JPH01277367A - Fm demodulating device - Google Patents

Abstract

PURPOSE:To improve an SN ratio by separating and adding the lower wave of an FM signal and an upper wave obtained by passing the FM signal through a limiter by a low pass filter and a high pass filter respectively having complementary characteristics. CONSTITUTION:An FM signal is restricted at its amplitude by a limiter 8, the amplitude-limited FM signal is supplied to a high pass filter 9, the FM signal is also supplied to a low pass filter 11 having a comprementary characteristic with the high pass filter 9 and output signals from the filters 9, 11 are added and supplied to an FM demodulator 6. Thereby, the low wave of the FM signal and the upper wave obtained by passing the FM signal through the limiter 8 are separated and added through the low pass filter 11 and the high pass filter 9 and the added signal is demodulated. Consequently, the levels of the lower and upper waves are increased, the degree of modulation is increased, the noise level of the signal supplied to the FM demodulator 6 is reduced because of the equalization of the upper and lower waves and the SN ratio is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulator suitable for use in, for example, reproduction threads of video tape recorders.

[Summary of the invention]

The present invention provides the lower side wave of the FM signal and the remy/
The S/N ratio is improved by separating and adding the upper side waves obtained throughout the evening using a low-pass filter and a high-pass filter having complementary characteristics, and then demodulating the signals.

(Prior Art) FIG. 4 shows the structure of recycled yarn for a video tape recorder.

In the figure, (1) is a magnetic tape, and (2) is a reproduction magnetic head. Reproducing do M obtained from the magnetic head (2)
The signal is supplied to an equalizer (4) via a reproduction amplifier (3). In this case, the reproduced FM signal supplied to the equalizer (4) is transmitted to the carrier JO as shown in FIG. 5A.
The level of the lower side wave J-1 has increased in the same company/l. When Jo≦J-tX2, an overmodulation state occurs, so the equalizer (4) attenuates the level of the lower side wave J-L, as shown in FIG. 5B. Here, J+1 is an upper side wave, and N shown in the diagonal diagram is noise. When the level of the lower side wave J-1 is attenuated by the equalizer (4), the level of the noise N in the lower side band is also attenuated.

In addition, the output signal of the equalizer (4) is transmitted to the limiter (5).
A reproduced video signal is supplied to an FM demodulator (6) via an output terminal (7) derived from the output side of the FM demodulator (6).

As shown in FIG. 5C, the noise N contained in this video signal increases in level in proportion to the frequency (triangular noise). In this case, the level of the noise N in the lower sideband is attenuated by the equalizer (4), so the level of the noise N in the lower sideband J-1 decreases as shown by the broken line in C in the same figure. .

Therefore, various proposals have been made to improve the S/N ratio by increasing the utilization rate of the lower side waves.

For example, in a device using a playbank network circuit, the FM signal is passed through a low-pass filter that drops the frequency linearly in a phase straight line to increase the utilization rate of the lower side wave. For example, when using a double limiter, limiting is done with a bandpass filter whose center frequency is the carrier frequency, and then the lower side wave is added to change the crystal region, increasing the utilization rate of the lower side wave. It is something that makes you

(Problem to be solved by the invention) However, by increasing the utilization rate of the lower side wave in this way, S/
According to those aiming at improving the S/N, there is a limit to the improvement of the S/N when the lower side wave is used, and there is also a drawback that distortion occurs.

Taking these points into consideration, the present invention provides S
The purpose is to improve /N.

(Means for Solving the Problems) The present invention limits the amplitude of an FM signal with a limiter (8) and then supplies it to a high-pass filter (9), and passes the FM signal through a low-pass filter having complementary characteristics to the high-pass filter (9). (11), and adds the output signals of the high-pass filter (9) and low-pass filter (11) and supplies the resultant signal to the FM demodulator (6).

(Function) In the above configuration, the lower side wave of the FM signal and the upper side wave obtained by passing the FM signal through the limiter (8) are separated and added by the low-pass filter (11) and the high-pass filter (9) having complementary characteristics. Since the signal is demodulated after being demodulated, the levels of the lower side wave and upper side wave are increased, the modulation depth is amplified, and the noise level of the signal supplied to the FM demodulator (6) is reduced by equalization of the upper and lower side waves. , the S/N ratio can be improved.

Furthermore, since the low-pass filter (11) and high-pass filter (9) having complementary characteristics are used, it is possible to suppress distortion to a small level.

Furthermore, since the levels of the lower side wave and the upper side wave supplied to the FM demodulator (6) are close to each other, the system is resistant to overmodulation.

