JP3078703B2 - Digital dropout compensator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dropout compensator, and more particularly, to a digital dropout compensator for compensating for a dropout of an FM signal reproduced by a VTR by digital processing.

[0002]

2. Description of the Related Art Generally, in a VTR, when an FM signal obtained by FM-modulating a luminance signal from a video tape is reproduced, a signal read from the video tape causes a reading leak due to the adhesion of a scratch or dust on the tape, so-called dropout. For this reason, a dropout compensator is provided.

For example, a dropout compensator disclosed in Japanese Patent Laid-Open No. 58-222682 discloses a limiter 1 for limiting the amplitude of a reproduced FM signal as shown in FIG.
And a peak detection circuit 2 for extracting an envelope of the FM signal, and a demodulation circuit 3 for demodulating the reproduced FM signal into a luminance signal.
A Schmitt circuit 4 for detecting a dropout when a detection output falls below a predetermined level and generating a detection pulse, a delay circuit 5 for delaying the demodulated luminance signal by one horizontal synchronization period, and usually a demodulation circuit And a switch circuit 6 for selecting a luminance signal output from 3 and for selecting a luminance signal delayed by one horizontal synchronization period by a delay circuit 5 based on a detection pulse generated by a Schmitt circuit 4.

Next, the operation of the conventional example will be described.

[0005] The reproduced FM signal is amplitude-limited by the limiter 1, and the amplitude-limited FM signal is demodulated by the demodulation circuit 3 to become a luminance signal.
The signal envelope is taken out and supplied to the Schmitt circuit 4. When the detection output falls below a predetermined level by the Schmitt circuit 4, dropout is detected and a detection pulse is generated. The luminance signal normally output from the demodulation circuit 3 is selected by the switch circuit 6, and if there is a dropout, the luminance signal delayed by one horizontal synchronization period by the delay circuit 5 based on the detection pulse generated by the Schmitt circuit 4 The selection compensates for the dropout.

[0006]

Recently, analog circuits have been digitized, and video signal processing circuits such as VTRs have also been digitized, and Y / C processing circuits and the like have been digitized. As described above, the conventional dropout compensator is configured by an analog circuit. In the analog circuit, when the peak detection of the FM signal subjected to the limiter is performed, a rectifier circuit using an analog low-pass filter is used. Therefore, when the dropout compensator is digitized, the time constant of the analog low-pass filter is relatively large. If the low-pass filter is digitized as it is, the circuit scale becomes large.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a digital dropout compensation which can be digitized without increasing the circuit scale by simplifying a filter for detecting an envelope. It is intended to provide a device.

[0008]

According to the present invention, an object of the present invention is to provide an A / D converter for A / D-converting a reproduced FM signal, and a luminance signal from the A / D-converted FM signal. Demodulation calculation means for demodulating the A / D converted FM
Envelope detecting means for extracting an envelope of a signal, comparing means for detecting a dropout error pulse from the extracted envelope, adjusting means for adjusting the length of the dropout error pulse, and usually the demodulation calculating means select the luminance signal demodulated by, when the error is provided with a selection switch means for selecting the luminance signal delayed by one horizontal synchronizing period by the delay means based on the drop-out error pulses the adjusting di
A digital dropout compensator, wherein the envelope
Loop detecting means for determining a predetermined value of the A / D-converted FM signal.
Data below the level is treated as negative data and
Absolute value calculating means for calculating the frequency
Averaging means for averaging and outputting the envelope of the FM signal
This is achieved by the digital dropout compensator of claim 1 , comprising:

According to the present invention, the aforementioned object is achieved by adjusting
Means for detecting the fall of said dropout error pulse
With a clipping function to start counting from
Counter means, and the value of the counter means is set in advance.
3. The apparatus according to claim 2, further comprising another comparing means for comparing the value with a predetermined value.
Digital dropout compensator.

