JPH01190185A - Dropout compensating circuit - Google Patents

Abstract

PURPOSE:To eliminate the difference from brightness of a dropout compensation part through the addition of on inexpensive circuit and to improve picture quality by using a luminance signal in response to the brightness of a video reproduced screen just before the occurrence of dropout as a reproducing signal. CONSTITUTION:An output of a video head 1 becomes a reproduced luminance signal whose high and low frequencies are shaped by a de-emphasis circuit 6 and a low pass filter 7 respectively via a preamplifier 2 and on FM demodulator 4, mixed with a color signal from a chroma demodulator 3 at a luminance color mix circuit 12 through a terminal 8a of a changeover switch 8 and the result is outputted from a terminal 13. Then the reproduced luminance signal in response to the lightness of the inputted reproduced screen is inputted to a sample and hold circuit 11 via a mean brightness level detection circuit 9 from the filter 7. If dropout takes place, the luminance signal just before is outputted via the switch 8 from the circuit 11 by a command of a dropout detector 5 to compensate the reproducing signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a dropout compensation circuit for a video signal processing circuit of a video tape recorder (referred to as a VTR).

[Prior Art] As a method of compensating for video signal dropouts during VTR playback, there is a method of using an IH delay line;
Because of the high manufacturing costs, lower cost dropout compensation circuits that do not use IH delay lines have traditionally been used.

FIG. 4 is a block diagram showing such a conventional example,
1 is a video head of the VTR, 2 is a preamplifier connected to the video head 1 and amplifies the luminance signal of the video signal, 3 is a chroma demodulator connected to the video head 1 and demodulates the color signal of the video signal into a television signal, 4 is a chroma demodulator FM demodulator 5 is connected to the output side of preamplifier 2 and performs FM demodulation; 5 is a dropout detector connected to the output side of preamplifier 2 and detects dropout; 6 and 7 detect the signal output from FM demodulator 4; These are a de-emphasis circuit for waveform shaping and a low pulse filter. Reference numeral 8 denotes a changeover switch whose contact is switched to terminal 8a or terminal 8b in response to the output signal of dropout detector 5. Terminal 8a is connected to the output side of low pulse filter 7, and terminal 8b is connected to power supply 14. . 12 is the output terminal of the changeover switch 8 and the chroma demodulator 3
The output end of the brightness/color mixing circuit 12 is connected to the television signal input end of the video playback screen.

In the conventional video signal processing circuit configured as described above, under normal conditions when video signal dropout does not occur, the luminance signal of the video signal output from the video head 1 is amplified by the preamplifier 2, and then subjected to FM demodulation. F in vessel 4
After being demodulated, the signal is waveform-shaped by a de-emphasis circuit 6 and a low pulse filter 7 to become a reproduced luminance signal.

This reproduced luminance signal is input via the terminal 8a of the changeover switch 8 to the luminance/color mixing circuit 12 to which the color signal output from the chroma demodulator 3 is input. The brightness/color mixing circuit 12 mixes these brightness signals 9 and outputs the mixture as a television signal. On the other hand, when dropout of the video signal occurs, the dropout detector 5 detects that the output waveform of the preamplifier 2 becomes an abnormal waveform, and outputs a signal to switch the changeover switch 8 from the terminal 8a to the terminal 8b. Terminal 8b has a fixed gray brightness level (approximately 50
1RE) is input, and the selector switch 8
is connected to the terminal 8b, the reproduced luminance signal input to the luminance/color mixing circuit 12 at the time of dropout is a luminance signal of a constant luminance level, and the image quality at the time of dropout of the video playback screen is adjusted. I was correcting it.

Note that when the dropout of the video signal disappears, the output waveform of the preamplifier 2 becomes normal and the dropout detector 5 no longer outputs a signal, so the changeover switch 8 returns to the terminal 8a again. Therefore, the brightness/color mix circuit 12
The luminance signal inputted to the output terminal becomes a normal reproduced luminance signal output from the low pulse filter 7.

[Problems to be Solved by the Invention] However, in the conventional dropout compensation circuit described above, when a dropout occurs when the brightness of the video playback screen is bright or dark, the brightness signal output from the dropout compensation circuit is Since the brightness level is constant, there is a problem in that the brightness of the video playback screen and the brightness of the dropout compensation portion are significantly different, making dropouts noticeable and deteriorating image quality.

The present invention has been made to solve the above-mentioned problems, and by changing the brightness level of the brightness signal output from the dropout compensation circuit in accordance with the brightness level of the video playback screen immediately before the dropout occurs, An object of the present invention is to provide an inexpensive dropout compensation circuit capable of improving image quality when dropout occurs.

