JP3158537B2 - Inversion prevention circuit - Google Patents

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 application to, for example, a reproduction system of a video tape recorder.

[0002]

2. Description of the Related Art Hitherto, a high-quality standard of 8 mm video (Hi8 standard) has been proposed in response to a demand for a high-quality image for a consumer VTR. According to the Hi8 standard, the FM carrier frequency is changed from 5 MHz to 7 for higher resolution and higher S / N.
MHz and the frequency shift is from 1.2 MHz to 2 MHz. Therefore, the shortest recording wavelength is about 0.7 μm
(5.4 MHz) to about 0.5 μm (7.7 MHz). FIG. 10 shows frequency allocation of the conventional 8 mm video standard, and FIG. 11 shows Hi8
It shows the frequency allocation of the standard.

As described above, in the Hi8 standard, an inversion phenomenon due to a separation loss (spacing loss) at the time of reproduction tends to occur due to a reduction in the shortest recording wavelength.

FIG. 12 shows an example of a conventional inversion prevention circuit. In the figure, a luminance FM signal YFM reproduced from a magnetic tape has a -si
The signal is supplied to an adder 23 via a series circuit of an n ² filter 21 and a limiter 22. Also, the luminance FM signal YFM is
The sideband component is supplied to an adder 23 through a series circuit of a cos ² filter 24 and a coefficient unit 25 that pass the sideband component.

[0005] The adder 23 subtracts the output signal of the limiter 22 from the output signal of the coefficient unit 25. Then, the output signal of the adder 23 is output via the low-pass filter 26.

In the above configuration, by adjusting the gain K of the coefficient unit 25 so that K <1, an equalizing characteristic for suppressing the sideband is obtained. The limiter 22 acts to average the upper and lower sidebands when the level of the luminance FM signal YFM is sufficient, and acts as a variable gain amplifier when the level decreases.

As a result, a dynamic equalizing characteristic in which the equalizing amount for the sideband is varied according to the amplitude level of the luminance FM signal YFM is obtained, and the occurrence of the inversion phenomenon is prevented.

[0008]

By the way, separation loss and the like often occur instantaneously and largely. When the level of the carrier is greatly reduced, the level compensation cannot be sufficiently compensated by the level compensation characteristic (sin ² ) in the example of FIG. 12, and it is difficult to prevent the occurrence of the inversion phenomenon.

The separation loss Ls is expressed by the following equation (2). It is also said that separation loss occurs during recording, and that the coefficient is about 100.

[0010]

(Equation 2)

In the Hi8 standard, a level drop of -9 dB occurs at a separation loss (.lambda. = 0.37 .mu.m) of d = 0.06 .mu.m, but a level drop of about -18 dB is also expected in consideration of recording.

Further, when an FM signal is recorded / reproduced by a VTR or the like, a lower sideband enhancement effect occurs. This emphasis of the lower sideband occurs steadily, but is a factor of the inversion phenomenon.

According to the present invention, it is possible to preferably prevent the reversal phenomenon due to the emphasis of the lower sideband and the separation loss.

[0014]

SUMMARY OF THE INVENTION The present invention provides an input FM signal.
Signal and the output signal of this adder
And the frequency to be compensated to prevent inversion is 1 / 2τD
When a, have a transfer function A of the equation 3 (omega), the output F
A filter you output M signal, the output signal <br/> of the filter, and supplies non-linear gain to the adder feedback Le
And means for forming a loop.

[0015]

(Equation 3)

[0016]

[Operation] The level of the input FM signal is large and the gain K2 ≒
When it becomes 0, a linear phase filter is formed. By adjusting a and b to equalize the amplitude in the linear phase, it is possible to prevent the reversal phenomenon due to the lower sideband emphasis function, and it is possible to minimize the waveform distortion.

When the level of the input FM signal is low and the gain K2 satisfies 0 <K2≤1 / (a + b), a boost operation of the frequency f for which inversion is to be compensated is performed. Therefore, it is possible to compensate for the carrier level reduction due to a large instantaneous separation loss, and to sufficiently prevent the inversion phenomenon. In this case, the phase does not become linear but instantaneous phase distortion occurs. However, there is no problem because instantaneous phase distortion is hardly perceived.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. This example is Hi8 standard 8mmVT
This is an example applied to an R reproduction system.