[0010]

[0011]

According to the digital dropout compensator of the first aspect, the A / D conversion means converts the reproduced FM signal into an A / D signal.
D-converted, the demodulation operation means demodulates the luminance signal from the A / D-converted FM signal, and outputs the demodulated luminance signal via the selection switch means. The envelope detection means is A /
The envelope is extracted from the D-converted FM signal, and the comparing means detects a dropout error pulse from the extracted envelope. The adjusting means adjusts the length of the dropout error pulse. When dropout occurs, the selection switch means is switched by the adjusted dropout error pulse, and 1
A luminance signal delayed in the horizontal synchronization period is output, and the dropout is compensated. Therefore, the filter for envelope detection can be simplified by adjusting the length of the dropout error pulse, and the digitization of the dropout compensator can be realized on a small scale. Also, envelope detection
Means for controlling the A / D-converted FM signal to a predetermined level or less;
Perform absolute value operation by treating the data below as negative data
Absolute value calculation means and averages the FM signal taking the absolute value
Averaging means for outputting the envelope of the FM signal
So, with a simple configuration, it is the FM band of 8mm video
3MHz to 10MHz can be supported.
You can take out the envelope of the issue .

According to the digital dropout compensating apparatus of claim 2, the adjusting means includes the dropout error.
-Clippy that starts counting from the falling edge of the pulse
Counter means having a counting function;
With other comparing means for comparing the value with a predetermined value set in advance.
The length of the dropout error pulse
Can be extended to the actual dropout period
You.

[0013]

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a digital dropout compensator according to the present invention will be described below with reference to FIG. An object of the present embodiment is to provide a digital dropout compensator digitized on a small circuit scale.

In this embodiment, an A / D converter 11 as A / D conversion means for A / D converting a reproduced FM signal,
A demodulation operation unit 12 as a demodulation operation means for demodulating a luminance signal from the converted FM signal, and an envelope detector 13 as an envelope detection means for extracting an envelope of the FM signal A / D converted by the A / D converter 11
And a comparator 1 as first comparing means for detecting a dropout error pulse from the extracted envelope.
4, an error pulse length adjuster 15 as an error pulse length adjusting means for adjusting the length of the drop-out error pulse, and a luminance signal normally demodulated by the demodulation calculator 12 are selected. A switch 17 as selection switch means for selecting a luminance signal delayed by one horizontal synchronization period by the delay unit 16 based on the error pulse;
Is provided.

As shown in FIG. 2, the envelope detector 13 treats data below a predetermined level of the A / D-converted FM signal as negative data and performs absolute value calculation as absolute value calculation means for performing absolute value calculation. Vessel 18 and the absolute value of F
An eight-point averaging unit 19 as averaging means for averaging the M signal and outputting the envelope of the FM signal. The output of the eight-point averaging unit 19 is input to the input 1 of the comparator 14, and the reference values A and B input to the input 2 depending on whether the output of the comparator 14 is high or low.
Is provided. The error pulse length adjuster 15 includes a counter 21 as a counter having a clipping function of starting counting from the falling edge of the error pulse, and a counter 21 that compares the value of the counter 21 with a preset error pulse extension value C. And a comparator 22 as a comparing means. The eight-point averaging unit 19 includes three delay units 2 as shown in FIG.
3, 24, 25 and three adders 26, 27, 28,
And a coefficient multiplier 29.

Next, the operation of this embodiment will be described.

The A / D converter 11 converts the reproduced FM signal to A
/ D conversion. For example, when conversion is performed by an 8-bit A / D converter 11 at a sampling frequency of 28 MHz, an analog FM signal as shown in FIG.
When input to the / D converter 11, in FIG.
The sampling data indicated by the black dots is output. What is important here is that the level of the center of the amplitude of the digital FM signal is 128, which is the center of the 8-bit data. This is necessary for obtaining the envelope of the FM signal. The level of the center of the amplitude in the case becomes important. Therefore, the A / D converter 11
Although a clamp circuit is necessary before or after, the description is omitted because it is not directly related to the present invention, and the following description will be made on the assumption that clamped data is handled. The A / D-converted FM signal is sent to a demodulation calculator 12 and an envelope detector 13, and the demodulation calculator 12 demodulates the luminance signal from the A / D-converted FM signal. The demodulated luminance signal is output from the output terminal. At this time, A / A
The envelope of the D-converted FM signal is extracted.

