JPH05298823A - Reversal preventing circuit - Google Patents

Abstract

PURPOSE:To reduce a circuit configuration, to avoid the difficulty in a phase matching and to prevent an adverse effect caused by high frequency noises. CONSTITUTION:An input FM signal YFMi is supplied to a delaying device 4 and an adder 7 through a limiter 2 and a reversal buffer 3 and also is supplied to an adder 5. The output of the delaying device 4 is supplied to the adder 5, the output of the adder 5 is supplied to an adder 7 through a delaying device 6 and an output FM signal YFMo is obtained from the adder 7. A coefficient K of the limiter 2 becomes larger when the signal YFMi has smaller amplitudes and becomes smaller when the amplitudes become larger. If one sets a central carrier frequency fcs=1/4tau, the higher region is emphasized when the signal YFMi has smaller amplitudes and a reversal phenomenon is prevented. Since no HPF and LPF are used, the circuit configuration becomes small and the difficulty in matching phases is avoided. Since the signal YFMi directly supplied to the limiter 2, FM signals, whose carrier signals are depressed, are not inputted to the limiter 2, the increase in zero crosses due to high frequency region noises is reduced and good demodulated waveforms are obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an inversion prevention circuit suitable for being applied to a luminance signal reproducing system of a VTR that performs M modulation and recording.

[0002]

2. Description of the Related Art FIG. 7 shows the configuration of a luminance signal reproducing system of a conventional VTR. In the figure, the FM luminance signal YFM reproduced by the magnetic head 21 from the magnetic tape 20 is amplified by the reproduction amplifier 22, and then the FM demodulator 26 via the reproduction equalizer 23, the inversion prevention circuit 24 and the limiter 25.
Is supplied to. Then, the luminance signal Y output from the FM demodulator 26 is led to the output terminal 27.

Here, when an FM signal having a frequency spectrum as shown in FIG. 10A is recorded due to the nature of the electromagnetic conversion system (spacing or the like), the low frequency band is emphasized during reproduction and the high frequency band is suppressed. FM signal (shown in FIG. 7B). In FIG. 10, fc is the carrier frequency,
fm is the modulation frequency.

Since the high frequency band is suppressed in this way, when demodulating as it is, zero crossing is lost in the high frequency band,
It may be considered as a low frequency and demodulated. This is the so-called reversal phenomenon. That is, the recorded FM signal is as shown in FIG. 11A.
In such a case, the high frequency band is suppressed due to the nature of the electromagnetic conversion system, so that the reproduced FM signal lacks a zero cross as shown in FIG.

In order to prevent such an inversion phenomenon, an inversion prevention circuit 24 is provided in the example of FIG.
The inversion prevention circuit 24 is conventionally configured as shown in FIG. 8, for example. In the figure, the input FM luminance signal Y
FMi is supplied to the adder 33 via the high pass filter 31 and the limiter 32, and is also supplied to the adder 33 via the low pass filter 34. Then, the inversion-compensated FM luminance signal YFMo is output from the adder 33.

The gain of the limiter 32 changes according to the amplitude of the signal. Therefore, the frequency characteristic of the inversion prevention circuit 24 in the example of FIG. 8 becomes a characteristic in which the higher frequency band is emphasized as the amplitude becomes smaller (see FIG. 9), and the inversion phenomenon is prevented.

[0007]

By the way, in the example of FIG. 8, the high-pass signal extracted by the high-pass filter 31 and the low-pass signal extracted by the low-pass filter 34 are combined by the adder 33. It was difficult to match the respective phases at the time of synthesis due to the delay caused by the filters 31 and 34.

Therefore, an object of the present invention is to reduce the circuit scale, avoid the difficulty of phase matching, and reduce the influence of noise in the high frequency range.

[0009]

According to the present invention, there is provided an inversion prevention circuit composed of a filter consisting of a delay device, an adder and a coefficient device, in which a coefficient device whose coefficient changes non-linearly according to an input level of an FM signal. It is characterized in that the FM signal is directly supplied to the coefficient unit.

[0010]

For example, by constructing a filter using two delay elements, and making ¼τ (τ is the delay time of the delay element) substantially coincide with the center carrier frequency of the FM signal, a high frequency band at a small amplitude is obtained. The carrier signal can be boosted, the band wave can be suppressed, and the inversion phenomenon can be prevented.

