JPS6214908B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dropout erasing device for a video disc player.

A conventional dropout erasing device for a video disc player has a configuration as shown in FIG.
That is, in the same figure, 1 is the input FM signal, 2
is an FM demodulator, 3 is a low-pass filter for FM carrier suppression, 4 is a de-emphasis circuit, 5 is a dropout canceller switch, 6 is a reproduced video signal, 7 is a horizontal scanning period delay circuit, and 8 is a dropout detector. . The operation will be explained below with reference to FIG. In the figure, a is the waveform of the input video signal 9 of the de-emphasis circuit 4 described above. T _H in the figure is the horizontal scanning period, which is approximately 63.5 μsec. Further, 13 is a defect, that is, a dropout portion. Next, the figure b shows the de-emphasis circuit 4.
This is the waveform of the output video signal 10 of . The de-emphasis circuit 4 reproduces the original video signal by attenuating the high frequency components, but along with this, the waveform of the dropout section 13 is
The waveform of the dropout portion 13' changes to a rounded shape. Next, c in the same figure shows the waveform of the output pulse 11 of the dropout detector 8. The dropout canceller switch 5 in FIG. 1 is normally closed on the upper side of the figure, but it is closed on the lower side only during the period when the waveform c is at a high potential.
Replace defective parts with past video signals. next,
FIG. 2d shows the waveform of the reproduced video signal 6 obtained as the output of the dropout canceller switch 5. As can be seen from this waveform d, a residual trace 13'' of the defect remains in the reproduced video signal 6. The length of this residual trace 13'' is approximately 1 μsec, but when viewed on the reproduction monitor screen, it is approximately This was recognized as a 1cm horizontal stripe-like interference, which was deteriorating the image quality. The above points are the problems of the prior art in the dropout erasing device.

SUMMARY OF THE INVENTION An object of the present invention is to provide a dropout erasing device that can solve the problems of the prior art described above. To achieve this objective, the present invention aims to minimize the occurrence of residual traces as described above by providing a de-emphasis circuit and a pre-emphasis circuit within the dropout canceller loop. .

A basic first embodiment of the present invention will be described below with reference to FIG. In this figure, parts having the same functions as those in FIG. 1 are indicated by the same numbers, and their explanations will be omitted. Note that 12 is a de-emphasis circuit, which is provided after the dropout canceller switch 5. Further, 14 is a pre-emphasis circuit for emphasizing the high frequency components attenuated by the de-emphasis circuit 12 and restoring them to their original state. In other words, the purpose is to flatten the frequency versus amplitude characteristic of the dropout canceller loop. The operation will be explained below with reference to FIG. In the figure, waveform a is the waveform of the output 9 of the low-pass filter 3 for FM carrier suppression, and a portion 13 represents a defect, that is, a dropout portion. waveform b
is the waveform of the output pulse 11 of the dropout detector 8. Only in the high potential portion of this output pulse 11, the dropout canceller switch 5 closes to the lower side of the figure. In addition, waveform c in the same figure is output 1 of dropout canceller switch 5.
5 waveform. In this waveform, there are fewer residual traces of defective parts than in the prior art, but this is due to the dropout before the de-emphasis circuit 12.
This is because the provision of the canceller switch 5 avoids the effects of waveform rounding in the prior art. Waveform d in the figure is the reproduced video signal 6' obtained as the output of the de-emphasis circuit 12.
It can be seen that the residual traces are significantly reduced.

The basic embodiment has been described above, and modifications will now be described. The characteristics of the de-emphasis circuit in the basic embodiment described above may be divided into two parts, one of which may be placed in front of the dropout canceller switch 5, and the other part placed behind the dropout canceller switch 5, as in the prior art. Specifically, let the de-emphasis frequency characteristic be D(f), and an example thereof is given by the following equation.

This can be divided into two as follows.

Then, the characteristics of the factors in the first half of the right side of equation (2) are placed in front of the dropout canceller switch 5, and the equation
Place the characteristics of the factors in the second half of the right-hand side of (2) backwards. Correspondingly, the pre-emphasis circuit 14 shown in FIG. 3 has a characteristic that is the reciprocal of the factor in the second half of the right side of equation (2). In the above modified example, the residual traces in the output of the dropout canceller switch and the waveform of the reproduced video signal corresponding to waveforms c and d shown in FIG. 4 are different from those in the basic first embodiment described above. However, compared to the conventional technique, the length of the trace can be reduced to about 1/4, resulting in a large improvement effect.