Here, the operation of the envelope detector 13 will be described in detail.

The absolute value calculator 1 of the envelope detector 13
8, as shown in FIG. 5A, 128, which is the level at the center of the amplitude of the FM signal, is set to 0, and the absolute value of negative data of 128 levels or less is taken, and as shown in FIG. An FM signal having an absolute value is output.

For example, when the FM signal including the dropout is subjected to the absolute value processing by the absolute value calculator 18, the signal as shown in FIG. The FM signal that has been input and takes the absolute value is averaged by the 8-point averaging unit 19, and the envelope of the FM signal as shown in FIG. 6B is output. The eight-point averaging operation is the same as the principle of converting an AC current to a DC current. The reason why the eight-point averaging is used is to configure a low-pass filter as simple as possible. In order to cope with a certain 3 MHz to 10 MHz, the filter is a minimum filter capable of extracting the envelope of the FM signal within this band. Note that the sampling frequency is 28 MHz.

The envelope thus taken out is inputted to the input 1 of the comparator 14 and compared with the value inputted to the input 2. The comparator 14 outputs high when input 1 <input 2 and outputs low when input 1> input 2. When the switch 20 outputs high from the comparator 14, the reference value B is selected, and when low, the reference value A is selected.

For example, when the envelope of the FM signal as shown in FIG. 7A is input to the comparator 14, when the envelope of the FM signal is larger than the reference values A and B, a low signal is output from the comparator 14. The reference value A is output and the switch 20 selects the reference value A. When the envelope of the FM signal becomes lower than the reference value A at the time T1, the comparator 1 outputs a high signal. At time T2, when the envelope of the FM signal becomes higher than the reference value B, a low is output from the comparator 14, the switch 20 selects the reference value A, and the state returns to the initial state. Therefore, the period from time T1 to time T2 at which the comparator 1 outputs high is an error period. By this series of operations, stable detection of dropout is performed by the comparator 14 having a hysteresis characteristic.

The operation of the error pulse length adjuster 15 will now be described.

When an error pulse as shown in FIG. 8A is input from the comparator 14 to the counter 21, the counter 21 generates a clock having the same frequency as the sampling frequency of the A / D converter 11 in the low part of the error pulse. Is counted up. In the example shown in FIG. 8B, since the value of the counter 21 has already reached the clipping value D,
The counter 21 keeps outputting the value D. Note that the clipping value D is set to be larger than the extension value C of the error pulse input to the input 2 of the comparator 22. This value D
Is output from the counter 21 and is input to the input 1 of the comparator 22. Since D> C, input 1> input 2 is satisfied. As shown in FIG. Is output. While the comparator 22 outputs a low signal, the switch 17 is connected to the demodulation operation unit 12, and the luminance signal from the demodulation operation unit 12 is output.

When the output of the comparator 14 becomes high, the counter 21 is reset, and the output of the counter 21 becomes 0. Then, 0 is input to the input 1 of the comparator 22, input 1 <input 2, and the comparator 22 outputs high. From the moment the output of comparator 14 falls low,
The count-up by the counter 21 starts, and the counter output is increased to 1, 2, 3,. While the comparator 22 is outputting high, the switch 17 is connected to the delay unit 16 side, and the luminance signal delayed by one horizontal synchronization period is output by the delay unit 16 to compensate for dropout. At the moment when the value of the counter 21 exceeds the value C, the comparator 2
The output of 2 falls low. The error pulse extended by C clocks in this way is output from the comparator 22.

Next, the reason for adjusting the length of the error pulse will be described.

