JPH0427287A - Drop-out compensating circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of a gradation difference in the vicinity of a compensating part cause by a clamp processing by inputting a necessary DC level to a clamp circuit at the time when drop-out is detected by a first drop-out compensating switch, and outputting it after executing its 1H delay. CONSTITUTION:When drop-out of a reproducing luminance signal is detected by a drop-out detecting circuit 14, a first and a second drop-out compensating switches 8, 11 are switched to a contact (b). Subsequently, even if a DC voltage of a level corresponding to an average value of the reproducing luminance signal from a voltage source 9 by the switch 8 is applied and an abnormal waveform is generated by drop-out, a DC waveform being free from distortion is clamped to a clamp circuit 10, and outputted through a 1H delay element 12 of the switch 11. Accordingly, even if a drop-out processing is executed by the clamp, there is no waveform distortion, and it is prevented that a gradation difference appears in the vicinity of a drop-out compensating part caused by the clamp.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a video luminance signal processing circuit included in a video signal magnetic recording/reproducing device such as a video recorder, which performs dropout compensation in a luminance signal reproduction system. The present invention relates to a dropout compensation circuit.

[Conventional technology]

When a video signal is reproduced with a conventional video tape recorder (hereinafter abbreviated as VTR), if there is a scratch on the surface of the magnetic tape, information on the screen at the location of the scratch is lost. This is called dropout.

When this dropout occurs, a trail appears on the screen. To compensate for this, typically
When a dropout occurs using a 1H delay element that delays a video signal by one horizontal period, signal compensation is performed by switching from the original signal to a signal delayed by 1H period. This compensation operation is generally called dropout compensation.

FIG. 3 shows a block diagram of a luminance signal reproduction system of a conventional video luminance signal processing circuit equipped with a dropout compensation circuit. In FIG. 2, a reproduced FM luminance signal input to input terminal 1 is supplied to limiter circuit 2. In FIG. The FM signal recorded on the magnetic tape of the VTR is amplified by a head amplifier circuit (not shown), but this signal, that is, the signal input to the limiter circuit 2, is amplified by the frequency characteristics of the electromagnetic conversion system and head amplifier circuit. becomes an AM modulated signal. However, when AM modulation components are included, the C/N ratio (
(ratio of carrier component to noise component) deteriorates. The limiter circuit 2 described above removes this AM modulation component to improve the C/N ratio.

The output of this limiter circuit 2 is supplied to an FM demodulation circuit 3. The output of this FM demodulation circuit 3 is supplied to a first low pass filter (hereinafter abbreviated as LPF) 4 for removing FM carriers. This FM demodulation circuit 3 and the first LPF 4
The carrier component of the reproduced FM luminance signal is removed, and the reproduced video signal is demodulated and supplied to the de-emphasis circuit 5.The video signal is subjected to an emphasis circuit (not shown) before being FM modulated during recording. Since the signal components in the high frequency range are emphasized to improve the S/N ratio deterioration caused by triangular noise in the FM modulation #A/demodulation system, the de-emphasis circuit 5 converts the video signal into a frequency characteristic of the original signal. The high-frequency signal components are attenuated in order to return the signal to the non-linear de-emphasis circuit 6.

Furthermore, since the high-frequency low-level components of the signal supplied to the nonlinear de-emphasis circuit 6 are easily lost due to the frequency characteristics of the electromagnetic conversion system of the VTR, F
Before being subjected to M modulation, a non-linear emphasis circuit (not shown) emphasizes the frequency characteristics of low-level signal components in the high range, and during playback, a non-linear de-emphasis circuit 6 is used to restore the original signal. Processing is performed with frequency characteristics opposite to those during recording, and the signal is supplied to the second LPF 7.

This second LPF 7 performs timing alignment with the reproduced chroma signal and removes carrier leak during FM demodulation, and supplies the signal to the clamp circuit 10 . The output of the clamp circuit 10 is input to a first input terminal a of a dropout compensation switch 11, and the output of this dropout compensation switch 11 is input to an output terminal 13 and a 1H delay element 12 that delays the video signal by one horizontal period. Supplied. 1H
The output of the delay element 12 is supplied to the second input terminal of the dropout compensation switch 11.

In addition, the reproduced FM luminance signal input from input terminal 1 is
The signal is supplied to the dropout detection circuit 14. The output of the dropout detection circuit 14 is supplied as a control input to the dropout compensation switch 11, and when a dropout of the reproduced FM signal is detected, the first input terminal selection state is switched to the second input terminal selection state. ,
The signal at the second input terminal, that is, the output signal from the 1H delay element 12 is supplied to the output terminal 13. When dropout of the reproduced FM luminance signal is not detected, the signal at the first input terminal a, that is, the output signal of the clamp circuit 10 is supplied to the output terminal 13.

