JPS63303587A - Video signal processing circuit - Google Patents

Abstract

PURPOSE:To suppress a moire, which varies according to the change of a regenerated video signal to be FM modulated and is caused by the secondary distortion of the regenerated video signal to be FM modulated by controlling the limit level of a limiter circuit based on the detection output of a duty detection circuit. CONSTITUTION:The regenerated video signal to be FM modulated is supplied from a regenerative equalizer 2 to a limiter 3, and a pulse train corresponding to the edge of a square wave signal outputted from the limiter 3 is outputted from a pulse counter 5. The said pulse train is converted into an analog signal by an LPF 6, and outputted through a de-emphasis circuit 7. The output of the pulse counter 5 is frequency divided by 1/2 by a frequency division circuit 9, and the duty of its output signal is detected by a charge pump 10. The limit level of the limiter circuit 3 is controlled based on the output of the said charge pump 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video signal processing circuit suitable for application to a reproduction system of a VTR.

[Summary of the invention]

The present invention relates to a video signal processing circuit, which limits a reproduced FM modulated video signal to obtain a rectangular wave signal, forms a pulse train corresponding to the edges of the rectangular wave signal, converts the pulse train into an analog signal, and converts the pulse train into an analog signal. By dividing the frequency of the pulse train into 1/2, detecting the duty of this frequency-divided pulse train, and controlling the limit level based on the detected duty, changes in the reproduced FM modulated video signal can be prevented. This makes it possible to effectively suppress moiré caused by second-order distortion of the reproduced FM modulated video signal, which changes based on the frequency.

[Conventional technology]

In a VTR, it is common practice to perform FM modulation on a video signal before recording it on tape. When reproducing the FM modulated video signal recorded on tape in this way, the 'f&F
Second-order distortion of the M-modulated video signal causes moiré in the video signal obtained by FM demodulating the FM-modulated video signal.

The causes of moire include second-order distortion in the recording system, second-order distortion around the tape head, second-order distortion in the reproduction system, second-order distortion due to head magnetization, and the like.

Therefore, in the past, a pulse counter type demodulator was used as the FM demodulator to suppress the occurrence of moiré.

[Problem that the invention seeks to solve]

However, many of the causes of moiré are changes in the state, such as differences in the tape, magnetization of the head, and changes in the FM modulated video signal, and conventional pulse counter type demodulators cannot detect these changes. Because this was ignored, moiré could not be effectively suppressed in some cases, leading to a decline in the playback quality.

In view of this, the present invention proposes a video signal processing circuit that can effectively suppress moiré caused by second-order distortion of a reproduced FM modulated video signal that changes based on changes in the reproduced FM modulated video signal. That is.

[Means for solving problems]

The video signal processing circuit according to the present invention includes a limiter circuit (3) to which an FM modulated video signal to be reproduced is supplied, and a rectangular wave signal output from the limiter circuit (3) to which a pulse train corresponding to the edge of the limiter circuit (3) is supplied. a conversion circuit (6) that converts the pulse train output from the pulse forming circuit (5) into an analog signal; and a conversion circuit (6) that converts the pulse train output from the pulse forming circuit (5) into A frequency dividing circuit (9) that divides the frequency into 2 and a duty detection circuit (10) that detects the duty of the output signal of the frequency dividing circuit (9).
), and the limit level of the limiter circuit (3) is controlled based on the detection output of the duty detection circuit (10).

[Effect]

According to the present invention, by controlling the limit level of the limiter circuit (3) based on the detection output of the duty detection circuit (10), the reproduction target that changes based on the change in the reproduced FM modulated video signal is controlled. Moiré based on second-order distortion of the FM modulated video signal is suppressed.

〔Example〕

The present invention will be described below with reference to FIG.
Let us explain in detail an embodiment that is applicable to a TR regeneration system. T
P is a magnetic tape, and a rotating magnetic head is used to record an FM modulated video signal (in this case, an FM modulated composite color video signal, an FM modulated luminance signal, and an FM modulated black and white signal) so as to form an inclined track. (Video signals, etc. are also possible) are recorded. H indicates a rotating magnetic head, which allows the magnetic field recorded on the tape TP to be
Regenerate the M modulated video signal. This reproduced FM modulated video signal is amplified by a preamplifier (1) and then supplied to a reproduction equalizer (2) where it is equalized into an optimal waveform.

