JPH0210633B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording device for recording video signals using FM.

Since the VTR has a pre-emphasis circuit,
When a square wave is input, an overshoot occurs in the transient response, and when this becomes excessive, an inversion phenomenon occurs in the FM reproduction output (a phenomenon in which the reproduced image is broken at the sharp rise of the signal). For this reason, the VTR is equipped with a white clip circuit, which limits the FM input signal from exceeding the white clip level.

By the way, if the white clip level is set low, the effect of preventing the inversion phenomenon will be increased, but the waveform distortion at high frequencies will be increased. On the other hand, if the white clip level is set high, waveform distortion will be reduced, but the inversion phenomenon will be more likely to occur. For this reason, the white clip value for general VTRs is
It is set to be the optimum value for signals within EIA standards. In order to reduce the inversion phenomenon or waveform distortion, it is conceivable to simply change the DC white clip level according to changes in the input signal level, but it is difficult to prevent both the inversion phenomenon and waveform distortion. .

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a magnetic recording device that can limit both the reversal phenomenon and waveform distortion.

To achieve the above object, the present invention includes a pre-emphasis circuit connected to a recording signal transmission path;
a phase inversion circuit connected to the pre-emphasis circuit and formed to invert the phase of a recording signal output from the pre-emphasis circuit; A band pass filter formed to extract an alternating current component from the output, and a band pass filter connected to the pre-emphasis circuit and the band pass filter, and having a level for clipping the recording signal output from the pre-emphasis circuit. a white clip circuit formed to change in response to changes in the alternating current component output from the band pass filter; a frequency modulation circuit connected to the white clip circuit; and a frequency modulation circuit connected to the frequency modulation circuit. The present invention relates to a magnetic recording device comprising a magnetic recording head.

According to the above invention, even if an overshoot occurs due to passage through the pre-emphasis circuit, the clip level changes in accordance with the AC component by providing the corresponding phase-inverted AC component to the white clip circuit. However, the waveform in the overshoot portion becomes narrower. Therefore, the inversion phenomenon, which is a problem in frequency modulation (FM) recording and reproduction, is less likely to occur.

The video signal recording system of the VTR according to the embodiment of the present invention shown in FIG. 1 includes a known video signal input terminal 1, a low-pass filter 2, and
It has a pre-emphasis circuit 3, a coupling capacitor 4, a clamp circuit 5, a white clip circuit 6, an FM modulation circuit 7, a recording amplifier 8, a recording/reproduction changeover switch 9, and a rotary head 10, and further includes a white clip level modulation according to the present invention. It has a circuit 11.

The white clip level modulation circuit 11 is connected to the branch line 12 of the output stage of the pre-emphasis circuit 3.
An inverting amplifier circuit 13 consisting of resistors R ₁ , R ₂ , R ₃ and a transistor Q ₁ is connected, and this inverting amplifier circuit 13
A bandpass filter 14 consisting of an inductance element L1 and a capacitor _C1 is connected between the output terminal of the transistor _Q1 , that is, the collector of the transistor _Q1 , and the white clip level setting line of the white clip circuit 6.
It is formed by connecting. The base of the transistor _Q1 of the inverting amplifier circuit 13 is a resistor.
connected to pre-emphasis circuit 3 via R ₁ ;
Its collector is connected to the positive power supply +V _CC through a resistor R ₂ and its emitter is grounded through a resistor R ₃ . Moreover, the band pass filter 14 is
It consists of a series circuit of an inductance element L ₁ of 100 μF and a capacitor C ₁ of 220 pF.

The reproduction system that obtains reproduction signals from a magnetic tape 15 that travels diagonally along a guide drum and is scanned diagonally by a head 10 includes a known rotary head 10, a changeover switch 9, a reproduction amplification circuit 16, and a limiter 17, which are connected in sequence. , an FM demodulation circuit 18, a de-emphasis circuit 19, a video amplification circuit 20, and a reproduction output terminal 21.