Embodiment] Hereinafter, an embodiment of the present invention will be described with reference to FIG. This example is an example that is applicable to the playback system of a video tape recorder. In FIG. 1, parts corresponding to those in FIG. 4 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In the figure, the output signal of the equalizer (4) is supplied to a limiter (8) and limited. In this case, as shown in FIG. 2C, due to the limiter effect, the level of the lower side wave J-1 decreases and the level of the upper side wave J+x increases. At the same time, the noise N in the upper sideband is generated in the lower sideband. Here, FIGS. 2A and B show the reproduced FM signal supplied to the equalizer (4) and the output signal of the equalizer (4), respectively, and FIGS. 5A and B
This is similar to the one shown in .

In addition, the output signal of the limiter (8) is filtered by a high-pass filter (
9). This high-pass filter (9) has a frequency of 6 at the frequency fc of the carrier JO, as shown in the curve in Figure 3.
It has a characteristic of dB attenuation. In this case, the high-pass filter (9) outputs the lower side wave J as shown in the second diagram.
A -1 attenuated signal is output.

The output signal of this high-pass filter (9) is sent to the adder (1
0).

Further, the output signal of the equalizer (4) is supplied to a low-pass filter (11). This low-pass filter (11
) has a characteristic such that it is attenuated by 6 dB at the frequency fc of carrier Jo, as shown by curve E in FIG. That is,
This low-pass filter (11) is assumed to have complementary characteristics to the above-described high-pass filter (9).

In this case, from the low-pass filter (11), E
As shown in FIG. 2, an attenuated signal of the upper side wave Jet is output. The output signal of this low-pass filter (11) is supplied to an adder (lO).

Further, in the adder (10), a high pass filter (
9) and the output signals of the low-pass filter (11) are added to output a signal as shown in FIG. 2F. That is, when compared with the output signal of the equalizer (4) shown in FIG. 4B, the lower side waves J-1. The level of the upper side wave Jo1 increases, and the degree of modulation becomes amplified. Further, the levels of the lower side wave J-1 and the upper side wave J41 become close to each other.

Also, the output signal of the adder (10) is sent to the equalizer (12).
The equalizer (12) attenuates the levels of the lower side wave J-1 and the upper side wave J+1, as shown in FIG. 2G, to avoid overmodulation.

At the same time, the level of the noise N in the lower sideband and upper sideband is also attenuated.

Furthermore, the output signal of the equalizer (12) is transmitted to the limiter (5).
) is supplied to the FM demodulator (61), and a reproduced video signal is obtained at the output terminal (7) derived from the output side of this FM demodulator +j.

In this way, according to this example, the lower side wave J-1 of the reproduced FM signal
and the upper side wave J+1 obtained by passing the reproduced FM signal through the limiter (8) are passed through a low-pass filter (
11) The signals are separated by a high-pass filter (9), added by an adder (10), and then demodulated by an FM demodulator (6). Therefore, as shown in FIG. 2F, the lower side wave J-1. of the output signal of the adder (10). Since the level of the upper side wave Jo1 increases and the modulation degree increases,
As shown in figure G, the lower side wave J generated by the equalizer (12)
-1. By equalizing the upper side wave J,1, the level of noise N of the signal supplied to the FM demodulator (6) is reduced, and the S/N ratio can be improved. That is, FIG. 2H is FM'14
This shows the noise N contained in the output signal of the i-adjuster (6), which can be made lower than the level of the conventional example shown by the broken line.

Further, since the low-pass filter (11) and the high-pass filter (9) having complementary characteristics are used, the runout can be kept small.

Further, as shown in FIG. 2G, the levels of the lower side wave J-1 and the upper side wave Jet of the signal supplied to the FM demodulator (6) are close to each other, so that it can be made resistant to overmodulation.

Although the above-described embodiment is applicable to a reproduction system of a video tape recorder, the present invention can be similarly applied to other FM signal demodulation systems.

〔Effect of the invention〕

According to the present invention described above, the lower side wave of the addition signal.

As the level of the upper side wave increases and the modulation depth increases,
By equalizing the upper and lower side waves, the noise level of the signal supplied to the FMfjtm device is reduced, and the S/N ratio can be improved. Further, since a low-pass filter and a high-pass filter having complementary characteristics are used, distortion can be suppressed to a small level. Further, since the levels of the lower side wave and the upper side wave of the signal supplied to the FMfjt modulator are close to each other, it can be made resistant to overmodulation.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Figs. 2 and 3 are diagrams for explaining the example in Fig. 1, Fig. 4 is a block diagram of a conventional example, and Fig. 5 is a conventional example. FIG. (1) is a magnetic tape, (2) is a reproduction magnetic head, (3)
is a reproduction amplifier, (4) and (12) are equalizers, (
5) and (8) are limiters, (6) is an FM demodulator, (
7) is the output terminal, (9) is the high pass filter, (1o)
is an adder, and (11) is a low-pass filter.

Claims (1)

[Claims] The FM signal is amplitude-limited by a limiter and then supplied to a high-pass filter, the FM signal is supplied to a low-pass filter having complementary characteristics to the high-pass filter, and the output signals of the high-pass filter and the low-pass filter are added. F
M demodulator.