JPH07203370A - Inversion prevention circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] It is an object to provide an inversion prevention circuit capable of digitally performing an inversion prevention process. [Structure] A high-pass filter 1 that allows only high-pass components of an input video signal to pass, a low-pass filter 2 that allows only low-pass components of an input video signal to pass, and a value of an envelope for the output signal of the high-pass filter is derived. Envelope value deriving means 11, 12, 13 and the envelope value deriving means 1
A gain control amplifier 13 that amplifies the output signal of the high-pass filter 1 so that the amplitude level of the output signal of the high-pass filter 1 is made uniform based on the outputs of 1, 12, 13; It is provided with an adder 5 for adding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording / reproducing apparatus (hereinafter referred to as VTR) for performing FM recording and reproduction of video signals.
The present invention relates to an inversion prevention circuit used in, and more specifically, as processing for removing distortion of a reproduced FM signal,
The present invention relates to an inversion prevention circuit that amplifies a high frequency component of a signal with a gain according to an amplitude level and mixes the high frequency component with a low frequency component to obtain a reproduced FM signal without distortion.

[0002]

2. Description of the Related Art Conventionally, in order to prevent a reversal phenomenon called black blurring which occurs in a reproduced video signal of a VTR, a reproduction F
Various contrivances have been made in the signal processing before demodulation of the M signal.

A conventional inversion prevention circuit of this type, which is commonly called a double limiter, will be described below with reference to FIGS. 3 and 4. Incidentally, FIG. 3 is a block diagram of a conventional inversion prevention circuit, and FIG. 4 shows each part (a) to FIG.
It is a waveform diagram in (f).

As shown in FIG. 3, a conventional inversion prevention circuit has a high-pass filter (hereinafter referred to as HPF) 1 having a characteristic of passing only a component having an FM carrier frequency or more, and an FM carrier frequency or less. Low-pass filter (hereinafter referred to as LPF
2) and the above H. P. For the first limiter 3 that equalizes the amplitudes of the output signals of F1 and the output signal of the first limiter 3, the L. P. The phase correction circuit 4 that performs phase matching with the output signal of F2, the output signal of the phase correction circuit 4 and the L.P. P. An adder 5 for mixing the output signal of F2,
A second limiter 6 for adjusting the amplitude of the output signal of the adder 5 is provided.

The operation of the inversion prevention circuit configured as described above will be described. First, when a reproduced FM signal having a distorted waveform causing an inversion phenomenon shown in FIG. 4A is input. , H .; P. Only a high frequency component is passed through at F1, and a signal having a waveform that exceeds 0 level even in a period in which the signal amplitude is reduced as shown in FIG. 4B is output. On the other hand, L. P. At F2, only the low frequency component of the input reproduced FM signal is passed and a signal having the waveform shown in FIG. 4C is output.

H. P. FM signal passed through F1 (see FIG.
In (b), the amplitude is made uniform by the first limiter 3, and FIG.
The waveform is as shown in (d). Then, the L.P. P. After phase matching with the output signal of F2, the adder 5 adds the L.F. P. The waveform is mixed with the output signal of F5 and has the waveform shown in FIG. Since the FM signal having the waveform shown in FIG. 4E still has uneven amplitude, the amplitude is made uniform by the second limiter 6 to obtain the waveform shown in FIG.

In this way, by obtaining the reproduced FM signal without distortion, the inversion phenomenon can be prevented.

[0008]

However, in the above-described inversion prevention circuit, when it is attempted to perform digital processing, the first limiter 3 and the second limiter 6 have very high levels of the third high frequency and the fifth order. Since a high frequency is generated and it becomes aliasing noise to the FM signal band, there is a problem that it cannot be directly applied to digital processing.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an inversion prevention circuit capable of digitally performing an operation for preventing an inversion phenomenon.

[0010]

In order to solve the above-mentioned problems, an inversion prevention circuit according to the present invention provides a high-pass filter that passes only the high frequency component of an input video signal and only a low frequency component of the input video signal. A low-pass filter to be passed, an envelope value deriving means for deriving a value of an envelope for the output signal of the high-pass filter, and an amplitude level of the output signal of the high-pass filter based on the output of the envelope value deriving means. Amplifying means for performing uniform amplification and an adder for adding the output signal of the amplifying means and the output signal of the low-pass filter are provided.

[0011]

In the inversion prevention circuit according to the present invention, the amplitude level of the reproduced FM signal is made uniform by using the gain control amplifier without using a limiter. Therefore, the inversion phenomenon is prevented without causing any trouble due to the high-frequency folding noise. can do

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the inversion prevention circuit of the present invention will be described below.
1 and 2, the same parts as those of the above-described conventional example are designated by the same reference numerals, and the description thereof will be omitted. still,
FIG. 1 is a block diagram of an inversion prevention circuit according to the present embodiment, and FIG. 2 is a waveform diagram in each part (a) to (g) in FIG.