In FIG. 1, a luminance FM signal YFM reproduced from a magnetic tape 1 by a magnetic head 2 is supplied to a high-frequency compensation circuit 4 via a reproduction amplifier 3.

The high-frequency compensation circuit 4 compensates for high-frequency attenuation due to reproduction loss. This compensation amount is switched according to the electromagnetic conversion characteristics of the magnetic tape 1, the recording track width, and the like.
For example, when the magnetic tape 1 is an MP tape, the high-frequency attenuation increases, so that the compensation amount is larger than when the magnetic tape 1 is an ME tape.

The luminance FM signal YFM whose high-frequency attenuation has been compensated by the high-frequency compensation circuit 4 is supplied to an adder 12 constituting an inversion prevention circuit 11 via an AGC circuit 5. This adder 1
2 is supplied to the filter 13. The filter 13 has an amplitude characteristic shown in Expression 4 and is configured to generate a delay of τD.

Here, τD is given by f when the frequency to be compensated for preventing inversion is f (for example, about 8 MHz).
It is set so as to satisfy f = 1 / 2τD.

[0023]

(Equation 4)

The output signal of the filter 13 is supplied to the adder 12 via a series circuit of a limiter 14 having a non-linear gain and an attenuator 15. Adder 12
In, the output signal of the filter 13 is subtracted from the luminance FM signal YFM from the AGC circuit 5.

The transfer function A (ω) of the filter 13 of the inversion prevention circuit 11 is expressed by Expression 5.

[0026]

(Equation 5)

Further, the limiter 14 and the attenuator 15
Assuming that the total gain is K2, the frequency characteristic (amplitude characteristic) | G (ω) | of the inversion prevention circuit 11 is expressed by Equation 6.

[0028]

(Equation 6)

In the equation (6), at a frequency f = 1 / 2τD where the denominator term cosωτD = −1, sin ² (ωτD / 2) is 1
And the amplitude | G | is expressed by Expression 7.

[0030]

(Equation 7)

It should be noted that the frequency characteristic | G (ω) | obtained by normalizing the frequency characteristic | G (ω) |
(ω) | ′ is as shown in Expression 8.

[0032]

(Equation 8)

The inversion prevention circuit 11 controls the luminance FM signal Y
When the FM level is large and K2 ≒ 0, it becomes a linear phase filter. Therefore, a and b are adjusted and the amplitude is equalized in the linear phase, whereby the occurrence of the inversion phenomenon due to the lower sideband emphasizing action can be prevented, and the waveform distortion can be suppressed to the minimum.

In addition, in the inversion prevention circuit 11, when the level of the luminance FM signal YFM is small and 0 <K2 ≦ 1 / (a + b), a boost operation of the frequency f for which inversion is to be compensated is performed. As a result, it is possible to sufficiently compensate for the carrier level drop due to a large instantaneous separation loss, and to prevent the occurrence of the inversion phenomenon. In this case, the phase does not become linear but instantaneous phase distortion occurs. However, there is no problem because instantaneous phase distortion is hardly perceived.

Brightness FM output from inversion prevention circuit 11
The signal YFM is supplied to a limiter 7. In the limiter 7,
The AM component included in the luminance FM signal YFM is removed. The luminance FM signal YFM output from the limiter 7 is supplied to an FM demodulation circuit 8 where the luminance signal is demodulated. This luminance signal is led to an output terminal 10 via a de-emphasis circuit 9.

Next, a specific example of the filter 13 of the inversion prevention circuit 11 will be described with reference to FIGS. In these figures, parts corresponding to those in FIG. 1 are denoted by the same reference numerals.

First, the example of FIG. 2 will be described. AGC
The luminance FM signal YFM from the circuit 5 is supplied to the adder 12. The output signal of the adder 12 is supplied to the adder 103 via a series circuit of delay circuits 101 and 102 constituting the filter 13, and is also supplied directly to the adder 103. The delay times of the delay circuits 101 and 102 are set to τD.

The output signal of the adder 103 is supplied to the adder 105 via the attenuator 104, and the output signal of the delay circuit 101 is supplied to the adder 105. In the adder 105, the output signal of the attenuator 104 is subtracted from the output signal of the delay circuit 101.

