JPS612405A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To suppress the leakage of video FM modulation signal components to a video signal and to demodulate said components by quadrupling carrier frequency through phase shifting circuits, limiter circuits, double wave differentiation circuits, and an adder and separating an original signal at its frequency from the guadrupled modulation signals through an integration circuit to modulate the FM modulation signals. CONSTITUTION:The outputs of the double wave differentiation circuits 5, 6 are thin/ dense pulse waves doubled at their carrier frequency. Since video FM modulation signals of which phases are relatively different each other by 90 deg. over the whole frequency areas are inputted from odd type and even type transversal filters 1, 2 to the circuits 5, 6, the phases of the thin/dense pulse waves are relatively different by 90 deg.. Therefore, the adder 7 adds the thin/dense pulse waves having relatively different phases by 90 deg. and inputs thin/dense pulse waves quadrupled at its carrier frequency to the integration circuit 8. The integration circuit 8 separates the frequency of the video signal from that of the video FM modulation signals and outputs only the video signal. Consequently, the video FM modulation signal components can be prevented from being leaked to the video signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention particularly relates to an FM demodulation circuit that demodulates a reproduced video FM modulated signal from a magnetic recording/reproducing device (for example, a video tape recorder) that modulates and records a video signal. .

Configuration of conventional example and its problems Conventional FM demodulation circuit 2 For example, in a rotating head type video tape recorder (hereinafter referred to as VTR), the reproduced video FM modulation signal is passed through a limiter circuit as shown in FIG. When it passes through, it becomes a rectangular compression wave like (-), and when the waveform of (a) is differentiated by a differentiating circuit, it becomes like (b), and when this is detected (double-wave rectification), it becomes a pulse like (C). It becomes a compression wave. Furthermore, by passing this through an integrating circuit, the original waveform as shown in (d) can be reproduced.

By the way, in FM modulation and demodulation of a VTR, as the name low carrier FM indicates, the high frequency component of the video signal and the carrier wave frequency are very close to each other, and when demodulating without changing the carrier frequency, the FM modulation signal component This leaks into the original signal (that is, the video signal), producing small striped patterns and beats on the screen. For this reason, conventional FM demodulation circuits currently double the carrier frequency by performing double-wave rectification to suppress the amount of the FM modulation signal component leaking into the video signal. In a normal home type VTR, for example, the video band is 1.5, the frequency deviation ratio is 1.0, and the carrier frequency is 3.9. Jl, ■2nd etc. 1st lower wave measurement, 2nd
Lower side wave measurement etc. are not included in the video signal and do not pose a problem.

However, the conventional FM demodulation circuit has high resolution and high S/N.
When used in an FM modulation/demodulation system in which the transmission video band is wide and the frequency deviation or emphasis is large, the second lower wave measurement and the third lower wave measurement are the original signal. , that is, it is mixed into the video signal. In particular, in conventional FM demodulation circuits, the carrier wave frequency is doubled, so the frequency deviation is doubled, the power of the second lower wave measurement increases, and a beat pattern or beat is generated on the screen, resulting in a video signal. edge part S/N
It is causing the problem to deteriorate.

OBJECTS OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the conventional FM demodulation circuit described above, and is capable of suppressing leakage of video FM modulation signal components into video signals and demodulating them. The present invention aims to provide an FM demodulation circuit.

Composition of the Invention The FM demodulation circuit of the present invention has characteristics characterized by broadband frequency characteristics and group delay characteristics, and includes a phase shift circuit that generates an FM modulation signal whose phase is relatively different from the FM modulation signal by 90 degrees; The phase shifting circuit and the limiter circuit include two limiter circuits, two double-wave differentiating circuits, an adding circuit and an integrating circuit.

