JPS6349310B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dropout detection device for an optical playback device for a video disc or the like.

If there is a dropout of a recording signal on a video disk or the like, it will disturb the reproduced image, so it is necessary to detect and compensate for this. The present invention is for detecting dropout in an FM demodulation circuit using a pulse count type FM detector.

Figure 1 is a block diagram of a pulse count type FM detector.The FM signal In enters the limiter 6,
The waveform is shaped into an output signal A, which is supplied to the line receiver 7 (non-inverted) and the line receiver 8 (inverted). The output signal B of the line receiver 7 is sent to the delay circuit 9 and the NAND circuit 12, and the output signal C of the line receiver 8 is sent to the delay circuit 10.
Each is supplied to the NAND circuit 11. Delay circuits 9 and 10 delay input signals RO and H, respectively, to output signals RO' and H'.
NAND circuits 11 and 12 output a NAND of their respective input signals L' and H or L and H'.
The outputs of NAND circuits 11 and 12 are further NAND
The signal is supplied to a low-pass filter 14 via a circuit 13, and taken out as a detection output _O1 . on the other hand
Part of the output signal of NAND circuits 11 and 12 Y
and X are supplied to the dropout detection device 15 according to the present invention, and outputted as a dropout correction signal _O2 . A block diagram of the dropout detection device 15 is shown in FIG. X, Y are
Since it is a pulse train signal that corresponds to the zero cross point of the rising or falling edge of the FM signal, both can be combined.
By passing it through the NAND circuit 16, it is possible to obtain a pulse train signal corresponding to the zero-crossing points of both the rising and falling edges of the FM input signal waveform as shown in FIG. This signal is supplied to the detector circuit 17 (the output of the NAND circuit 13 may be added to the detector circuit 17), but the detector circuit 17 is composed of a retrigger type one-shot multi, such as a monostable multivibrator. Ans(+,
2nd) outputs a high level signal.

Therefore, if the signal pulse does not exist for a period of Ans or more, it shifts to a low level, so the output signal has a waveform as shown in FIG. 3, and the low level period can be treated as the first control signal. is its opposite phase output signal. The signal is supplied to a switch circuit 18 composed of a retrigger type one-shot multi such as a monostable multivibrator, and an output signal is obtained which transitions from a low level to a high level at the rising edge of the signal and then maintains the Bns high level. However, the output signal is an output signal with an opposite phase, and the low level period can be treated as a second control signal.

On the other hand, the output signal is such that only the trailing edge of the first control signal is Cns by means 19 for delaying the trailing edge.
It is delayed and becomes an output signal via the inverter 20. As means for delaying only the trailing edge in this manner, well-known means such as an instantaneous action delay return relay described in Japanese Utility Model Publication No. 40-20070 can be used. signal and signal
Output signal by supplying to NAND circuit 21
O ₂ (dropout correction signal) can be obtained. Therefore, the high level time of the output signal _O2 is T _D -A+B (A<B), where T _D is the dropout time. Here, A is a value slightly larger than 1/2fmin, where fmin is the lowest frequency of the input FM signal. B is the time during which the detection output is slightly delayed as it passes through the low-pass filter 14 and other subsequent predetermined circuits, and the noise generated in the video signal itself due to dropout occurs even after the dropout period _TD has elapsed. This is the time (approximately 1 μs to 2 μs) to correct the Nao C
Actually, the switch circuit 18 responds with a slight delay from the rising edge of the signal, so this is the time required to compensate for this. If the trailing edge of the signal is made into a signal without being delayed by Cns by the delaying means 19, the signal _O2 obtained by supplying the signals and to the NAND circuit 21 will be slightly different from the output signal of the switch circuit 18. It falls with a delay, resulting in a false low level region in the middle.

As described above, according to the present invention, a dropout correction signal that accurately corresponds to the dropout period can be obtained, so that dropout can be detected regardless of whether the dropout period is long or short. In addition, since the same signal that is supplied to the low-pass filter of the pulse count FM detector is used as the detection signal, the dropout noise that actually appears in the detection output O ₁ and the dropout correction signal O ₂ There is a good correlation between the dropout correction signal and the dropout correction signal. Moreover, the time required for dropout detection is a little more than 1/2 of the longest period of the FM input signal, so the response is quick, and an accurate correction signal can be obtained without including pulse noise in the dropout correction signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a pulse count type FM detector, FIG. 2 is a block diagram of a dropout detection device of the present invention, and FIG. 3 is an explanatory diagram of waveforms. ~...Each waveform diagram, 6...Limiter,
7, 8... Line receiver, 9, 10... Delay circuit, 11 to 13... NAND circuit, 14...
Low pass filter, 15...dropout circuit, 16...NAND circuit, 17...detector circuit, 18...switch circuit, 19...means for delaying the trailing edge of the input waveform, 20...inverter, 21...NAND circuit , O ₁ ...Detection output, O ₂
...Dropout correction signal, In...FM signal.

Claims (1)

[Claims] 1 means for generating a pulse train signal corresponding to each zero cross point in response to an FM input signal and outputting a pulse train signal; a first control signal generating means that detects when the next pulse is not supplied for a long time and generates the first control signal from this point until the next pulse is supplied; a second control signal generating means for generating a second control signal for a predetermined period of time from the end point of the first control signal; and means for delaying the trailing edge of the first control signal for a predetermined period of time; A dropout detection device comprising logic circuit means for generating an output signal while the first control signal output and the second control signal output are being generated.