Further, since the FM signal is directly supplied to the coefficient unit 2 of the filter, the carrier signal is not suppressed or the band wave is not emphasized. For example, when a limiter is used for the coefficient unit 2, It is possible to reduce the increase / decrease of zero cross due to noise in the limiter, and it is possible to obtain a good demodulation waveform.

Further, since the filter is constructed by using the delay device, the adder and the coefficient device, the phase matching becomes easier as compared with the conventional configuration using the high pass filter or the low pass filter. It is possible to reduce the circuit scale.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. This example is an example applied to a luminance signal reproducing system of a VTR (see FIG. 7), and is composed of a 3-tap transversal filter.

In the figure, an input FM luminance signal YFMi supplied to an input terminal 1 is a coefficient unit 2 composed of a limiter.
And the delayer 4 having the delay time τ via the inverting buffer 3 and the adder 7, and the direct adder 5
Is supplied to.

The output signal of the delay unit 4 is supplied to the adder 5, and the output signal of the adder 5 is supplied to the adder 7 via the delay unit 6 having a delay time τ. Then, the output signal of the adder 7 is led to the output terminal 8 as the output FM luminance signal YFMo.

In the above configuration, the coefficient of the coefficient unit 2 is set to K.
Then, the transfer characteristic YFMo / YFMi becomes as shown in Formula 1.

[0017]

[Equation 1]

Since the coefficient unit 2 is composed of a limiter, the coefficient K (limiter gain) is equivalently large when the FM luminance signal YFMi has a small amplitude. On the other hand, when the amplitude is large at the normal level, the coefficient K becomes small. As a result, as shown in FIG. 2, the FM luminance signal YFMi
The frequency characteristic changes according to the amplitude level of.

In this case, when the central carrier frequency of the FM luminance signal YFMi is fcs, by setting the delay time τ of the delay units 4 and 6 so that fcs = 1 / 4τ,
It becomes possible to obtain high-pass characteristics. This gives F
When the M luminance signal YFMi has a small amplitude, the high frequency region is boosted, so that the inversion phenomenon can be prevented.

Further, by setting the delay times of the delay units 4 and 6 so that fcs = 1 / 2τ, the bandpass characteristic can be obtained. As a result, the FM luminance signal YFM
When i has a small amplitude, it works in the direction of suppressing both sidebands, so that the inversion phenomenon can be prevented.

In this example, unlike the example of FIG. 8, the high pass filter 31 and the low pass filter 34 are not used, so that the circuit scale can be reduced and
The difficulty of phase matching can be avoided.

By the way, although the example of FIG. 1 has a transposed configuration, it can be considered to configure it as a normal transversal filter as shown in FIG. In FIG. 3, parts corresponding to those in FIG. 1 are designated by the same reference numerals.

In the case of the configuration as shown in FIG. 3, the frequency characteristic at the output point (point a) of the adder 5 becomes a band elimination characteristic (illustrated in FIG. 4). Therefore, the adder 5 suppresses the carrier signal and emphasizes the sideband F.
The M luminance signal is output. Further, the FM luminance signal YFMi has a lot of high frequency noise, but this noise is not suppressed.

When such an FM luminance signal output from the adder 5 is input to the coefficient unit 2 including a limiter, there is a possibility that the zero cross may increase or decrease due to the above-mentioned noise. Will be larger than. Therefore, the demodulated waveform may deteriorate in the example of FIG. 3 as compared with the example of FIG.

Next, another embodiment of the present invention will be described with reference to FIG. This example also uses a VTR luminance signal reproduction system (see FIG. 7).
This is an example applied to the reference), and is configured by a 5-tap transversal filter.

In the figure, an input FM luminance signal YFMi supplied to an input terminal 1 is supplied to a delay unit 11 having a delay time τ and an adder 18 via a coefficient unit 9 having a coefficient K1 and is directly added. Is supplied to the container 14. The value of the coefficient K1 is constant.

The input FM luminance signal YFMi has a coefficient K2.
Is supplied to the adders 12 and 16 via the coefficient unit 10 having The value of the coefficient K2 is variable, and the coefficient unit 10 is composed of a limiter in this example.

The output signal of the delay device 11 is the adder 12
The output signal of the adder 12 is supplied to the adder 14 via the delay device 13 having a delay time τ. The output signal of the adder 14 is a delay device 1 having a delay time τ.
5 is supplied to the adder 16, and the output signal of the adder 16 is supplied to the adder 18 via the delay device 17 having a delay time τ. Then, the output signal of the adder 18 is led to the output terminal 8 as the output FM luminance signal YFMo.

