JPH0363874B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is directed to the prevention of dropouts and noise bands in video signals in playback circuits related to recording and playback devices such as helical scan type magnetic recording and playback devices (hereinafter abbreviated as "VTR"). The present invention relates to a video signal processing circuit for reducing the problem.

[Background of the invention]

FIG. 1 is a block diagram showing an example of a video signal processing circuit of a conventional home VTR, in which 1 is a magnetic head, 17 is a preamplifier, 2 is a high-pass filter and peaking circuit, 3 is a dropout compensation circuit, and 4 is an FM 5 is a demodulation circuit, 5 is a mixing circuit 6 is a low-pass filter, and 7 is a frequency conversion circuit. The signal read out by the magnetic head 1 is amplified by a preamplifier 17, and its output a is divided by a high-pass filter and peaking circuit 2 and a low-pass filter 6 into an FM luminance signal b and a low-frequency conversion carrier color signal i. The FM luminance signal b is supplied to the FM demodulation circuit 4 via a dropout compensation circuit (hereinafter abbreviated as "DOC") 3, where a luminance signal c is obtained.
On the other hand, the low frequency conversion carrier color signal i is converted into the carrier color signal d in the standard frequency band by the frequency conversion circuit 7, and the carrier color signal d and the luminance signal c are mixed in the mixing circuit 5 to form the color video signal v. can get.

An example of the DOC circuit 3 is shown in detail in FIG. 2 within the dashed lines. In FIG. 2, 11 is an amplitude detection rectifier circuit, 12 is a comparator, 13 is a switch drive circuit, 14 is a switching circuit, and 15 is a
This is a 1H (one horizontal scanning period) delay line. The detection rectifier circuit 11 and the comparator 12 constitute a dropout detection circuit (hereinafter abbreviated as "DO-DET"). The FM brightness signal b is subjected to amplitude detection by a detection rectifier circuit 11, and a comparator 12 detects a portion of the FM brightness signal b that has decreased below a predetermined level, that is, a dropout. Detection level is usually -
It is set to about 20 dB, and is determined by the comparator 12, for example. When a dropout occurs, the dropout detection signal, which is the output of the comparator 12, is applied to the switching circuit 14 via the drive circuit 13, thereby switching the switching circuit 14 to the opposite position as shown in the figure, and delaying it by 1H by the delay line 15. Replace it with a signal.
In this way, the DOC circuit 3 outputs a luminance signal h without dropout.

By the way, in addition to normal playback that plays back at the standard speed, the so-called special playback function that runs the tape at a speed different from the standard speed and plays it back is an important function, but in this case, noise may be locally concentrated. (hereinafter referred to as "noise bands"), which impairs image quality. For example, in the case of high-speed picture search (hereinafter referred to as "search"), as shown in Figure 3,
In order to diagonally cross tracks A and B, which were recorded alternately with two heads with different azimuths, and to play them back alternately with two heads with different azimuth angles (in the example shown, track B is recorded alternately with the head and azimuth). Because the angles are different, a noise band portion is generated in the reproduced signal as a portion close to a dropout, as shown in FIG. 4a. However, the conventional DOC circuit described above cannot detect and compensate for this noise band. This is because, as mentioned above, in the conventional dropout compensation circuit, the dropout detection level is set as deep as about -20 dB, so it is usually not possible to detect a level drop due to a noise band of about -10 dB to -6 dB.
In addition, in Fig. 1, the reproduction peaking center frequency of the peaking circuit 2 is normally the white carrier of the FM luminance signal (the carrier frequency corresponding to the white peak level).
Since it is set near the white carrier, among the noise components in the noise band or dropout portion, those near the white carrier are emphasized. For this reason, Fig. 4b
As shown in , a white noise band appears in the demodulated video signal.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a video signal processing circuit for a recording/playback apparatus that overcomes the drawbacks of the prior art described above and reduces noise bands during special playback with high performance and low cost.

[Summary of the invention]

In order to achieve this object, the present invention provides, in addition to the conventional dropout compensation circuit, a noise band detection circuit that detects a noise band at a level shallower than the detection level of the dropout compensation circuit. The present invention is characterized in that the amplitude of the video signal output from the dropout compensation circuit is limited by the noise band detection signal obtained from the band detection circuit, and the noise band of the video signal is removed.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a block diagram of one embodiment of the present invention, in which the same parts as in FIG. 1 are given the same numbers.
The difference from the conventional example shown in the figure is that a dropout detection circuit 8 (hereinafter referred to as the DO-DET circuit) for noise band detection, a special reproduction mode detection circuit (selection circuit) 9, and an amplitude limiting circuit 10 for the reproduced luminance signal are added. That's what I did. An example of setting the detection level is shown in FIG. The detection level of the noise band detection circuit 9 is
The detection level (for example, -10dB to -20dB) of the DO-DET circuit in DOC circuit 3 is shallower (for example, -20dB).
6dB), so noise bands can be detected sufficiently.