The output signal of the adder 105 is supplied to the limiter 7 as an output signal of the filter 13 and is also supplied to the adder 12 via a series circuit of the limiter 14 and the attenuator 15.

In the above configuration, the attenuator 104
Is the gain of K1, the transfer function A (ω) of the input and output of the filter 13 is expressed by equation (9). When Equation 9 is compared with Equation 5 described above, a = 1-2K1 and b = 4K1.

[0041]

(Equation 9)

Further, the normalized frequency characteristic | G (ω) | '
Is represented by Expression 10.

[0043]

(Equation 10)

The gain K2 of the limiter 14 changes nonlinearly according to the input level. Therefore, when the input level is large and K2 ≒ 0, | G (ω) | ′ ≒ (1-2K1cosωτD) / (1 + 2K1), and the frequency characteristic can be set without phase distortion by changing K1.

FIG. 3 shows a change in the frequency characteristic when K1 is changed. When K1 = 0.5, it becomes a sine-square filter, and when K1 = 0, the frequency characteristic becomes flat.

On the other hand, when the input level is small and 0 <K2 ≦ 1
In the case of / (1 + 2K1), the lower sideband level is suppressed as the gain K2 of the limiter 14 increases. As a result, the initially set characteristics operate as a dynamic equalizer, thereby compensating for the carrier level reduction. Note that oscillation occurs when K2 = 1 / (1 + 2K1).

FIG. 4 shows a change in the frequency characteristic when the input level changes from a large input level to a small input level when K1 = 0. FIG. 5 shows a change in the frequency characteristic when the input level changes from a large input level to a small input level when K1 = 0.2. FIG. 6 shows a change in the frequency characteristic when the input level changes from a large input level to a small input level when K1 = 0.5.

When K1 = 0.5 and K2 = 0, a sine-square characteristic is obtained. In the example of FIG. 12 described above, no further compensation characteristics can be obtained, but in this example, the lower the input level, the more the boost compensation is performed around f = 1 / 2τD.

Next, the example of FIG. 7 will be described. AGC
The luminance FM signal YFM from the circuit 5 is supplied to the adder 12. The output signal of the adder 12 is supplied to the adder 113 via a series circuit of delay circuits 111 and 112 constituting the filter 13 and is also directly supplied to the adder 113. The delay times of the delay circuits 111 and 112 are set to τD.

The output signal of the adder 113 is reduced to a half level by the attenuator 114,
5,116. These adders 115 and 116
Is supplied with an output signal of the delay circuit 111.

In the adder 115, the output signal of the attenuator 114 is subtracted from the output signal of the delay circuit 111.
The output signal of the adder 115 is supplied to the adder 118 via the limiter 117.

In the adder 116, the output signal of the delay circuit 111 and the output signal of the attenuator 114 are added.
The output signal of the adder 116 is supplied to the adder 118 via the attenuator 119.

In the adder 118, the output signal of the limiter 117 is subtracted from the output signal of the attenuator 119. The output signal of the adder 118 is supplied to the limiter 7 as an output signal of the filter 13 and is also supplied to the adder 12 via a series circuit of the limiter 14 and the attenuator 15.

In the above configuration, the attenuator 119
Assuming that the gain of the filter 13 is K1 and the gain of the limiter 117 is K3, the input / output transfer function A (ω) of the filter 13 becomes
It is represented by Comparing Equation 11 with Equation 5 above, a =
2K1, b = 2 (K3-K1).

[0055]

[Equation 11]

Further, the normalized frequency | G (ω) | '
It is expressed by Equation 12.

[0057]

(Equation 12)

The gain K2 of the limiter 14 changes nonlinearly according to the input level. Therefore, increase the input level, when the K2 ≒ 0, | G (ω ) | '≒ (K3sinω 2 τD / 2 + K1cos 2 ωτD / 2) / K3 becomes, can set the carrier amplitude by changing the K3, also K1 Can be adjusted to adjust the sideband amplitude, and can be equalized in linear phase. Here, when K1 = 0, the filter becomes a sine-square filter, and when K1 = K3, the frequency characteristic becomes flat.

On the other hand, when the input level is small and 0 <K2 ≦ 1
/ 2K3, the gains K2,
It operates as a dynamic equalizer under the control of K3. Note that oscillation occurs when K2 = 1 / 2K3.