FIG. 2 shows an example of the connection relationship between the white clip level modulation circuit 11 and the white clip circuit 6 in FIG. 1. The white clip circuit 6 has a series circuit of a white clip level adjusting resistor R ₄ and a voltage dividing resistor R ₅ connected between the +V _CC power supply and ground. Resistance R ₄
The voltage obtained from transistor Q ₂ is applied via resistor R ₆ and white clip setting input line 22.
connected to the base of. Since a transistor Q ₂ , another transistor Q ₃ , and a resistor R ₇ are connected in sequence between the +V _CC power supply and the ground, the resistance value of the transistor Q ₂ corresponding to the white clip setting input and the transistor Q A voltage determined by the voltage division ratio of the resistance value of _{R 3} and the sum of the value of resistor R ₇ is obtained at point P ₁ . On the other hand, the video signal clamped by the clamp circuit 5 shown in FIG. 1 is input from the line 23 to the base of the transistor Q ₄ via the resistor R ₈ . Base and point of transistor Q ₄
Since diodes D ₁ and D ₂ are connected between P ₁ and P 1, when a video signal that is higher than the potential at P ₁ by the forward voltage drop of diodes D ₁ and D ₂ is input, diode D ₁ , D ₂ is turned on and clipped. Therefore, a voltage higher than a certain level (white clip level) is not input to the base of transistor _Q4 . Therefore, a video signal of a certain level or higher is not output to the white clip output line 24 derived from the emitter of the transistor _Q4 .

By the way, in this embodiment, the output line 2 of the white clip level modulation circuit 11 is
5 is coupled to the white clip setting input line 22. Therefore, the bandpass filter 14
The AC component obtained from the current is added to the DC white clip level.

FIG. 3 is a voltage waveform diagram illustrating states A to F in FIG. 1. Now, pre-emphasis circuit 3
When a dot-shaped video signal having an extremely short width (for example, 200 nsec) is inputted to the circuit, the waveform shown in FIG. 3B is obtained by the action of the pre-emphasis circuit 3. This waveform is sent to the white clip circuit 6 via the clamp circuit 5, and is also sent to the inverting amplifier circuit 13 via the branch line 12, where it is converted into a phase inverted signal as shown in FIG. 3C. When the signal shown in FIG. 3C is sent to the bandpass filter 14, an AC component as shown in FIG. 3D is obtained on its output line, which is added to the DC white clip level setting input and White clip level V _R modulated as shown in Figure E
is obtained. The video signal V _S shown by the dotted line and solid line in _FIG . You will get the output shown in .

If the conventional flat white clip level is used, the white clip output will be as shown in FIG. Therefore, as is clear from the comparison between FIG. 3F and FIG. 4, according to the modulation white clip level of the present invention, the top of the clipped waveform is not flat but tapered. That is, the white clip V _R level gradually decreases from the point near when the video signal waveform intersects the white clip level V _R ,
Since the signal is then modulated to rise, the overshot video signal waveform is clipped so that its width becomes narrower. In this way, when the clipped waveform becomes tapered, the excessive level signal period becomes shorter, and the high frequency period of the FM modulated wave also becomes shorter. In addition, the reversal phenomenon in the reproduced output is less likely to occur. If clipping is performed at a flat white clip level V _R1 as shown in FIG. 4, the excessive level signal period will become longer, the high frequency period of the FM modulated wave will also become longer, and there is a possibility that an inversion phenomenon of the reproduced output will occur. Therefore, in order to prevent the reversal phenomenon using the conventional white clip method, the white clip level had to be further lowered as shown in V _R2 in FIG. For this reason, in the past, attempts to prevent the inversion phenomenon inevitably resulted in increased waveform distortion at high frequencies, and also resulted in poor signal-to-noise ratios since the signal level could not be increased.

5 to 7 show the relationship between the white clip level and the reversal phenomenon. Even with the conventional white clip method, as shown in Figure 5A, if the level of the video input signal V _S is low and the overshoot due to pre-emphasis does not significantly exceed the white clip level V _R1 , an inversion phenomenon will occur. Does not occur. That is, in this case, since the excessive level period is short, the high frequency period of the FM modulated wave shown in FIG. 5B is also short. Therefore, due to poor frequency phase characteristics of the VTR tape, head system, amplifier system, etc.
As shown in the figure, even if the reproduction output of the high frequency part decreases,
Recover immediately. Therefore, there is little possibility that a zero cross will be missed and an inversion phenomenon will occur. On the other hand, in the conventional method, the input signal is
As the level of V _S increases, the amount by which the overshoot portion crosses the white clip level V _R1 increases, and the high frequency period becomes longer, as shown in FIG. 6B. Therefore, as shown in FIG. 6C, the reproduced output is in a state where a zero cross is missing, and an inversion phenomenon occurs. Therefore, in order to prevent the reversal phenomenon, the white clip level must be further lowered.
Must be V _R2 . However, this results in an increase in high-frequency waveform distortion and a decrease in the signal-to-noise ratio.