The carrier wave frequency is multiplied by 4 using a double-wave differentiating circuit and an adding circuit, and the original signal is frequency-separated and demodulated from the 4-multiplied FM signal using an integrating circuit. This enables demodulation while suppressing leakage of FM modulated signal components into the original signal.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 is a block diagram of main parts showing an embodiment of the present invention. In the same figure, 1 and 2 are transformers and refills that constitute the phase shift circuit, 1 is an odd type, and 2 is an odd type.
is of even type. Video FM conversion signal, 112fi) When input to the transformer filter 1 and the even type transformer filter 2, the output from the transformer filter 1.2 is the full frequency Two video FM modulated signals having a relative phase difference of 900 degrees in the region and a constant group delay are obtained and input to limiter circuits 3 and 4 having the same configuration, respectively. In the limiter circuit 3°4, the video FM modulation signal becomes a rectangular wave as shown in Fig. 1 (-),
The signal is input to a double-wave differentiator circuit 6.6 having the same configuration. The outputs of the double-wave differentiating circuits 5 and 6 become compression waves of pulses shown in FIG. Here, the main block configuration of the limiter circuit 31 double-wave differentiator circuit 5 or the limiter circuit 41 double-wave differentiator circuit 6 is the same as the main block configuration of a conventional FM demodulation circuit, and the double-wave differentiator circuit 6 ,6
The output from the converter becomes a compression wave of pulses whose carrier frequency is doubled. Still, the outputs of the two-wave differentiator circuits 5 and 6 are inputted from the transversal filters 1 and 2 with video FM modulation signals that have a relatively 900 phase difference in the entire frequency domain, so It becomes a pulse compression wave. Therefore, in the adder circuit 7, the relative phase is 90
The compression waves of 0 different pulses are added to create a pulse compression wave whose carrier frequency is multiplied by 4, and is input to the integrating circuit 8. The integrating circuit 8 frequency-separates the video signal and the video Fivl modulation signal and outputs only the video signal.

In this way, by multiplying the carrier wave frequency by 4, it is possible to prevent the video FM modulation signal components up to the third lower wave measurement from leaking into the video signal. Proving this using the formula is as follows.

The carrier frequency is f. , if the video signal band is fab,
The frequency f3w at which the third lower wave measurement occurs when the carrier frequency is multiplied by 4 is expressed by the following equation.

f-4f-3fab・・・・・・・・・(1)3w
c. The condition that the third lower wave measurement does not leak into the video signal is as follows.
Since 13w>fab, when substituted into this, 13w -'fc -3fab>fab 90
0. --(2)" fc > fab
... River = (3) This always holds true for normal VTRs. It is possible to prevent the video FM modulation signal component from leaking into the video signal at the lower wave measurement section of the air filter 3.

Next, FIG. 3(a) shows a transversal filter with one stage of delay elements (delay amount T). That is, the coefficient is +1
In the case of , an even type transversal filter with coefficients −
1 indicates an odd type transversal filter. Let the outputs of the even type and odd type transversal filters be fo and fl, respectively.

If the input signal is fa, the signal after passing through the delay element is fb, and the angular velocity is ω, then the vector relationship is as shown in FIG. 3(b), and fo and fl are always in quadrature phase.

Therefore, at all frequencies, the outputs of the even and odd transversal filters are relatively in quadrature phase.

Next, if the transfer function of FIG. 3 (,) is found, it will be as follows.

Even-numbered transversal filter fdω)-fノω)tenfb←) Odd 11ZfJ Transversal filter f1(ω)
-f a(J) -fdt-r) Formula (4) above, (6)
The second term in the equation indicates the frequency characteristic, and the third term indicates the phase characteristic.

Therefore, the phase characteristic becomes a linear phase and the second group delay becomes constant. Therefore, when the video FM modulated signal is passed through the transversal filter, group delay distortion will not occur.

