JPH0695750B2 - FM demodulation circuit for video signal - Google Patents

Description

The present invention relates to an FM demodulation circuit for a video signal used in a reproducing circuit of a video tape recorder or the like.

[Outline of Invention]

The present invention relates to an FM demodulator circuit for a video signal, and when a signal is supplied to an FM demodulator via a so-called soft limiter and hard limiter, a bandpass filter is inserted between them to expand the pulse width and substantially demodulate the signal. This allows the demodulation limit of the device to be increased.

[Conventional technology]

In a video tape recorder, so-called video signal recording is generally performed by FM demodulation. That is, FIG. 5 is a waveform diagram of the video signal at each stage. First, when there is a baseband signal rising from black to white as shown in A, the signal is as shown in B in FIG. Pre-emphasis is applied to improve S / N. Then, this signal is modulated so that the frequency is low on the black side as shown in FIG.
A high FM signal is formed on the white side. This FM signal is recorded on the magnetic tape by the magnetic head.

However, when such a signal is recorded and reproduced, the tape
High-frequency components are attenuated due to the spacing loss and frequency characteristics of the head system. For this reason, the reproduced signal becomes as shown by D in the above waveform diagram, and the zero-cross of the carrier wave may be lost due to the decrease in the amplitude of the high frequency due to the rise of the video signal. If such a zero-cross is lost, the signal when this signal is FM demodulated rises and the signal that should be originally white becomes black, as shown in FIG. 8E, and a so-called black inversion phenomenon occurs.

Therefore, in order to suppress the black inversion phenomenon, it is possible to raise the frequency characteristic of the circuit system before FM demodulation on the high frequency side, for example. However, simply performing such lifting is not preferable because the S / N of the demodulated signal deteriorates.

On the other hand, a circuit called a so-called soft limiter as shown in FIG. 6 has been proposed. In the figure, the FM signal supplied to the input terminal (61) is supplied to the soft limiter circuit (62) which operates at a signal level high enough to save the above-mentioned missing portion of the zero cross, and the FM signal of the limiter circuit (62) is supplied. The output is supplied to a high pass filter (63) that attenuates low frequency components.

Therefore, in this circuit, the signal of the part where the zero crossing is lost passes through the limiter circuit (62) as it is and is supplied to the high-pass filter (63), and this high-pass filter (63).
Restores the zero cross. In addition, the normal level signal is limited and the sidebands are averaged to prevent S / N deterioration due to the high pass filter (63).

Then, the signal from the high pass filter (63) is supplied to the FM demodulator (65) through the hard limiter circuit (64) which operates at a signal level lower than that of the limiter circuit (62).

In this way, the zero-cross is restored, and good FM demodulation without S / N deterioration due to the high-pass filter (63) can be performed.

However, due to phase distortion of the tape system, the width T of the restored pulse may become extremely narrow as shown in FIG. 7, for example. When this width T exceeds the demodulation limit of the demodulator (65), this pulse is ignored and the black inversion phenomenon similar to the above occurs.

On the other hand, it is possible to raise the demodulation limit of the demodulator (65) to a sufficiently high frequency. However, in that case, a white inversion phenomenon, which is the opposite of the above, may occur.

That is, when there is a baseband signal that falls from white to black as shown in FIG. 8A, this signal is pre-emphasized and the FM-modulated signal is as shown in FIGS. . When this signal is recorded / reproduced, the reproduced signal is distorted in the low frequency portion as shown in FIG. 4D due to so-called third-order distortion during recording / reproduction. On the other hand, if the demodulator (65) has a high demodulation limit, as shown in the figure, demodulation as a high frequency is performed when the distortion exceeds a zero level, and as shown in FIG. The signal that should become white becomes white, and a so-called white inversion phenomenon occurs.

Therefore, in the above circuit, the black inversion phenomenon occurs when the demodulation limit of the demodulator is lowered, and the white inversion phenomenon occurs when the demodulation limit is increased, and it is impossible to suppress these at the same time.

[Problems to be solved by the invention]

As described above, the conventional technique has a problem that the black inversion phenomenon and the white inversion phenomenon cannot be suppressed well at the same time.