During special playback, if a noise band occurs in the FM luminance signal as shown in Figure 4a, the DO-DET circuit 8 with a shallow detection level as described above will
Noise bands are detected. Note that as an example of the DO-DET circuit 8, a simple circuit as shown in FIG. 7 is used. Since the DO-DET circuit 8 has a shallow detection level, it detects not only noise bands but also dropouts during normal reproduction, and therefore even a signal for which dropout compensation has been normally performed may be amplitude limited. Therefore, in order to prevent this, a special reproduction mode detection circuit 9 is provided. This special playback mode detection circuit 9 has a gate signal f that is low level during normal playback and high level during special playback.
is applied, whereby the noise band detection signal e output from the DO-DET circuit 8 is selected as the noise band detection signal g only during special reproduction.
The operation of the reproduced luminance signal amplitude limiting circuit 10 is controlled by the selected noise band detection signal g, and the eighth
The white level noise band portion of the input luminance signal of the amplitude limiting circuit 10 shown in FIG. 8A is amplitude limited as shown in FIG. Ru.

Note that if you only want to detect noise bands, the first
In the conventional circuit shown in the figure, DO− in DOC circuit 3
Although it is conceivable to simply make the detection level of the DET circuit shallower, using only a detection circuit with such a shallow detection level is not preferable because it causes a synchronization shift or an unstable state. Therefore, as described above, two detection circuits with different detection levels are used.

In the embodiment shown in FIG. 5, the DO
- When DET circuit 8 is added, DO-
The DET circuit 8 is composed of a circuit similar to the detection rectifier circuit 11 and a second comparator, but instead of this, as shown in FIG.
A relatively shallow level (-10 dB to -
A comparator 16 set to 6 dB) may be connected.

Although the above embodiment deals with a search as special playback, the present invention can be similarly applied to other special playbacks (for example, slow, still, reverse, triple speed, etc.).

〔Effect of the invention〕

As described in detail in the embodiments above, according to the present invention, dropouts during normal playback are compensated for without any problems, and the noise band during special playback is set to a level shallower than the normal dropout detection level. Since it can be detected by a band detection circuit, the amplitude of the output video signal of the dropout compensation circuit in this noise band portion is controlled to be limited by the detection signal of the noise band detection circuit, and only during special playback. This provides effects such as being able to effectively reduce the effects of noise bands on the image at low cost.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional VTR video signal reproduction processing circuit, Fig. 2 is a block diagram of an example of a dropout compensation circuit, and Fig. 3 is an example of a special playback video track and video head running line in the case of search. Schematic diagram showing the relationship between
5 is a block diagram of an embodiment of the present invention, FIG. 6 is a dropout detection level setting diagram, and FIG. 7 is a dropout detection level diagram. 8a and 8b are waveform diagrams showing the noise band reduction effect in the embodiment of FIG. 5, and FIG. 9 is a block diagram of another embodiment of the present invention. be. 2...High pass filter and peaking circuit,
3... Dropout compensation circuit, 4... FM demodulator,
5...Mixing circuit, 6...Low pass filter, 7...Frequency conversion circuit, 8...Noise band detection circuit, 9...Special reproduction mode detection circuit, 10...Reproduction luminance signal amplitude limiting circuit.

Claims (1)

[Claims] 1. A dropout method in which a dropout portion of a frequency modulated video signal reproduced from a recording medium is detected at a predetermined detection level to obtain a dropout detection signal, and the dropout portion of the video signal is replaced by a delayed video signal without dropout based on the dropout detection signal. a compensation circuit; a noise band detection circuit that detects the noise band portion of the video signal at a level shallower than the dropout detection level to obtain a noise band detection signal; and a selection circuit that selects the noise band detection signal only during special playback. and an amplitude limiting circuit that receives the output video signal of the dropout compensation circuit and limits the amplitude of the noise band portion of the output video signal only during special playback using the noise band detection signal selected by the selection circuit. Video signal processing circuit for playback equipment.