In the above description of the modification, it has been stated that the de-emphasis circuit is at least partially located behind the dropout canceller switch 5, but the drop-out canceller switch and the rear de-emphasis circuit are integrated, and further,
It is possible to eliminate the need for the pre-emphasis circuit 14. This will be explained as a second embodiment with reference to FIG. In this figure, parts having the same functions as those in FIG. 3 are designated by the same numbers, and their explanations will be omitted.
Further, 4' in the same figure is a prefix portion of the de-emphasis characteristic related to the above-mentioned equation (2), and this may be deleted as in the case of the basic first embodiment. Resistor R ₁ , resistor R ₂ , and capacitor C constitute a de-emphasis circuit, and if its frequency characteristic is D ₂ (f), this is expressed by the following equation.

D ₂ (f)=1+j2πC(R ₁ +R ₂ )f/1+j2π
CR ₂ f (3) Therefore, it can be seen that by appropriately selecting the values of R ₁ , R ₂ and C, any de-emphasis characteristic can be provided. The reason why the pre-emphasis circuit can be omitted is that the input correction signal to the dropout canceller switch 5 is converted into a constant voltage source by an emitter follower in FIG. 5. The dropout canceller switch used in both the prior art and the present invention is an analog switch based on C-MOS IC technology that can be obtained at low cost.

Finally, we will discuss the issue of placement of the de-emphasis circuit. In FIG. 1, the de-emphasis circuit 4 is positioned after the dropout canceller loop, that is, further behind the point where the reproduced video signal 6 is branched to the horizontal scanning period delay circuit 7.
Although it is conceivable to arrange them independently, this is not preferred because of the following disadvantages. That is, when the de-emphasis circuit is placed after the dropout canceller loop, the high frequency component in the input signal to the horizontal scanning period delay circuit 7 becomes
Since it remains in an emphasized state, the amplitude becomes large, and therefore saturation distortion etc. are likely to occur in the dropout canceller loop due to constraints on the dynamic range of the horizontal scanning period delay circuit 7 and associated amplifiers. . On the other hand, if a small signal level is transmitted in an attempt to avoid this distortion, fluctuations in the DC level due to changes in ambient temperature, etc. will cause deviations between the input DC voltages of the dropout canceller switch 5, and the desired dropout canceller will This is because it becomes impossible to perform the action. Therefore, although there are drawbacks to the arrangement in which the de-emphasis circuit is placed after the dropout canceller loop, this drawback can also be avoided by the present invention.

As described above, according to the present invention, in the dropout erasing device, the length of the residual trace when the dropout erasing circuit operates, which was a problem in the prior art, can be reduced to about 1/4 or less. The effect of improving the quality of reproduced images is significant, and the structure can be realized with almost no increase in price compared to conventional techniques, so it has high industrial value.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the configuration of a dropout erasing device according to the prior art, FIG. 2 is a waveform diagram for explaining the operation of the device in FIG. 1, and FIG. 3 is a first diagram of a dropout erasing device according to the present invention. FIG. 4 is a waveform diagram for explaining the operation of the apparatus of FIG. 3, and FIG. 5 is an explanatory diagram showing the configuration of a second embodiment of the present invention. Explanation of symbols, 1...Input FM signal, 2...FM
demodulator, 3... low-pass filter, 4, 4'... de-emphasis circuit, 5... dropout canceller switch, 6, 6'... reproduced video signal,
7...Horizontal scanning period delay circuit, 8...Dropout detector, 12...De-emphasis circuit, 1
3, 13'...Dropout section, 13''...Residual trace, 14...Pre-emphasis circuit.

Claims (1)

[Claims] 1. In a dropout erasing device of an image signal reproducing device, which includes a dropout detector, a dropout erasing switch, and a horizontal scanning period delay circuit, a deemphasis circuit that shares at least a part of the characteristics of the video deemphasis circuit,
What is claimed is: 1. A dropout erasing device, which is inserted between an output terminal of a dropout erasing switch and an input terminal of a horizontal scanning period delay circuit, and is characterized in that the frequency versus amplitude characteristic of the loop of the dropout erasing circuit is substantially flattened.