Next, the frequency characteristics will be described. The frequency characteristics are as described above (
As can be seen from the second terms of equations (4) and (6), they are sine waves or cosine waves. Regarding the frequency characteristics, video F
Considering the influence on the M modulation signal, it is desirable that the frequency characteristics be constant. To achieve this, a multi-stage transversal filter is used. FIG. 4 shows a five-stage connected transversal filter as an example. If this is used, the frequency characteristics can be made constant in the band from 2 stones to 1611 II+. Finally, regarding the doubling circuit of this embodiment, a pulse counter type one consisting of a limiter circuit, two-wave differentiator circuit, and an adder circuit has been described, but other methods may also be used in quadrature detection FM demodulators. There is a method in which the carrier wave frequency is multiplied by 4 and demodulated using a multiplier, which can achieve the same effect as the present embodiment.

Effects of the Invention As detailed above, the present invention demodulates the video FM modulation signal by multiplying the carrier frequency by 4 using a transversal filter, thereby achieving high resolution.

Even in an FM modulation/demodulation system that attempts to obtain a high S/N ratio, that is, an FM modulation system with a wide video bandwidth and a large amount of frequency deviation or emphasis, the video FM modulation signal component leaks into the video signal. can be suppressed and demodulated.

Therefore, even with an FM modulation/demodulation system that provides high resolution and high S/N, it is possible to prevent small striped patterns and beats on the screen, and improve the S/N of the edge portion of the video signal. can. In addition, since two video FM modulation signals with a relative phase of 90° are generated using a transversal filter, the relative phase is 90° in the entire frequency range.
Since the angle does not change and the phase is linear, the group delay is constant and no group delay distortion occurs. Furthermore, with regard to frequency characteristics, by using a multi-stage transversal filter, it is possible to satisfy the condition that the frequency characteristics are constant over a wide band, so that distortion of the video signal due to the frequency characteristics does not occur.

[Brief explanation of the drawing]

Figure 1 (a), (b), (c) l (d) is a principle waveform diagram showing the principle of FM demodulation, Figure 2 is a block diagram of main parts showing an embodiment of the present invention, and Figure 3 ( a) , (b
) is a main part block diagram and phase vector diagram showing a transversal filter connected to one stage of delay elements, and Fig. 4 shows delay element 5.
FIG. 2 is a block diagram of main parts showing an example of a stage-connected transversal filter. 1...Odd number type transversal filter, 2.
・・・Even number W transversal filter, 3, 4・
...Limiter circuit, 6,6...Double-wave differentiator circuit, 7...Addition circuit, 8...Integrator circuit,
9...Delay element, 10...Coefficient circuit,
10'...Addition circuit, 11.12゜13.14
．． 15...Delay element, 16-27...5
Stage-connected transversal filter coefficient circuit, 28.29
...Addition circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person 1st
Figure 2 Figure 4

Claims (3)

[Claims] (1) Has characteristics that keep broadband frequency characteristics and group delay characteristics constant, and the phase is 9 relative to the FM modulated signal.
first and second phase shift circuits that generate two FM modulation signals that differ by 0°; first and second limiter circuits that follow the first and second phase shift circuits; first and second double-wave differentiating circuits that follow the second limiter circuit, an adder circuit that adds the outputs of the first and second double-wave differentiator circuits, and an integrating circuit that integrates the output of the adder circuit. The carrier wave frequency is multiplied by 4 using the phase shift circuit, the limiter circuit, the double-wave differentiating circuit, and the adding circuit, and the original signal is frequency-separated from the 4-multiplied FM modulation signal by the integrating circuit and demodulated. An FM demodulation circuit characterized in that it is configured as follows. (2) The phase shift circuit is composed of a transversal filter including a delay element, a divisor circuit, and an adder circuit,
Two Fs whose phases differ by 90° relative to the FM modulation signal
The FM demodulation circuit according to claim 1, wherein the FM demodulation circuit generates an M modulation signal. (3) The transversal filter is symmetrical,
FM demodulation according to claim (2), characterized in that the FM demodulation is configured by delay elements and parsimonious stage connections, and generates two FM modulation signals whose phases are relatively different by 90 degrees from the FM modulation signal. circuit.