If there is an FM signal including a dropout as shown in FIG.
9B has a waveform as shown in FIG. 9B, and the dropout error pulse detected from this envelope is slightly shorter than the dropout period as shown in FIG. 9C. I will. However, in the analog dropout compensator, the filter for extracting the envelope uses a filter having a relatively large time constant, and the waveform of the envelope is shown in FIG.
The waveform becomes gentle as shown in FIG. 9D, and the dropout error pulse detected from this envelope becomes sufficiently long as shown in FIG. 9E. Although the waveform diagrams of the FM signal and the error pulse shown in FIG. 9 are shown on the assumption that the timing has been adjusted, actually, since the error pulse is sent from the input FM signal and detected, Timing adjustment is required. However, if a filter having a large time constant is digitized, the circuit scale becomes large. Therefore, it is necessary to simplify the filter as much as possible. Therefore, in the present invention, by extending the length of the error pulse, the configuration of the eight-point averaging unit 19, which is a filter for envelope detection, can be simplified.

Thus, the digitalization of the digital dropout compensator can be realized, the digitalization of the Y / C processing section of the VTR, the digitalization of the entire VTR,
LSI integration is possible. If the entire VTR can be digitized, an adjustment circuit and a correction circuit peculiar to an analog circuit are not required, and integration and high performance of a video device can be realized.

[0030]

According to the digital dropout compensator of claim 1, the filter for envelope detection can be simplified by adjusting the length of the dropout error pulse, and the digitization of the dropout compensator can be reduced. Can be realized on a scale. Also, with a simple configuration, 8mm video
It can support the FM band of 3 MHz to 10 MHz.
You can extract the envelope of the FM signal within the band
You .

According to the digital dropout compensator of claim 2, the length of the dropout error pulse is
It can be extended to the actual dropout period .

[0032]

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a digital dropout compensator of the present invention.

FIG. 2 is a block diagram showing a configuration of a digital dropout compensator of the present invention.

FIG. 3 is a block diagram showing a configuration of an eight-point average calculator of the digital dropout compensator of the present invention.

FIG. 4 is a diagram illustrating an A / D conversion operation according to the present embodiment.

FIG. 5 is a diagram illustrating an absolute value calculation process according to the embodiment.

FIG. 6 is a diagram illustrating an envelope detection operation of an FM signal according to the present embodiment.

FIG. 7 is a diagram illustrating an error pulse detection operation of the present embodiment.

FIG. 8 is a diagram illustrating an error pulse length adjusting operation according to the present embodiment.

FIG. 9 is a diagram illustrating a difference between detection error pulse periods due to a difference between filters.

FIG. 10 is a block diagram showing a configuration of a conventional analog dropout compensator.

[Explanation of symbols]

 11 A / D converter 12 Demodulation calculator 13 Envelope detector 14 Comparator 15 Error pulse length adjuster 16 Delayer 17 Switch 18 Absolute value calculator 19 8-point average calculator 20 Switch 21 Counter 22 Comparator

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/91-5/596 H04N 9/79-9/898

Claims (2)

(57) [Claims] An A / D converter for A / D converting a reproduced FM signal.
/ D conversion means, demodulation operation means for demodulating a luminance signal from the A / D converted FM signal, envelope detection means for extracting an envelope of the A / D converted FM signal, and Comparison means for detecting a dropout error pulse, and adjustment means for adjusting the length of the dropout error pulse,
Normally, there is provided a selection switch means for selecting a luminance signal demodulated by the demodulation operation means and selecting a luminance signal delayed by one horizontal synchronization period by a delay means based on the adjusted dropout error pulse in the case of an error. A digital drop-out compensating device , wherein the envelope detecting means includes the A / D-converted F
Data below the predetermined level of the M signal is treated as negative data.
Absolute value calculating means for performing absolute value calculation by
Averaging FM signal and outputting envelope of FM signal
A digital dropout compensator comprising: 2. The method according to claim 1 , wherein said adjusting means is configured to control said dropout.
Click to start counting from the falling edge of the error pulse.
Counter means having a tapping function;
Another comparison method for comparing the value of the step with a predetermined value set in advance.
The digital dropout compensator of claim 1, comprising a stage .