JPS5940355B2 - Dropout compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a so-called dropout compensation circuit that compensates for short-time C signal defects (dropouts) from a signal source with a simple circuit configuration.

An example in which the present invention is applied to a video disc player will be described below using the block diagram of FIG. In Figure 1, 1 is a pickup circuit that picks up the signal from the video disc, and 2 is a pickup circuit that passes the signal (generally FM waves) obtained from the pickup circuit through the bandwidth required for video playback (for example, 2 to 8 MIIz). It is a band pass filter (BPF) that allows

This signal is supplied to the transmitter 3, and then the counter type F
It is demodulated by a demodulation circuit 4 such as an M demodulator and supplied to a low pass filter (LPF) 5. The signal at this point (the FM wave recorded on the disc) is converted into the AM wave normally used in TV sets. However, in order to effectively utilize the frequency bandwidth, the color signal is processed using a color printing carrier wave embedding method (BurriedSubcarrieColor).
Encoding) is combined with the luminance signal (
For this reason, the color subcarrier frequency is, for example, 1.534091.
MH2 is selected). After the signal converted to the AM wave passes through a low-pass filter (LPF) 5, it passes through a comb-shaped (co-shaped)
mb) It is supplied to a comb filter 12 through a filter 10 and an inverter 11, and is also added to an AM modulator T.

In this AM modulation circuit 7, the signal supplied from the low-pass filter 5 is oscillated by a carrier wave (carrier) oscillator 6 in order to convert it into a high-frequency signal necessary for passing through the IH (one horizontal period) delay line (IHDL) 8. For example, 5.
It is AM modulated with a carrier wave of 113636 MH2. This modulated signal is supplied to a comb filter 13 and also added to a 1H delay line 8. An AM demodulation circuit 9 consisting of a low-pass filter is a carrier wave oscillator 6
It functions to extract the 5.11MH2 carrier wave component generated in the above and demodulate and reproduce the original AM signal (however, this AM wave is delayed by IH). The comb-shaped filter 10 functions to add the output from the low-pass filter (LPF) 5 and the IH-delayed signal from the AM demodulation circuit 9 to extract a luminance signal.

That is, the color printing carrier wave embedding method functions to remove color signal components contained in the luminance signal. As is well known, this operation effectively removes color signal components that mainly exist near the color subcarrier frequency, but it has the disadvantage that it also attenuates the luminance signal that mainly exists in the low frequency range. perform an action. Therefore, here, the comb-shaped filter 12
and a low-pass filter (LPF) 16, which mainly corrects the luminance signal component for low frequencies (for example, 0.5 MHz or less).

That is, in order to compensate for the low frequency luminance signal component attenuated by the comb filter 10, the comb filter 12 transfers the output from the low pass filter (LPF) 5 to the inverter 1.
1 and the signal delayed by 1H from the AM demodulation circuit 9 are added, and this addition output is further passed through a low pass filter (LPF) 16 to extract only the low frequency component. The adder circuit 15 mainly uses the luminance signal component obtained from the comb filter 10 via the delay circuit (DL) 14 and the low pass filter (LPF) from the comb filter 12.
16 and the low frequency luminance signal component obtained via the switching circuit 19, and as a result, the addition circuit 15 obtains a luminance signal corrected for the low frequency component.

The frequency characteristics of this adder circuit 15 are shown in FIG.
Note that in this figure, part A is due to low frequency correction. Now, if a dropout occurs and the output of the low-pass filter (LPF) 5 disappears, the output of the comb filter 10 will not be able to operate normally, that is, the embedded color subcarrier cannot be removed, and the unnecessary color will be removed along with the luminance signal. Since the subcarrier is also passed through, the quality of the reproduced image is degraded, but it is sufficient as a very short-term dropout signal (as a dropout correction signal).

The same thing happens after the next 1H, and this time the signal from 1H before is defective. This means that without the low-frequency correction comb filter 12, the image quality will deteriorate, but short-term dropouts can be corrected even without providing a dropout correction circuit. However, in order to prevent image quality deterioration during normal operation, a comb-shaped filter for low frequency correction is normally required, and since the output phase of this comb-shaped filter 12 is opposite to the output phase of the comb-shaped filter 10, the addition circuit 15
(When a dropout occurs), one side of the canceling signal is missing, and if the output is the same polarity as the normal synchronization signal polarity, it is good, but if it is output with the opposite phase, the black and white image will be reversed. , if a dropout occurs during a certain 1H period, a dropout corresponding to that period will always appear on the screen (more precisely, if a dropout occurs during a certain 1H period, it will affect the 2H period screen because a comb filter is used, As mentioned above, the image quality will deteriorate, but it will be corrected).

This shows that if dropout occurs, turning off one of the two comb-shaped filters will correct the dropout as described above. That is, as shown in FIG. 1, it is sufficient to turn off the output of the comb filter in either the switching circuit 19 or 19' which outputs a signal with a polarity opposite to that in normal operation.

Therefore, the present invention has been developed by paying attention to this principle for the first time, and examples will be specifically described below.

That is, the switching circuit 19 is a circuit inserted to perform dropout correction in the present invention based on the above principle.

In FIG. 1, this switching circuit 19 is inserted between the low-pass filter (LPF) 16 and the adder circuit 15, but it may be inserted at position 19' as shown by the broken line in FIG. The selection is made depending on the polarity of the synchronization signal included in the luminance signal. The dropout detection circuit 26 is a circuit that detects the occurrence of dropout in the signal of the demodulation circuit 4, and controls the switching circuit 19 using this output.

For example, the output generated when the dropout detection circuit 26 detects the occurrence of a dropout controls the switching circuit 19 to turn it off.

Therefore, a dropout-corrected signal is obtained at the output of the adder circuit 15 in accordance with the above principle.

Note that the comb-shaped filter 13 is for color signal extraction;
7 is a band pass filter (BPF), 18 is a color signal amplification circuit, an adder circuit 20 adds a luminance signal, a color signal, and a synchronization signal to obtain a signal that can reproduce an image on, for example, an NTSC TV set; is a synchronous separation circuit, 22
A synchronizing signal is obtained by a synchronizing signal generator 22, a bandpass filter (BPF) 23, and an audio circuit 24 provide an audio output.

As described above, according to the dropout compensation circuit of the present invention, the switching circuit prevents either the luminance signal or the low frequency correction signal from being supplied to the adding circuit for adding them when a dropout occurs. This has the effect that correction can be performed appropriately.

[Brief explanation of the drawing]

FIG. 1 shows a program diagram of a video disc player embodying the present invention, and FIG. 2 shows the characteristics of a comb filter with low frequency correction. 1: Pickup circuit, 4: Demodulation circuit, 7: AM modulation circuit, 8: 1H delay circuit, 15 and 20: Addition circuit, 19
: Switching circuit, 26: Dropout detection circuit.

Claims (1)

[Claims] 1 A comb filter circuit is configured using a delay that delays the horizontal scanning time (1H), and a low frequency correction circuit is configured to separate the luminance signal and color signal, and perform low frequency compensation of the comb filter for luminance signal sampling. The provided signal regenerator is characterized in that a switching circuit for turning on and off either the luminance signal or the low frequency signal based on the dropout detection output is provided before the addition circuit for the luminance signal and the low frequency correction signal. dropout compensation circuit.