[Problem to be solved by the invention]

As mentioned above, the dropout compensation is performed by the clamp circuit 10.
In the configuration performed in the latter stage, if a dropout occurs due to scratches on the magnetic tape, the information of the reproduced FM luminance signal is lost, and the noise generated in the reproduced FM luminance signal processing path is FM demodulated. Therefore, the demodulated video signal exhibits an abnormal waveform when dropout occurs. If the video signal with this abnormal waveform is clamped as it is in the clamp circuit 10 and the dropout compensation switch 11 is switched to compensate with the video signal delayed by 1H, the switching point of the dropout compensation switch 11 (the compensation Since a discontinuous point is created in the DC component at the switching point in the direction of release, a large difference in shading appears at that part on the TV screen.
There was a problem that it was noticeable on the TV screen.

FIG. 4 is a time chart showing how dropout compensation is performed and how a discontinuous point of DC component occurs in a video signal due to dropout compensation. FIG. 4(5) shows the reproduced FM luminance signal, and T shows the dropout period. FIG. 4(g)) shows the reproduced luminance signal after FM restoration, and the waveform becomes abnormal during the dropout period T. FIG. 4 fc] shows the reproduced luminance signal after the clamping process, in which the DC potential suddenly changes due to the occurrence of dropout. FIG. 4 fdl shows the output signal of the 1H delay element 12. FIG. 4 F81 shows the reproduced luminance signal appearing at the output terminal 13, in which the DC potential is discontinuous due to the occurrence of dropout.

An object of the present invention is to provide a dropout compensation circuit that can prevent differences in shading from occurring near a dropout compensation location on a screen due to clamp processing.

[Means to solve the problem]

The dropout compensation circuit of the present invention includes a first dropout compensation switch to which a reproduced luminance signal after FM demodulation is supplied to a first input terminal, and an average value of the reproduced luminance signal after FM demodulation. a voltage source that supplies a DC potential to a second input terminal of the first dropout compensation switch; and a second voltage source that supplies the output of the first dropout compensation switch to the first input terminal through a clamp circuit.
a dropout compensation switch; and a 1H delay element that delays the output of the second dropout compensation switch by one horizontal period of the video signal and supplies the delayed output to the second input terminal of the second dropout compensation switch. , playback F before demodulation
a dropout detection circuit that detects a dropout of the M luminance signal and switches the first and second dropout compensation switches from a first input terminal selection state to a second input terminal selection state during a dropout detection period. .

[For production]

According to the configuration of the present invention, when dropout of the reproduced FM luminance signal is not detected by the dropout detection circuit, the first and second dropout compensation switches are switched to the first input terminal side. As a result, the reproduced luminance signal after FM demodulation is subjected to clamp processing by the clamp circuit and then output as a reproduced luminance signal.

On the other hand, when a dropout of the reproduced FM luminance signal is detected by the dropout detection circuit, the first and second
The dropout compensation switch is switched to the second input terminal side. As a result, when dropout is detected, the output of the 1H delay element is output as a reproduced luminance signal, and dropout compensation is performed. At this time, the clamping process in the clamp circuit is performed on the DC potential corresponding to the average value of the reproduced luminance signal after FM demodulation, and the reproduced luminance signal after demodulation may have an abnormal waveform due to dropout occurrence. Even if the first and second dropout compensation switches are switched, there is little variation in the DC component of the signal after the clamp processing, and after the dropout is eliminated, the reproduced luminance signal after the FM demodulation is not subjected to the clamp processing. This reduces fluctuations in the DC component of the reproduced luminance signal when it is output.
This makes it possible to reduce waveform distortion caused by dropout compensation. In other words, it is possible to prevent a difference in shading from occurring near the dropout compensation location on the screen due to the clamp processing.

〔Example〕

FIG. 1 shows a block diagram of a luminance signal reproduction system of a video luminance signal processing circuit equipped with a dropout compensation circuit according to an embodiment of the present invention. In FIG. 1, the reproduced luminance signal output from the LPF 7 is as follows: Dropout compensation switch 8
is supplied to the first input terminal a of. The second input terminal of the dropout compensation switch 8 is connected to a voltage source 9 that generates a DC potential corresponding to the average value of the reproduced luminance signal input to the first input terminal a of the dropout compensation switch 8.
Voltage is applied from The output of the dropout compensation switch 8 is then supplied to the clamp circuit 1°. The output of this clamp circuit 10 is supplied to a first input terminal a of a dropout compensation switch 11. The output of the dropout compensation switch 11 is output from the output terminal 1.
3 and 1H delay element 12, and 1H delay element 1
2 output is the second output of the dropout compensation switch 11.
Supplied to the input terminal. Further, the reproduced FM luminance signal supplied to the input terminal 1 is supplied to the dropout detection circuit 14, and the output of the dropout detection circuit 14 is supplied to the dropout compensation switch 811 as a control input.