[Means for Solving the Problems] The present invention provides an average brightness level detection circuit for detecting the average brightness level of a reproduced brightness signal, and a sample for holding the output of the average brightness level detection circuit in a video signal processing circuit of a VTR. A hold circuit, a dropout detector that detects dropouts in the played video signal, and when a dropout occurs during video signal playback, the output signal of the dropout detector detects the average held in the sample and hold circuit. The present invention is characterized by comprising a circuit that outputs a brightness level during a dropout period.

[Operation] The average brightness level detection circuit detects the average brightness level of the reproduced brightness signal, outputs this average brightness level to the sample and hold circuit, and holds this average brightness level in the sample and hold circuit for a predetermined period of time.

Therefore, when a dropout occurs in a video signal, the average brightness level input to the sample and hold circuit is the average brightness level immediately before the dropout occurs. The dropout detector detects dropout and outputs a signal to switch the contact so that the sample and hold circuit outputs the signal. Therefore, when a dropout occurs, the sample-and-hold circuit outputs a luminance signal at the average luminance level immediately before the dropout occurs, and the luminance of the video playback screen and the luminance of the dropout compensator become almost the same.

[Example 1] In FIG. 1, 1 is the video head of a VTR, 2 is a preamplifier connected to the video head 1 and amplifies the luminance signal of the video signal, and 3 is connected to the video head 1 and converts the color signal of the video signal into the TV signal. 4 is an FM demodulator connected to the output side of preamplifier 2 and performs FM demodulation; 5 is a dropout detector connected to the output side of preamplifier 2; the output side of dropout detector 5 is switched. Connected to switches 8 and 10. 6 and 7 are a de-emphasis circuit and a low pulse filter that are connected to the FM demodulator 4 and shape the waveform of the output signal of the FMBH unit 4. A terminal 8a of a changeover switch 8 and an average brightness level detection circuit 9 are connected to the output side of the low pulse filter 7. Reference numeral 11 denotes a sample hold circuit that holds the signal of the average brightness level detection circuit 9, and is connected to the average brightness level detection circuit 9 via a changeover switch IO that is turned off by the output signal of the dropout detector 5. The output of the sample hold circuit 11 is input to the terminal 8b of the changeover switch 8. 12 is a brightness/color mixing circuit connected to the selector switch 8 and the output side of the chroma demodulator 3. Luminance·
The output end of the color mix circuit 12 is connected to the television signal input end of the video playback screen.

In the video processing circuit of a VTR configured as described above,
The luminance signal of the video signal output from the video head 1 is input to the preamplifier 2 and amplified. On the other hand, the chrominance signal is input to a chroma demodulator 3, and the video signal is demodulated into a television signal. The luminance signal amplified by preamplifier 2 is
The FM demodulator 4 performs FM demodulation, and the demodulated signal is waveform-shaped in the high frequency range in the de-emphasis circuit 6 and in the low frequency range in the low pulse filter 7 to become a reproduced luminance signal.This reproduced luminance signal is transmitted to the selector switch. Via the terminal 8a of 8, the brightness
The color mix circuit 12 mixes the demodulated color signal output from the chroma demodulator 3 and outputs the luminance/color mix circuit 12.
The output is input to the video output terminal 13. Further, the average brightness level of the reproduced brightness signal output from the low pulse filter 7 is detected by the average brightness level detection circuit 9,
The average brightness level detection circuit 9 outputs a signal corresponding to the brightness of the input playback screen, that is, a brightness signal that is high when the playback screen is bright and low when the playback screen is dark. Further, the average brightness level detection circuit 9 is configured to output the average brightness level signal with a slight delay in time from the timing at which the video signal is input from the low pulse filter 7.

When a video signal dropout occurs, the output waveform of the preamplifier 2 disappears from the dropout occurrence time T to the dropout disappearance time 17, as shown in waveform 20 in FIG. 2(a). At this time, the reproduced luminance signal is
The waveform becomes distorted as shown in waveform 21 in figure (b), but
As shown in the waveform 22 of FIG. 2(c), the signal output from the average brightness level detection circuit 9 is delayed in time as described above, so that the signal output from the sample 4-waveform 22 at the point when dropout occurs. The average brightness level input to the circuit 11 is the average brightness level of the reproduced brightness signal immediately before dropout occurs. A dropout detector 5 detects the occurrence of dropout in the reproduced luminance signal and outputs a dropout detection signal. This dropout detection signal causes the selector switch 8 to switch from the terminal 8a to the terminal 8b, and the selector switch IO to switch from ON to OFF. Therefore, when a dropout occurs, the signal held in the sample and hold circuit 11 is applied to the luminance/color mixing circuit 12 via the switch 8. That is, the reproduced luminance signal inputted manually to the luminance/color mixing circuit 12 is as shown in FIG.
This is not the reproduced luminance signal that has dropped out as shown in the waveform 21 of b), but the reproduced luminance signal at the average luminance level immediately before dropout that is input to the sample hold circuit 11 via the terminal 8b of the changeover switch 8. Yes, the sample hold circuit 11 has waveform 2 in FIG. 2(d).
3, the signal is output until the dropout disappears. Furthermore, when the dropout disappears,
The dropout detector 5 detects this and stops generating the signal. As a result, the selector switch 8 is switched from the terminal 8b to the terminal 8a, and the selector switch 10 is switched from OFF to ON. Therefore, the reproduced luminance signal input to the luminance/color mixing circuit becomes a normal reproduced luminance signal without passing through the dropout compensation circuit.