As shown in FIG. 1, the inversion prevention circuit according to the present embodiment does not use the limiters 3 and 4 in the configuration of the above-described conventional inversion prevention circuit, but instead has a signal in phase with the input signal and π /. With respect to the outputs of the Hilbert transformer 11 that outputs signals that are two-phase shifted, the square adder 12 that squares and adds the two output signals of the Hilbert transformer 11, and the output of the square adder 12, respectively. A square root calculator 13 for calculating a square root, and an L.V. for attenuating noise components higher than the envelope band of the FM signal. P. F14 and the L.
P. For example, with a gain that is proportional to the inverse value of the output value of F14, P. A gain control amplifier (hereinafter, referred to as GCA) 15 for amplifying the output signal from F1 is provided.

The operation of the inversion prevention circuit configured as described above will be described. First, when a reproduced FM signal having a distorted waveform that causes the inversion phenomenon shown in FIG. 2A is input. , H .; P. Only a high frequency component is passed through at F1, and a signal having a waveform that causes zero crossing even in a distorted portion as shown in FIG. 2B is output. On the other hand, L. P. At F2, only the low frequency component of the input reproduced FM signal is passed and a signal having the waveform shown in FIG. 2C is output.

H. P. The FM signal that has passed through F1 (Fig. 2
In (b), the signal is input to the Hilbert transformer 11, and a signal delayed for a fixed time and a signal further delayed by π / 2 for a fixed time are obtained. Then, when the two output signals obtained by the Hilbert transformer 11 are input to the square adder 12,
Since a value obtained by adding the respective squared values can be obtained, H.264.
P. Information about the amplitude envelope for the output signal of F1 will be obtained.

However, since this information is the squared value of the value of the envelope, the square root calculator 13 derives the value of the envelope to obtain the waveform shown in FIG. 2 (d). This waveform has F
Since a noise component irrelevant to the envelope information is present due to the level fluctuation of the M signal and the noise component, L.M. P. F
By passing through 14, the smooth waveform without noise shown in FIG.

G. C. The H.264 input to A15. P. F1
Since the output signal of is amplified by a gain proportional to the envelope level, for example, a reciprocal value, a signal having a waveform shown in FIG. Become. And this G. C. A15
Output signal and L. P. By adding the output signal of F2 with the adder 5, a reproduced FM signal without waveform distortion as shown in FIG. 2 (g) can be obtained, and the inversion phenomenon after FM demodulation can be prevented. .

As described above, according to the inversion prevention circuit of the present embodiment, since the limiter is not used, there is no problem of interference due to high frequency aliasing noise.
The signal is divided into a high-frequency component and a low-frequency component once, and the same processing as in the case where the limiter is also used for the high-frequency component is performed, so it is equivalent to the inversion prevention circuit using the conventional double limiter described above. The effect can be obtained, and at the same time, it can be sufficiently applied to the case where the inversion phenomenon is prevented by digital processing.

[0019]

Since the inversion prevention circuit according to the present invention has a structure in which the amplitude level of the reproduced FM signal is made uniform by using the gain control amplifier without using a limiter, the inversion phenomenon does not occur due to the high-frequency folding noise. Can be prevented. Therefore, the same effect as that of the conventional inversion prevention circuit can be obtained by digital processing.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an inversion prevention circuit of the present invention.

FIG. 2 is a waveform chart in each part in FIG.

FIG. 3 is a block diagram showing a conventional inversion prevention circuit.

FIG. 4 is a waveform diagram in each part in FIG.

[Explanation of symbols]

 1 L. P. F 2 H. P. F 5 adder 11 Hilbert transformer 12 square adder 13 square root calculator 14 L.D. P. F 15 G.I. C. A

Claims (2)

[Claims] 1. A high-pass filter that passes only a high-frequency component of an input video signal, a low-pass filter that passes only a low-frequency component of an input video signal, and a value of an envelope for an output signal of the high-pass filter is derived. Envelope value deriving means, amplification means for amplifying the output signal of the high pass filter based on the output of the envelope value deriving means so that the amplitude levels of the output signals are made uniform, the output signal of the amplification means and the output signal of the low pass filter An inversion prevention circuit comprising an adder for adding and. 2. The inversion prevention circuit according to claim 1, wherein the envelope value derivation means has a phase difference between the input signal and a signal of the same phase.
Hilbert transformer that outputs a / 2 phase-shifted signal, a square adder that squares and adds the two output signals of the Hilbert transformer, and a square root of the output signal of the square adder An inversion prevention circuit comprising a square root calculator for