JPS6074884A - Drop-out compensating circuit of video tape recorder - Google Patents

Abstract

PURPOSE:To prevent the spark noises, the noises due to the incomplete compensation, etc. from appearing on a reproduced screen by replacing a signal of a 1H period containing a drop-out with a 1H preceding signal. CONSTITUTION:A switching circuit 15 is actuated by a drop-out detecting circuit 14 which detected a drop-out contained in a reproduced signal of a video tape recorder. Then the reproduced signal which is already delayed by a 1H period by the 1st delay circuit 11 is delayed again by a 1H period by the 2nd delay circuit 12. Thus the 1H period including the drop-out is totally replaced and therefore a phase difference produced between periods before and after replacement, the spark noises due to an action delay, etc. are all produced within a flyback time. Thus such spark noises, the noises due to the incomplete compensation, etc. give no evil effect to a reproduced screen. In this way, the satisfactory drop- out compensation is always ensured.

Description

TECHNICAL FIELD The present invention relates to a dropout compensation circuit for a video tape recorder, which compensates for dropouts in playback signals of the video tape recorder on a per scanning line basis.

BACKGROUND ART In video tape recorders that use magnetic tape as a signal recording medium, if the tape is scratched or has dust attached to it, so-called dropout occurs, in which part of the signal is lost during signal playback, and the signal is not displayed on the playback screen. Spark-like noise may occur. Therefore, as a countermeasure against this type of dropout, for example, the signal before dropout occurs is delayed by a delay circuit, and the original signal is switched to the delayed signal or mixed, etc., to prevent signal loss. A so-called dropout compensation circuit has been proposed, and attempts have been made to apply it to video tape recorders. Figure 1 shows an example of a conventional dropout compensation circuit for a video tape recorder. In the dropout compensation circuit/, a changeover switch 3 is provided in the signal line of the high-frequency reproduction signal sent from the reproduction circuit 2 in the video tape recorder. The changeover switch 3 is activated by the output of the dropout detection circuit that detects the presence or absence of the dropout.
In this case, the configuration is such that either the original signal or a delayed signal obtained by delaying a part of the original signal by one horizontal scanning period is selectively taken out. Delay circuit! will be activated only when the dropout detection circuit is activated, that is, at the point in time when a dropout occurs.

By the way, the reproduction signal of the reproduction circuit 2 includes a frequency-modulated luminance signal and a low-frequency converted color signal.
Among these signals, since the luminance signal is an FM signal, it is particularly important that phase continuity is maintained.

However, since conventional dropout compensation circuits are configured to replace the original signal with a delayed signal only when a dropout occurs, it is difficult to maintain the phase continuity of the luminance signal, and the phase difference that occurs during replacement is difficult to maintain. This has the disadvantage that white or black spark-like noise occurs on the playback screen.

Also, a delay circuit! Since the delay in its own operation cannot be ignored,
This has the drawback that noise from the uncorrected portion appears on the playback screen, making the screen even more unsightly.

DISCLOSURE OF THE INVENTION The present invention eliminates the above-mentioned drawbacks, and in response to dropouts occurring in the original signal, the signal of one horizontal scanning period including the dropout is completely replaced with the signal of one horizontal scanning period before. It is an object of the present invention to provide a dropout compensation circuit for a video tape recorder which is configured to compensate for dropouts and thereby prevents noise caused by dropout compensation from appearing on the playback screen. Achieving this objective In order to avoid this problem, the present invention provides the first . a λ-th delay circuit; provided between the second delay circuits;
a changeover switch that selectively inputs either the output of the delay circuit or the output of the second delay circuit as an input to the second delay circuit; and detects the presence or absence of a dropout in the input of the first delay circuit. A dropout detection circuit is provided, and a dropout detection signal is supplied from the dropout detection circuit, and during the horizontal scanning period following the horizontal scanning period including the dropout, the changeover switch is set such that the output of the second delay circuit is The gist of this is that it consists of a switching circuit that switches to the selected side.

According to the present invention, the dropout detection circuit detects a dropout in the playback signal of the video tape recorder, and the switching circuit operates, and the playback signal, which has already been delayed by one horizontal scanning period in the first delay circuit, is switched to the first delay circuit. By delaying the signal for one horizontal scanning period again using the delay circuit No. 2, the configuration completely replaces the signal for one horizontal scanning period including the dropout. Unlike a dropout compensation circuit, compensation failures due to a phase difference before and after replacement or a delay in one main operation all occur within the retrace period, and this causes spark-like noise and noise due to compensation failure to appear on the playback screen. There is no inconvenience such as unsightliness, and excellent effects such as good dropout compensation are always possible.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to FIG. 2 and subsequent figures. FIG. 2 is a circuit configuration diagram showing an embodiment of a dropout compensation circuit for a video tape recorder according to the present invention;
FIG. 3 ((R) to F) are signal waveform diagrams of various parts of the dropout compensation circuit shown in FIG. 2, respectively.

In FIG. 2, the dropout compensation circuit 10 includes a switch 13 for switching between the first delay circuit // and the second delay circuit 12.
The configuration is such that the changeover switch 13 is switched and controlled by the idle lag out detection circuit/da and the switching circuit/3. The first delay circuit // receives the output of the reproduction circuit λ as an input, and the output of the first delay circuit /l and the output of the second delay circuit 12 are selectively switched to the second delay circuit by a changeover switch 13. Delay circuit 1. ! A connection is made to the input.