As mentioned above, when dropout occurs, a compensation brightness signal is output to compensate for the reproduced brightness signal, but the average brightness level of this compensation brightness signal is not a constant level as in the conventional example, but when dropout occurs. Since this is the average brightness level of the reproduced brightness signal immediately before it is generated, the brightness of the video playback screen and the brightness of the dropout compensator are almost the same. Therefore, on the video playback screen, the brightness of the video playback screen and the brightness of the dropout compensation section are extremely different from each other, which was seen in the conventional example, and it is no longer noticeable that dropout compensation has been performed, and even if a dropout occurs, the video playback screen can maintain good image quality.

Incidentally, FIG. 3 is a circuit diagram showing a specific example of the above-mentioned average luminance level detection circuit 9, changeover switch IO, and sample and hold circuit 11. 31 is a coupling capacitor, and the output line 32 of the coupling capacitor 31 is connected in parallel with the output side of a diode 33 and one end of a capacitor 34. Note that the input side of the diode 33 and the other end of the capacitor 34 are grounded. 35 is FE
The output line 32 is connected to the source of the FET 35, the input side of a diode 36 is connected to the gate, and the positive end of an amplifier 39 is connected to the drain through one end of a capacitor 37 and a resistor 38. . Also, FE
A resistor 40 is connected between the source and gate of T35, and the negative end of the amplifier 39 is connected to the output side 41 of the amplifier 39. In the above configuration, the reproduced luminance signal from the low pulse filter 7 is inputted from the input terminal of the coupling capacitor 31, and the reproduced luminance signal is inputted from the input terminal of the coupling capacitor 31. Diode 33. The average brightness level of the input reproduced brightness signal is detected by the capacitor 34. By appropriately setting the capacitance of the capacitor 34, it is possible to temporally delay the output of the signal after detection with respect to the input reproduced luminance signal. The FET 35 functions as a switch that is OFF when a dropout occurs and ON otherwise (normally) in response to a non-dropout detection signal inputted from the output side of the diode 36. The voltage of the signal delayed in time is stored in the capacitor 37 and outputted to the terminal 8b of the changeover switch 8 via the amplifier 39.

[Effects of the Invention] As detailed above, according to the present invention, when a dropout occurs, a luminance signal corresponding to the luminance of the video playback screen immediately before the dropout occurs is used as the video playback signal, so that the brightness of the video playback screen and the dropout are used as the video playback signal. The brightness of the out-compensated portion will no longer differ greatly. By adding the above-described inexpensive circuit in this way, it is possible to improve the image quality when dropouts occur.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of a video signal processing circuit and a dropout compensation circuit of the present invention, and FIG.
) is a diagram showing the output waveform of the preamplifier, Figure 2(b) is a diagram showing the waveform of the reproduced luminance signal without dropout compensation, and Figure 2(c) is the output waveform of the average luminance level detection circuit. FIG. 2(d) is a diagram showing the waveform of the reproduced luminance signal when the dropout compensation circuit of the present invention is added, and FIG. 3 is a diagram showing a detailed example of the dropout compensation circuit of the present invention. The circuit diagram shown in FIG. 4 is a block diagram illustrating a video signal processing circuit and a conventional dropout compensation circuit. 5... Dropout detector, 9...
Average brightness level detection circuit, 11...sample hold circuit.

Claims (1)

[Claims] (1) In the video signal processing circuit of a VTR, there is an average brightness level detection circuit that detects the average brightness level of the reproduced brightness signal, a sample hold circuit that holds the output of this average brightness level detection circuit, and a droplet in the reproduced video signal. If a dropout occurs during video signal playback, the output signal of the dropout detector outputs the average brightness level held in the sample and hold circuit for the duration of the dropout. A dropout compensation circuit comprising a circuit.