The configuration from the limiter circuit 2 to the LPF 7 is the same as that of the conventional example.

Next, the operation of the dropout compensation circuit in this video luminance signal processing circuit will be explained.

Normally, when dropout of the reproduced FM luminance signal is not detected by the dropout detection circuit 14, the dropout compensation switch 8.11 is switched to the first input terminal a side. As a result, the reproduced luminance signal after FM recovery SFL,
That is, the output signal of the LPF 7 is subjected to clamp processing in a clamp circuit 810 and then sent to the output terminal 13 as a reproduced luminance signal.

On the other hand, when dropout of the reproduced FM luminance signal is detected by the dropout detection circuit 14, the dropout compensation switch 8.11 is switched to the second input terminal side. As a result, when dropout is detected, the output of the 1H delay element 12 is output as a reproduced luminance signal, and dropout compensation is performed. At this time, the clamping process in the clamp circuit 10 is performed on the DC potential corresponding to the average value of the demodulated reproduced luminance signal input to the first input terminal a of the dropout compensation switch 8. Even if the reproduced luminance signal after demodulation exhibits an abnormal waveform due to the occurrence of dropouts, there is little variation in the DC component of the signal after clamping before and after switching the dropout compensation switch 8.11, eliminating dropouts. After that, when the reproduced luminance signal after FM demodulation is subjected to clamp processing and output, it is possible to reduce fluctuations in the DC component of the reproduced luminance signal, thereby reducing waveform distortion due to dropout compensation. In other words, it is possible to prevent a difference in shading from occurring near the dropout compensation location on the screen due to the clamp processing.

FIG. 2 is a time chart showing the state of dropout compensation in this embodiment. Figure 2 (at is playback F
M indicates the luminance signal and T indicates the dropout period. Figure 2 (bl indicates the reproduced luminance signal after FM demodulation,
During the dropout period T, the waveform is abnormal. Figure 2 (C1 shows the output signal of the dropout compensation switch 8, and the waveform during the dropout compensation period is not abnormal and is fixed at a constant potential. Figure 2 (d)
indicates the output signal of the 1H delay element 12. Figure 2 t
el indicates the reproduced luminance signal appearing at the output terminal 13, and indicates that the DC potential does not become discontinuous even if dropout occurs.

〔Effect of the invention〕

In the dropout compensation circuit of the present invention, a dropout compensation switch is provided at the front stage of the clamp circuit, and during the dropout compensation period, a DC potential corresponding to the average value of the reproduced luminance signal after FM demodulation is input to the clamp circuit, and the dropout compensation circuit is Since the noise output that is FM demodulated at a high level due to the occurrence of out is prevented from being input to the clamp circuit,
Malfunctions due to noise in the clamp circuit can be prevented, and the second dropout compensation switch
Even if dropout compensation is performed by switching with the output signal of the delay element, the waveform will not be distorted, and it is possible to prevent differences in shading from occurring near the dropout compensation location on the screen due to clamp processing.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a video luminance signal processing circuit equipped with a dropout compensation circuit according to an embodiment of the present invention, FIG. 2 is a timing chart of each part of FIG. 1, and FIG. 3 is a conventional dropout compensation circuit. FIG. 4 is a block diagram of a video luminance signal processing circuit equipped with a video luminance signal processing circuit, and FIG. 4 is a timing chart of each part of FIG. 1...Input terminal, 2...Limiter circuit, 3...F
M demodulation circuit, 4... LPF, 5... De-emphasis circuit, 6... Non-linear de-emphasis circuit, 7
...LPF, 8...dropout compensation switch, 9...voltage source, lO...clamp circuit, 11...
・Dropout compensation switch, 12...1H delay element, 13...output terminal, 14...dropout detection circuit

Claims (1)

[Claims] a first dropout compensation switch to which a reproduced luminance signal after FM demodulation is supplied to a first input terminal; and a DC potential corresponding to an average value of the reproduced luminance signal after FM demodulation to the first dropout compensation switch. a voltage source to be applied to a second input terminal of the first dropout compensation switch; a second dropout compensation switch whose output from the first dropout compensation switch is supplied to the first input terminal through a clamp circuit;
The output of the dropout compensation switch is set to 1 of the video signal.
a 1H delay element that delays by a horizontal period and supplies the signal to the second input terminal of the second dropout compensation switch; and a dropout detection circuit that switches a second dropout compensation switch from a first input terminal selection state to a second input terminal selection state.