FIG. 7 shows the principle of the present invention.
As shown in FIG. 6A, even if the input signal V _S and the white clip level V _R1 are the same as those in FIG. 6A, the inversion phenomenon does not occur.
In the present invention, the maximum white clip level is the fifth level.
As in FIG. 6 and FIG. 6, V _{R1 is} not kept constant but is changed according to fluctuations in the input signal level. Therefore, the shaded area in FIG. 7A is removed by the modulation white clip level. As a result, the overshoot portion becomes tapered and the high frequency period becomes shorter as shown in FIG. 7B. Therefore, as shown in FIG. 7C, there is less possibility that zero crossings of the reproduced output will be missing. still,
Since the inversion phenomenon does not occur below the level V _R2 , the part to be removed at the modulation white clip level is V _R2
It is desirable to do more than that.

As is clear from the above, this embodiment has the following advantages.

(a) Modulate the white clip level to correspond to changes in the level of the input signal, and clip it so that the time width of the leading edge of the waveform after white clipping is substantially constant regardless of the input signal level. Therefore, the high frequency period of the FM modulated wave is shortened, making it difficult for the inversion phenomenon to occur.

(b) Since it is not necessary to lower the white clip level to prevent the inversion phenomenon, it is possible to reduce high frequency waveform distortion and improve the S/N ratio.

Although the embodiments of the present invention have been described above, the present invention is not limited thereto and can be further modified. For example, white clip circuit 6
may be modified as shown in FIG. In other words, the DC white clip level obtained from the white clip level setting resistors R ₄ and R ₅ is connected to the diode.
given to point P ₁ between D ₁ and capacitor C ₂ , and
An alternating current component may be applied here from the white clip level modulation circuit 11.

Further, the constant of the bandpass filter 14 may be changed depending on changes in the time width of the dot signal included in the recording signal.

In addition, in response to the sudden rise of the waveform, a time constant circuit such as a CR starts operating, and a modulation component is created so that the white clip level gradually decreases, and this is used to set the constant DC white clip. May be added to the level.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the video signal system of a VTR according to an embodiment of the present invention, Fig. 2 is a circuit diagram showing the white clip circuit of Fig. 1 in detail, and Fig. 3 shows the state of each part in Fig. 1. Figure 4 is a voltage waveform diagram showing a conventional white clip; Figure 5 is a voltage waveform diagram showing a conventional white clip;
6 and 7 are waveform diagrams showing the relationship between input signal level changes and reproduction output changes due to white clipping, and FIG. 8 is a circuit diagram showing a modification of the white clip circuit. In the symbols used in the drawings, 3 is a pre-emphasis circuit, 6 is a white clip circuit, 7 is an FM modulation circuit, 11 is a white clip level modulation circuit, 13 is an inverting amplifier circuit, and 14 is a band pass filter. .

Claims (1)

[Claims] 1. A pre-emphasis circuit connected to a recording signal transmission path; and a pre-emphasis circuit connected to the pre-emphasis circuit;
and a phase inversion circuit formed to invert the phase of the recording signal output from the pre-emphasis circuit; and a phase inversion circuit connected to the phase inversion circuit and configured to extract an alternating current component from the output of the phase inversion circuit. a band-pass filter formed therein; and a band-pass filter connected to the pre-emphasis circuit and the band-pass filter, the level of which is set at a level for clipping the recording signal output from the pre-emphasis circuit. A magnetic recording device comprising a white clip circuit formed to change in response to changes in an alternating current component, a frequency modulation circuit connected to the white clip circuit, and a recording magnetic head connected to the frequency modulation circuit. Device.