JPS62241483A - Fm demodulating circuit for video signal - Google Patents

Abstract

PURPOSE:To excellently suppress black-and-white inversion at the same time by inputting the output signal of a band-pass filter to an FM demodulator through the 2nd limiter circuit which operates at a signal level lower than that of the 1st limiter circuit. CONSTITUTION:The output of a limiter circuit 2 is supplied to a high-pass filter 3 which attenuates a low frequency component. The signal from this high-pass filter 3 is further supplied to a low-pass filter 4 which attenuates a frequency component higher than the highest frequency of a carrier. Then, the signal from this low-pass filter 4 is supplied to an FM demodulator 6 through a hard limiter circuit 5 which operates at the signal level lower than that of the limiter circuit 2. For example, when a pulse signal shown in a figure A is led out of the high-pass filter 3, this Signal is supplied to the low-pass filter 4 and then the output signal has its zero-cross part expanded in pulse width from T to T' as shown in a figure B. Consequently, the demodulation limit of the FM demodulator is increased substantially and black-and-white inversion is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulation circuit for video signals used in playback circuits of video tape recorders and the like.

[Summary of the invention]

The present invention relates to an FM demodulation circuit for video signals, and when a signal is supplied to an FM demodulator via a so-called soft limiter and a hard limiter, a bandpass filter is inserted between them to expand the pulse width and substantially reduce the pulse width. This makes it possible to increase the demodulation limit of the demodulator.

[Conventional technology]

In video tape recorders, so-called video signals are generally recorded using FM modulation.

In other words, FIG. 5 is a waveform diagram of a video signal at each stage. First, when there is a baseband signal rising from black to white as shown in AK, this signal has S as shown in FIG. Pre-emphasis is applied to improve /N. This signal is then modulated to form an FM signal with a low frequency on the black side (and a high frequency on the white side) as shown in Figure C. This F'M signal is recorded on the magnetic tape by a magnetic head.

However, when such a signal is recorded and played back, the tape
High frequency components are attenuated due to head system spacing loss and frequency a% characteristics. For this reason, the reproduced signal becomes as shown in D in the above-mentioned waveform diagram, and the amplitude of the high frequency wave becomes small at the rise of the video signal, and the zero cross of the carrier wave may be missing.

When the zero cross is missing in this way, the signal when this signal is FM demodulated turns black at the rising edge, as shown in FIG.

Therefore, in order to suppress this black reversal phenomenon, for example, FM
It is possible to improve the frequency characteristics of the circuit system before demodulation on the high frequency side. However, simply performing such lifting is undesirable because the S/N of the demodulated signal deteriorates.

In response to this, a so-called soft limiter circuit as shown in FIG. 6 has been proposed. In the figure, the FM signal supplied to the input terminal 100 days is supplied to a soft limiter circuit 64 that operates at a signal level high enough to preserve the missing portion of the zero cross mentioned above, and the output of this limiter circuit is a low frequency signal. It is fed into a bypass filter that attenuates the component.

Therefore, in this circuit, one of the parts where the zero cross is missing is
The δ signal passes through the limiter circuit (621) and is supplied to the bypass filter, and the zero cross is restored by the inno(s) filter. Also, the normal level signal is limited and the side bands are averaged. , S/N deterioration due to the bypass filter is prevented.

And this bypass filter (the signal from ti31 operates at a lower signal level than the limiter circuit 'Nrti4)]
- is supplied to the FLVI demodulator through the limiter circuit.

In this way, the zero cross is restored, and it is possible to perform excellent FM demodulation without deterioration of S/N due to the bypass filter.

However, in this circuit, if the order of the bypass filter IQ is increased to increase the effect of suppressing the inversion phenomenon,
Due to phase distortion of the tape system, for example, as shown in FIG. 7, the width T of the restored pulse may become extremely narrow. If this width T exceeds the demodulation limit of the demodulator φ, this pulse will be ignored and the same black reversal phenomenon as described above will occur.

On the other hand, it is conceivable to raise the demodulation limit of the demodulator to an optically higher frequency. However, in that case, there is a possibility that a white reversal phenomenon opposite to that described above may occur.

In other words, when there is a baseband signal that falls from white to black as shown in Figure 8A, the pre-emphasis is applied to this signal and the FM modulated signal becomes as shown in Figures B and C. ing. When this signal is recorded and reproduced, distortion occurs in the low frequency portion of the reproduced signal due to so-called third-order distortion during recording and reproduction, as shown in the figure. On the other hand, if the demodulation limit of the demodulator is high, as shown in the figure, the demodulation as a high frequency will be performed as soon as the distortion exceeds zero Vbell even slightly, and the original point 5 as shown in figure E will be demodulated. The signal that should be white becomes white, causing what is called a white inversion phenomenon.

