, the input composite video signal to be delayed is supplied to a first input terminal, the delayed composite video signal from the AGC circuit is supplied to a second input terminal, and a switching signal is released. a switching circuit that switches and outputs the composite video signal supplied to each input terminal, and a switching circuit that detects whether or not a dropout occurs in the input composite video signal that should be delayed, and if dropout does not occur, outputs the composite video signal that is supplied to each input terminal. a dropout detector that applies a switching signal to the switching circuit so as to output the composite video signal as is, and output the input composite video signal delayed by the upper variable delay element when a dropout occurs; a synchronization separation circuit that separates a horizontal synchronization signal from the output composite video signal; and a synchronization separation circuit that clamps the vedestal level of the input composite video signal based on a pulse generated from the output horizontal synchronization signal of the synchronization separation circuit. a second clamping means for clamping the vedestal level of the output delayed composite video synchronization signal of the AGC circuit based on a pulse generated from the output horizontal synchronization signal of the synchronization separation circuit; and the dropout detection means. A gate circuit is provided with the output signal of the device and the horizontal synchronization signal from the synchronization separation circuit, and outputs the horizontal synchronization signal as a sampling pulse only when no dropout occurs, and the sampling pulse from this gate circuit is used to a first sampling and holding means for sampling and holding a synchronizing signal portion in the output signal of the first clamping means; and a synchronizing signal in the output signal of the second clamping means using a horizontal synchronizing signal from the synchronization separation circuit; A second sampling and holding means that samples and holds the portion and both output signals of the first and second sampling and holding means are compared in level, and a signal corresponding to the difference is applied to the AGC circuit as the control signal. First
and a differential amplifier that controls the gain so that both output signal levels of the second sampling and holding means are equal to each other, so even if the input composite video signal has time axis fluctuations, it is delayed by following this. Therefore, it is possible to compensate for dropouts without causing a phenomenon such as double image shift, and when a charge transfer element is used as the variable delay element, an ultrasonic delay line using glass as a medium can be used. Compared to conventional delay devices, it is possible to compensate for dropouts by using delayed signals from direct current to high frequencies, thereby improving resolution and preventing inversion phenomena. When only a frequency modulated wave can be used as an input signal, it is possible to prevent the generation of noise due to phase discontinuity caused by replacing the output of a glass delay line when a dropout occurs as in the past.

The dropout can be compensated for, and the loop gain can be accurately and automatically adjusted to "1", and even if a dropout occurs in the horizontal sync signal part, in that case, the gate circuit can adjust the loop gain to "1". Since the sampling and holding means 1 continues to perform hold operation, it is possible to automatically and accurately control so that both input signal levels of the differential amplifier are equal, that is, the loop gain is 1. It has features such as the ability to BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a delay circuit constituting a dropout compensator according to the present invention.

Figure 2 shows the system diagram of the system according to the present invention. 1 is a block diagram showing an embodiment of a dropout compensator; FIG. 1...
...Video signal input terminal, 5...N-bit charge coupled device (N-bit CCD), 10...
...Delayed signal output terminal, 19...Demodulated video signal input terminal, 20...Switching circuit, 22...
- Reproduction frequency modulated video signal input terminal, 23...Dropout detector.

Figure 1 Figure 2

Claims (1)

[Claims] 1 A pulse generation circuit that generates pulses synchronized with a horizontal synchronization signal in an input composite video signal that may have time axis fluctuations reproduced from a recording medium, and the amount of delay is controlled by the output pulse of the pulse generation circuit. a variable delay element that delays a composite video signal outputted to an output terminal for a period that is an integral multiple of the horizontal synchronizing signal period; and a variable delay element that controls the level of the delayed composite video signal taken out from the variable delay element in accordance with a control signal. an AGC circuit, and the input composite video signal is a first
The delayed composite video signal from the AGC circuit is supplied to the input terminal of the AGC circuit, and the composite video signal supplied to either one of the input terminals is supplied to the second input terminal according to the applied switching signal. A switching circuit selectively outputs the signal to the output terminal and the variable delay element, and detects the presence or absence of dropout in the input composite video signal, and the switching circuit selects and outputs the signal to the first input terminal when dropout does not occur. a dropout detector for selectively outputting the incoming input composite video signal and applying a switching signal to the switching circuit so as to selectively output the delayed composite video signal that has entered the second input terminal when a dropout occurs; , a synchronization separation circuit that separates and filters a horizontal synchronization signal from the composite video signal output to the output terminal, and a pedestal level of the input composite video signal based on a pulse generated from the output horizontal synchronization signal of the synchronization separation circuit. a first clamping means for clamping, a second clamping means for clamping the pedestal level of the output delayed composite video signal of the AGC circuit based on a pulse generated from the output horizontal synchronization signal of the synchronization separation circuit, and the dropout detection. a gate circuit which is supplied with the output signal of the device and the horizontal synchronization signal from the synchronization separation circuit and outputs the horizontal synchronization signal as a sampling pulse only when no dropout occurs; a first sampling and holding means for sampling and holding a synchronizing signal portion in the output signal of the first clamping means; and a synchronizing signal portion in the output signal of the second clamping means by the horizontal synchronizing signal from the synchronization separation circuit. A second sampling and holding means that samples and holds the second sampling and holding means compares the levels of both output signals of the first and second sampling and holding means, and applies a signal corresponding to the difference to the AGC circuit as the control signal. A dropout compensating device comprising: a differential amplifier that performs gain control so that the output signal levels of the first and second sampling and holding means are respectively equal.