Dropout detection circuit/4! detects the output signal level of the reproduction circuit λ, and supplies a dropout detection signal to the switching circuit/3VC when the detected signal level falls below a predetermined signal level set in advance. The switching circuit/S consists of a horizontal sync separation circuit/, ffa, a 7 lip-flop circuit ISB, and a monostable multivibrator/
The switching of the changeover switch 13 is controlled by the output of the monostable multivibrator/3a.

The horizontal synchronization separation circuit /ja separates the horizontal synchronization signal included in the output of the reproduction circuit 2, and supplies the separated horizontal synchronization signal to the reset input terminal of the flip-flop circuit /jb. The flip-flop circuit 1Sb receives the dropout detection signal of the dropout detection circuit/Q to the set input terminal, and performs a single signal during the period from the time when a dropout occurs to the end of one horizontal scanning period including the dropout portion. Stable multivibrator/, supplies a set output to Sc. Monostable multi-bi break/j Q
f? , flip-frog circuit 1. The switch 13 is triggered by the trailing edge of the set output supplied by the second delay circuit t2 and supplies a switching signal to the changeover switch 13 during the horizontal scanning period so that the output of the second delay circuit t2 becomes the input of the second delay circuit t2. Switch.

Here, when there is no dropout, the changeover switch /3 is connected to the first delay circuit ii and the second delay circuit 12.
Therefore, the output of the reproducing circuit is delayed by two horizontal scanning periods within the dropout compensation circuit 10 and output.

Now, the output of the reproducing circuit 2, that is, the input of the dropout compensation circuit io, reaches the time τ as shown in FIG. 3(A).
8. τ2. It is assumed that dropouts occur at three time points τ, and dropouts at times τ and T3 occur in adjacent scanning lines.

When dropout occurs at time i, Figure 3(B)
The dropout detection signal of the dropout detection circuit shown in FIG.
Sb is a 0-flip lag flop circuit that remains set until it is reset by the horizontal synchronization signal from the horizontal synchronization separation circuit /la shown in FIG. 3(C) at the end of one horizontal scanning period including the dropout portion. The monostable multivibrator /3a supplied with the set output of j13 supplies a switching signal that lasts for one horizontal scanning period from the trailing edge of the set output to the changeover switch 13, as shown in FIG. 3(E)K. As a result, until then, the first delay circuit // and the second delay circuit /2 . The changeover switch 13, which had been connected to the changeover switch 13, is now connected to form a closed loop connecting the first delay circuit 12 and the changeover switch 13. Therefore, the signal related to the seven horizontal scanning lines before the horizontal scanning line including the dropout portion and which has just passed through the first delay circuit // is sent to the second delay circuit 12 for one horizontal scanning period. After being delayed, if it continues to be delayed in the second delay circuit 12, IF? For the horizontal scanning line in which a dropout has occurred, the delay circuit t2 of the delay circuit t2 replaces the -horizontal scanning period with the seven previous horizontal scanning lines. Therefore, unlike the conventional dropout compensation circuit in which only the dropout portion is replaced with the core portion of the seven previous horizontal scanning lines, replacement is performed over the entire -horizontal scanning period. Therefore, the boundary of the replaced signal is located within the retrace period and does not appear on the playback screen.However, since the correlation between two adjacent horizontal scanning lines is high, dropout compensation The reproduced image does not become unnatural, and it is possible to perform dropout compensation that looks natural and good.

Regarding the dropout that occurred successively at times t and τ3, a switching signal for two horizontal scanning periods from the next horizontal synchronizing signal at time T is supplied to the switching circuit /3, and the second delay circuit 12 The signal will be delayed up to three times before the first dropout occurs. That is, n
Regarding dropouts that occur continuously in the horizontal scanning lines of a book, the delay of the same signal by the second delay circuit 12 is repeated over 1 to n horizontal scanning periods.

Furthermore, regardless of the number of delays, the boundary of the replaced signal is located within the retrace period, so it does not appear on the playback screen, resulting in a natural playback image free of noise.

As explained above, according to the dropout compensation circuit 10 having the above configuration, the dropout detection circuit /<Z that detects a dropout in the playback signal of the video tape recorder activates the switching circuit /j, The reproduced signal, which has already been delayed by one horizontal scanning period in the delay circuit l/1, is
By delaying the signal again by the second horizontal scanning period using the second delay circuit I, the configuration completely replaces one horizontal scanning period including the dropout, so only the dropout portion is replaced with the signal from one horizontal scanning period ago. Unlike conventional dropout compensation circuits, compensation failures due to phase differences or operation delays before and after replacement occur all within the retrace period, and this causes spark-like noise and noise due to compensation failure to appear on the playback screen. There is no inconvenience such as unsightliness, and good dropout compensation is always possible.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing an example of a dropout compensation circuit for a conventional video tape recorder, and FIG. 2 is a circuit configuration diagram showing an example of a dropout compensation circuit for a video tape recorder according to the present invention. Figure 3 (A) or F')
are signal waveform diagrams of each part of the dropout compensation circuit shown in FIG. 2, respectively. 010... dropout compensation circuit, //... first delay circuit, 12... second delay circuit, 13... Changeover switch, /G... Dropout detection circuit, 13... Changeover circuit O

Claims (1)

[Claims] The first step is to delay the playback signal of the video tape recorder in which dropout is expected to occur by each horizontal scanning period. the first delay circuit; a changeover switch provided between the second delay circuits and selectively inputting either the output of the first delay circuit or the output of the second delay circuit; A dropout detection circuit detects the presence or absence of a dropout in the input of the delay circuit, and a dropout detection signal is supplied from the dropout detection circuit, during the horizontal scanning period following the horizontal scanning period including the dropout. changeover switch,
A dropout compensation circuit for a video tape recorder, comprising a switching circuit that switches the output of the second delay circuit to the selected side.