Therefore, in the above-mentioned circuit, if the demodulation limit of the demodulator and IJ@ device is lowered, a black inversion phenomenon will occur, and if the demodulation limit of the IJ@ device is lowered, a black reversal phenomenon will occur, and if the demodulation limit of the IJ@ device is lowered, a white melon phenomenon will occur, and these cannot be suppressed satisfactorily at the same time.

[Problem that the invention seeks to solve]

As described above, the conventional technology has problems such as the inability to satisfactorily suppress the black reversal phenomenon and the white reversal phenomenon at the same time.
there were.

[Means for solving problems]

The present invention provides an FM modulated video signal (input terminal (l)
) is operated at a high signal level by the i@1 limiter circuit (
2), the output signal of this band pass filter is inputted to the band pass filter ()/I pass filter %13), which attenuates the frequency components higher than the highest frequency of the FM modulation, and the low pass filter t4)'), and the output signal of this band pass filter is input to the above first filter. This is an FM demodulation circuit for a video signal which is input to an FM demodulator (6) via a second limiter circuit (5) which operates at a lower signal level than the limiter circuit shown in FIG.

[Effect]

According to this, the width of this pulse is expanded by attenuating the high frequency component of the pulse signal restored at the zero crossing, so that this pulse signal can be demodulated without raising the demodulation limit of the demodulator. Therefore, the non-inversion phenomenon is suppressed, and since the demodulation limit is not raised, there is no fear that a white inversion phenomenon will occur, and black and white inversion phenomena can be suppressed satisfactorily at the same time.

〔Example〕

In Fig. 1, the soft limiter circuit (2) operates at a high enough signal level that the FM signal supplied to the input terminal txt preserves the missing portion of the zero cross mentioned above.
The output of this limiter circuit (2) is supplied to a bypass filter (3) that attenuates low frequency components. The signal from this bypass filter (3) is further supplied to a low-pass filter (4) having a characteristic of attenuating frequency components higher than the highest frequency of the carrier wave. The signal from the low-pass filter (4) is supplied to the FM demodulator (6) through the limiter circuit (5), which operates at a lower signal level than the limiter circuit (2).

Therefore, in this circuit, the overall frequency characteristics of the bypass filter (3) and the low-pass filter (4) are as shown in FIG. 2, for example. Here, fc is the center frequency of the carrier wave, and the frequency fo? is approximately twice this fc. : A characteristic of a bandpass filter with a peak is formed.

And by providing a circuit with these five characteristics,
For example, when a pulse signal as shown in FIG. 3A is taken out from the bypass filter (3) and this signal is supplied to the low-pass filter (4), this output signal is shown in FIG.
As shown in Figure 5, the pulse width is T at the zero-crossing part of the signal.
It is expanded from T to T.

That is, the high frequency components of the pulse signal are attenuated, thereby making the signal waveform duller, thereby widening the pulse width at the zero-crossing portion. On the other hand, since the waveform due to the third-order distortion described above does not originally contain high-frequency components, the zero-crossing portion of this signal does not expand even if the signal exceeds the low-pass filter (4).

In this way, the FMM signal is demodulated. According to the circuit described above, the width of the pulse restored by the no-pass filter is expanded by the low-pass filter, and the FM signal is demodulated.
The demodulation limit of the demodulator can be substantially increased, the non-inversion phenomenon can be suppressed, and at this time, the pulse width of the signal due to third-order distortion etc. does not widen, so there is no possibility that the white inversion phenomenon will occur.

Note that for the above-described configurations of the bypass filter (3) and the low-pass filter (4), band-pass filters having the same frequency characteristics may be used.

Moreover, as shown in FIG. 4, the above-mentioned hard limiter t53
The output of is inputted again to the low-pass filter circuit (41) K,
This output is further input to the hard limiter circuit (6),
It is also possible to further widen the pulse width by repeating this configuration in cascade.

〔Effect of the invention〕

According to the present invention, the width of the pulse signal is expanded by attenuating the high frequency components of the pulse signal restored at the zero crossing, so that the pulse signal can be demodulated without raising the demodulation limit of the demodulator. become,
Since the non-reversal phenomenon is suppressed and the demodulation limit is not raised, there is no fear that a white reversal phenomenon will occur, and it is now possible to satisfactorily suppress black and white reversal phenomena at the same time.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an example of the present invention, FIGS. 2 to 4 are diagrams for explaining the same, and FIGS. 5 to 8 are diagrams for explaining the conventional technology. (1) is the input terminal, (2) is the soft limiter circuit, (3
) is a bypass filter, +4) is a low-pass filter,
+5)&”!hard limiter circuit, (6) is an FM demodulator.

Claims (1)

[Claims] An FM-modulated video signal is input to a bandpass filter that attenuates frequency components higher than the highest frequency of the FM modulation through a first limiter circuit that operates at a high signal level; An FM demodulation circuit for a video signal, wherein an output signal of the filter is input to an FM demodulator via a second limiter circuit that operates at a lower signal level than the first limiter circuit.