In this example, the operation is performed with the phase characteristics shown in FIG. 9 in relation to the gains K2 and K3 of the limiters 14 and 117 and the threshold level. For example, K
When 2 ≒ 0, if K3 ≠ 0, the range of equalization that can be operated in the linear phase region increases.

Next, the example of FIG. 8 will be described. AGC
The luminance FM signal YFM from the circuit 5 is supplied to the adder 12. The output signal of the adder 12 is supplied to the adder 123 via a series circuit of delay circuits 121 and 122 constituting the filter 13 and is also directly supplied to the adder 123. The delay times of the delay circuits 121 and 122 are set to τD.

The output signal of the adder 123 is set to a half level by the attenuator 124,
Supplied to The output signal of the delay circuit 121 is supplied to the adder 125.

In the adder 125, the output signal of the attenuator 124 is subtracted from the output signal of the delay circuit 121.
The output signal of the adder 125 is supplied to the adder 127 via the limiter 126. The output signal of the delay circuit 121 is supplied to the adder 127 via the attenuator 128, and is subtracted from the output signal of the limiter 126.

The output signal of the adder 127 is supplied to the limiter 7 as an output signal of the filter 13, and is also supplied to the adder 12 via a series circuit of the limiter 14 and the attenuator 15.

In the above configuration, the attenuator 128
If the gain of the filter 13 is K1 and the gain of the limiter 126 is K3, the input / output transfer function A (ω) of the filter 13 is given by
It is represented by Comparing Equation 13 with Equation 5 above, a =
The K1, b = -2K _3.

[0066]

(Equation 13)

The normalized frequency | G (ω) | ′ is
It is represented by Equation 14.

[0068]

[Equation 14]

Although detailed description is omitted, the same operation and effect as in the example of FIG. 7 can be obtained in the example of FIG.

In the examples of FIGS. 2, 7 and 8, when the peak of the input / output gain is normalized to 1, the position where the limiter 14 is inserted is different from the above-described embodiment, and the filter 13 May be before or after. However, it is assumed that the total gain of the limiter 14 and the attenuator 15 is K2.

In the above-described embodiment, the present invention is applied to the Hi8 standard 8 mm video reproduction system. However, the present invention can be similarly applied to other systems which need to prevent the inversion phenomenon. Of course.

[0072]

According to the present invention, when the level of the input FM signal is high, a linear phase filter is formed. Therefore, the amplitude is equalized in the linear phase to prevent the inversion phenomenon due to the lower sideband enhancement. And the waveform distortion can be minimized. Further, when the level of the input FM signal is low, the boost operation of the frequency for which inversion is to be compensated is performed, so that it is possible to compensate for the carrier level drop due to a large instantaneous separation loss and to sufficiently prevent the inversion phenomenon.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment.

FIG. 2 is a diagram illustrating a specific example of an inversion prevention circuit.

FIG. 3 is a diagram illustrating frequency characteristics of the example of FIG. 2;

FIG. 4 is a diagram illustrating frequency characteristics of the example of FIG. 2;

FIG. 5 is a diagram illustrating frequency characteristics of the example of FIG. 2;

FIG. 6 is a diagram illustrating frequency characteristics of the example of FIG. 2;

FIG. 7 is a diagram illustrating a specific example of an inversion prevention circuit.

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

FIG. 9 is a diagram illustrating a relationship between a threshold level, a limiter level, and a phase characteristic in the example of FIG. 7;

FIG. 10 is a diagram showing frequency allocation according to a conventional 8 mm video standard.

FIG. 11 is a diagram illustrating frequency allocation according to the Hi8 standard.

FIG. 12 is a configuration diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magnetic tape 2 Magnetic head 4 High frequency compensation circuit 7 Limiter 8 FM demodulation circuit 11 Inversion prevention circuit 12 Adder 13 Filter 14 Limiter 15 Attenuator

Claims (1)

(57) [Claims] 1. An adder to which an input FM signal is supplied, and an output signal of the adder, wherein when a frequency to be compensated for preventing inversion is set to 1 / 2.tau. (omega) have a, output filter you output an FM signal, an output signal of the filter, preventing inversion circuit comprising a means for forming a feedback loop to supply non-linear gain to said adder. (Equation 1)