[Means for solving problems]

The present invention is an FM-modulated video signal (input terminal (1))
Limiter circuit (2) that operates at a high signal level
Is input to a band pass filter (high pass filter (3), low pass filter (4)) for attenuating frequency components of the above-mentioned FM modulation or higher, and the output signal of this band pass filter is input to the first limiter circuit. Second limiter circuit (5) operating at a lower signal level
This is an FM demodulation circuit for a video signal which is input to the FM demodulator (6) via the.

[Action]

According to this, the width of this pulse is expanded by attenuating the high-frequency component of the pulse signal with the restored zero cross, so that this pulse signal can be obtained without increasing the demodulation limit of the demodulator. It becomes possible to demodulate, the black inversion phenomenon is suppressed, and since the demodulation limit is not raised, there is no fear of the white inversion phenomenon, and the black and white inversion phenomenon can be suppressed well at the same time.

〔Example〕

In FIG. 1, the FM signal supplied to the input terminal (1) is supplied to the soft limiter circuit (2) which operates at a signal level high enough to preserve the above-mentioned missing portion of the zero cross, and the limiter circuit (2 2) is supplied to a high pass filter (3) that attenuates low frequency components. The signal from the high-pass filter (3) is further supplied to the low-pass filter (4) having the characteristic of attenuating the frequency component of the carrier wave having the highest frequency or higher. Then, the signal from the low-pass filter (4) is passed through the hard limiter circuit (5) which operates at a lower signal level than the limiter circuit (2).
It is supplied to the FM demodulator (6).

Therefore, in this circuit, the total frequency characteristics of the high-pass filter (3) and the low-pass filter (4) are as shown in FIG. 2, for example. Here, f _c is the center frequency of the carrier wave, and the characteristics of the band pass filter having a peak at a frequency f _o that is about twice this f _c are formed.

And by providing a circuit with such characteristics,
For example, when a pulse signal as shown in FIG. 3A is taken out from the high pass filter (3) and this signal is supplied to the low pass filter (4), this output signal is
As shown in, the pulse width is T
To T '. That is, since the high frequency component of the pulse signal is attenuated, the waveform of the signal becomes dull, and thereby the pulse width of the zero cross portion is widened. On the other hand, in the waveform due to the above-mentioned third-order distortion, the zero-cross portion of this signal does not spread even after passing through the low-pass filter (4).

The FM signal is demodulated in this way, but the circuit described above substantially increases the demodulation limit of the FM demodulator by expanding the width of the pulse restored by the high-pass filter by the low-pass filter. As a result, the black inversion phenomenon is suppressed, and the pulse width of the signal due to the third-order distortion or the like does not widen at this time, so that there is no fear of the white inversion phenomenon.

The high pass filter (3) and the low pass filter (4) described above may use band pass filters having the same frequency characteristics.

Moreover, as shown in FIG. 4, the hard limiter (5) described above is used.
The pulse width can be further widened by re-inputting the output of (1) to the low-pass filter circuit (41), inputting this output to the hard limiter circuit (42), and repeating this configuration in cascade.

〔The invention's effect〕

According to the present invention, the width of this pulse is expanded by attenuating the high-frequency component of the pulse signal whose zero-crossing has been restored, so that the pulse signal can be expanded without raising the demodulation limit of the demodulator. Can be demodulated,
Since the black inversion phenomenon is suppressed and the demodulation limit is not raised, there is no fear of causing the white inversion phenomenon, and the black and white inversion phenomenon can be suppressed well at the same time.

[Brief description 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 a conventional technique. (1) is an input terminal, (2) is a soft limiter circuit,
(3) is a high-pass filter, (4) is a low-pass filter, (5) is a hard limiter circuit, and (6) is an FM demodulator.

Claims (1)

[Claims] 1. An FM-modulated video signal is input via a first limiter circuit operating at a high signal level to a bandpass filter for attenuating frequency components above the highest frequency of the FM modulation, and this bandpass filter is provided. Of the video signal which is adapted to be inputted to the FM demodulator via the second limiter circuit which operates at a signal level lower than that of the first limiter circuit.
FM demodulation circuit.