With the above configuration, the transfer characteristic YFMo / YF
Mi becomes as shown in Equation 2.

[0030]

[Equation 2]

Since the coefficient unit 10 is composed of a limiter, the coefficient K2 (gain of the limiter) equivalently increases when the FM luminance signal YFMi has a small amplitude.
On the other hand, when the amplitude is large at the normal level, the coefficient K2 becomes small. For example, K1 = -1 / 4, K2 = 0--1 / 4
In this case, the frequency characteristic when the FM luminance signal YFMi has a large amplitude (for example, K1 = -1 / 4, K2 = 0) is
As shown by the solid line in FIG. 6, the FM luminance signal YFMi
Has a small amplitude (for example, K1 = -1 / 4, K2 =-
The frequency characteristic of 1/4) is as shown by the broken line in the figure.

That is, when the FM luminance signal YFMi has a small amplitude, the bandpass characteristic becomes close to the highpass characteristic. Here, if ¼τ is set near the white peak, the sideband wave suppression degree can be increased when the FM luminance signal YFMi has a small amplitude, and the inversion phenomenon can be favorably prevented.

The frequency characteristic as shown in FIG. 6 can be obtained by connecting a bandpass filter and a variable frequency characteristic variable highpass filter for inversion prevention in series.
However, this example has the advantages that it can be configured as in the example of FIG. 5, the number of parts can be reduced, and a phase linear type can be obtained.

In the example of FIG. 5, the coefficient unit 10 is constituted by a limiter so that the coefficient K2 is changed according to the amplitude level of the FM luminance signal YFMi. Conversely, the coefficient unit 9 is changed by the limiter. Alternatively, the coefficient K1 may be changed, and both the coefficient units 9 and 10 may be configured by limiters.

Further, in the above-mentioned embodiment, the FIR filter is composed of 3-tap or 5-tap, but it may be composed of a FIR filter of a higher order than this, or may be composed of an IIR filter. ..

[0036]

According to the present invention, a filter including a delay device, an adder and a coefficient device is used, and a high pass filter or a band pass filter is used as in the prior art. Instead, the circuit scale can be reduced and the difficulty of phase matching can be avoided.

Further, since the FM signal is directly supplied to the coefficient unit of the filter, the carrier signal is not suppressed or the band wave is not emphasized. For example, when a limiter is used for the coefficient unit, the limiter It is possible to reduce the increase / decrease in zero cross due to noise and obtain a good demodulation waveform.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing frequency characteristics of the example of FIG.

FIG. 3 is a block diagram showing a configuration of an example of an inversion prevention circuit.

FIG. 4 is a diagram showing frequency characteristics of a main part of the example of FIG.

FIG. 5 is a block diagram showing the configuration of another embodiment of the present invention.

6 is a diagram showing frequency characteristics of the example of FIG.

FIG. 7 is a block diagram showing a luminance signal reproducing system of a conventional VTR.

FIG. 8 is a block diagram showing a configuration of an example of an inversion prevention circuit.

9 is a diagram showing frequency characteristics of the example of FIG.

FIG. 10 is a diagram showing a spectrum change due to a property of an electromagnetic conversion system.

FIG. 11 is a waveform diagram for explaining an inversion phenomenon.

[Explanation of symbols]

 1 Input terminal 2,9,10 Coefficient device 3 Inversion buffer 4,6,11,13,15,17 Delay device 5,7,12,14,16,18 Adder 8 Output terminal 22 Reproduction amplifier 23 Reproduction equalizer 24 Inversion Prevention circuit 25 Limiter 26 FM demodulator

Claims (4)

[Claims] 1. An inversion prevention circuit including a filter including a delay device, an adder, and a coefficient device, the coefficient device having a coefficient that nonlinearly changes according to an input level, and an FM signal is directly supplied to the coefficient device. An inversion prevention circuit characterized by: 2. The filter is configured by using two delay devices, and ¼τ (τ is a delay time of a delay element) is the F
The inversion prevention circuit according to claim 1, wherein the center carrier frequency of the M signal is substantially matched. 3. The filter is configured by using four delay devices, and ¼τ (τ is a delay time of a delay element) is the F
The inversion prevention circuit according to claim 1, wherein the high frequency carrier frequency of the M signal is substantially matched. 4. The inversion prevention circuit according to claim 1, 2 or 3, wherein the coefficient unit is composed of a limiter.