This equalized FM modulated video signal is processed by a limiter circuit (
3) and is limited.

The rectangular wave signal a (Fig. 2A) output from the limiter circuit (3) is sent to a pulse count type FM demodulator (4) consisting of a pulse counter (5) and a low-pass filter (6) connected to the next stage. A pulse train b consisting of pulses (with a pulse width of Δt) corresponding to the rising and falling edges of the rectangular wave signal a is supplied to the pulse counter (5) of the rectangular wave signal a.
(FIG. 2B) is formed. This pulse counter (5)
The pulse train output from the low-pass filter (6)
is supplied to the de-emphasis circuit (7) and converted into an analog signal, that is, a video signal (in this case, a composite color video signal) (kfld). This video signal is supplied to the de-emphasis circuit (7) and de-emphasized, and then sent to the output terminal ( 8).

The pulse train from the pulse counter (5) has a frequency division ratio of 1.
The signal is supplied to a /2 frequency divider (consisting of a one-stage flip-flop circuit) (9) and is frequency-divided by 1/2. The frequency-divided output C (FIG. 2C) of this frequency divider (9) includes second-order distortion. This frequency divided output C is the charge pump (10)
For example, during the high level period ta, the capacitor of the charge pump (10) is charged, and during the low level period tb, the capacitor is discharged.

This charge pump (10) is equipped with an offset variable resistor (10a), and this variable resistor (10a)
A desired offset is given to this charging/discharging current by adjusting .

As a result, the charging period t of this charge pump (10)
The output of the charge pump (10) controls the mint level of the limiter circuit (3) so that a and the discharge period tb have an arbitrary ratio. In this case, the duty is 50%
When the second-order distortion of the FM modulated video signal is O, and the duty deviates from 50%, the second-order distortion is included. In this case, just because the second-order distortion is 0 does not necessarily mean that the moire is O, and by selecting an appropriate amount of the second-order moire, the moire can be suppressed or eliminated.

Furthermore, by supplying the output of the charge pump (10) to the limiter circuit (3) via the low-pass filter (11), the period ta and tbO ratios are integrated over a certain period of time and are output to the limiter circuit (3). ). As a result, changes in second-order distortion caused by perpendicular fluctuations in one vertical period of the FM modulated video signal are absorbed.

According to such a video signal processing circuit, it is possible to detect secondary distortion of the reproduced FM modulated video signal that changes in various ways, such as differences in tape, magnetization of the head, fluctuations in the contact of the reproduced FM modulated video signal, and changes in the head output. , the moiré caused by the second-order distortion can be minimized by controlling the detected second-order distortion to a certain level.

Moreover, according to this video signal processing circuit, in particular, the reproduced FM
Moiré changes due to level fluctuations of the modulated video signal can be almost completely suppressed. Regarding other moiré, if it appears as second-order distortion in the reproduced FM modulated video signal, the second-order distortion is detected,
This can be effectively suppressed.

〔Effect of the invention〕

According to the present invention described above, it is possible to obtain a video signal processing circuit that can effectively suppress moiré based on second-order distortion of a reproduced FM modulated video signal that changes based on changes in the reproduced FM modulated video signal. Can be done.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a timing chart for explaining the same. TP is a magnetic tape, H is a rotating magnetic head, (3) is a limiter circuit, (4) is a pulse count type FMffi 8m
(5) is a pulse counter, (6) is a low-pass filter, (9) is a frequency divider, (10) is a charge pump, (1
1) is a low-pass filter.

Claims (1)

[Scope of Claims] A limiter circuit to which a reproduced FM modulated video signal is supplied; a pulse forming circuit to which a rectangular wave signal output from the limiter circuit is supplied and forms a pulse train corresponding to the edges thereof; A conversion circuit that converts the pulse train output from the pulse forming circuit into an analog signal, a frequency dividing circuit that divides the pulse train output from the pulse forming circuit into 1/2, and a duty ratio of the output signal of the frequency dividing circuit. A video signal processing circuit comprising: a duty detection circuit for detecting a duty, and controlling a limit level of the limiter circuit based on a detection output of the duty detection circuit.