JPS6120927B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording and reproducing device, and an apparatus that can prevent deterioration of the SN ratio of a video signal due to sliding noise and obtain high-quality images even when sliding noise occurs. The purpose is to provide.

An embodiment of the present invention will be described below in comparison with a conventional example.

Figure 1 shows the basic configuration of the video signal playback circuit and demodulation circuit system in a conventional VTR.
1 is a signal generator that records a single sine wave signal instead of the modulator output signal for the following explanation.Amplifier 2
In addition to Note that normally, the FM modulated wave of the video signal is amplified by a recording amplifier 2 and recorded on a video tape 4 by a recording head 3.

Next, the signal recorded on the video tape 4 is played back by the playback head 5, the playback output is amplified by the playback amplifier 7, the amplitude fluctuation component is removed by the limiter 9, and the signal is returned to the original video signal 12 via the demodulation circuit 11. demodulates to .

When demodulating an FM signal using the reproducing circuit and demodulating circuit system described above, the following problems occurred.

Currently, small VTRs use ferrite heads to extend the life of the head, and single-crystal,
Various types of ferrite materials such as HPF ferrite are used. However, when using this ferrite head, a problem arises in the sliding noise induced within the head as the head slides on the tape 4. This noise is particularly affected by the composition of the ferrite that makes up the head, the amount of protrusion of the head, the tape tension, the shape of the head, etc., and in severe cases, the noise level can increase by more than 10 [dB]. be.

In this case, the video S/N ratio deteriorates significantly and the image quality deteriorates significantly.

Next, various noise components will be explained using FIG. 2. Figure 2 shows measurements taken using a VTR with a head-to-tape relative speed of 5.8 [m/sec] and a reproducing amplifier with flat frequency characteristics. At this time, head 3 uses a single crystal ferrite head,
The videotape 4 used had magnetic properties of a coercive force H _c of 670 O _e and a residual magnetic flux density B _r of 1400 Gauss. The characteristic 17 in FIG. 2 shows that a 4MHz single sine wave signal corresponding to the center carrier wave is added from the signal generator in FIG. The outputs 13 and 15 were measured by connecting a spectrum analyzer to the regenerative amplifier output 8.
These are the signal and modulation noise, respectively. Further, 17 is a modulation noise measured at the limiter output 10 by adding this signal and modulation noise to a limiter, and this noise is demodulated to become a noise component that determines the video S/N.

Also, the carrier wave signal level C at the limiter output
and the noise component N, that is, C/N, is the video S/N.
It has been experimentally confirmed that there is approximately a one-to-one correspondence with N, and as this value becomes smaller, the video S/N deteriorates. 14 is device noise, and 16 is the aforementioned sliding noise. From this figure 2, 8MHz
It can be seen that the above modulation noise is mostly occupied by sliding noise.

Next, the influence of sliding noise on the C/N of the limiter will be explained using FIG.

Figure 3 shows a 4MHz
The signals are regarded as carrier signals and summarized as C/N. At this time, the spectra obtained by varying the protrusion amount of the head from the rotating cylinder surface to 25 μm, 50 μm, and 100 μm were 18 and 1, respectively.
It is 9,20. Further, with the above parameters, the limiter 9 output 10 corresponds to 21, 22, and 23, respectively.

As described above, in the output 10 of the limiter 9, as the sliding noise level increases, the C/N rapidly deteriorates in parallel movement. Therefore, in the conventional circuit configuration, if sliding noise occurs in the head, it will greatly affect the video S/N.

Next, the principle of the present invention that can avoid this problem will be explained using FIGS. 4 and 5. Through the applicant's experiments and theoretical analysis,
The following was revealed. Here, if the carrier signal frequency is _c and the frequency of the noise component is _N , then [ ] If O < _N < _c , the spectrum where the noise component is superimposed on the carrier component in the regenerative amplifier 7 [Figure 4 A]
When the light passes through the limiter 9, it becomes a spectrum as shown in FIG. 4B.

[] When _c < _N < 2 _c When the spectrum in which the noise component is superimposed on the carrier component (FIG. 4C) passes through the limiter 9, it becomes a spectrum as shown in FIG. 4D.

[] When 2 _c < _N < 3 _c When the spectrum [Fig. 4 E] in which the noise component is superimposed on the carrier wave component passes through the limiter 9, it becomes as shown in Fig. 4 F.

The following becomes clear from these FIGS. 4A to 4F.

Noise components existing in a band of 2 _c or more that is not necessary for signal transmission, such as 2 _c - _N in Figure 4F.
Components outside the required transmission band enter the signal transmission band (from 0 to 2 _c ), such as 4 _{c −N} . In addition, in Fig. 4 A to F, 2 _c < _N < 3
Although only the noise up to _c is shown, noise components beyond that also enter the signal transmission band and degrade the S/N ratio as noise components.

Further, the arrows in FIGS. 4A to 4F indicate the direction in which the frequency _N of the noise in the regenerative amplifier increases, and at that time indicate the direction in which the spectrum in the limiter output moves.

To explain more clearly, consider a case where the noise 24 and the carrier signal 25 having the distribution shown in FIG. 5A are superimposed. The noise components in Fig. 5A are 0<N< _N as shown in Fig. 5C, E, and G.
They are denoted by 26, 27, and 28 as noise components of _c , _c < _N < 2 _c , and 2 _c < _N < 3 _c , respectively.

By passing through the limiter, the noise components 26, 27, and 28 are separated into both sides of the carrier wave, as shown in FIG. Become. Therefore, considering the noise components within 0< _N < _3c , as shown in FIG.
2, the signal will appear within the required transmission band 0 to _2c .

In particular, sliding noise has a high level of noise components with 2 _c < _N , which causes deterioration of video S/N.

Therefore, in the present invention, as shown in FIG. 6, a low-pass filter 34 is provided between the regenerative amplifier 7 and the limiter 9 to remove noise components outside the signal transmission band, that is, 2 _c or more, thereby reducing the sliding noise. The circuit configuration is such that the influence of noise is greatly suppressed, and the sliding noise does not affect the S/N ratio.

Figure 7 was obtained in the same way as the spectrum measurement shown in Figure 3, but this is
Observation was made at the terminal in FIG.

During the measurement, the cutoff frequency of the low pass filter 34 was set to 8MHz, or _2c , as shown in FIG. Spectra 37, 36, and 35 in FIG. 7 correspond to 18, 19, and 20 in FIG. 3, and are obtained when the head protrusion amounts are 25 μm, 50 μm, and 100 μm. 40, 39, and 38 are the spectra at the limiter output, and the C/N does not change significantly due to the influence of sliding noise, which means that even if the sliding noise level becomes significantly high, It has been demonstrated that the S/N ratio hardly deteriorates.

Furthermore, as a result of experimenting by changing the cutoff frequency of the low-pass filter, it was found that from the viewpoint of obtaining a good quality image, a good quality image could be obtained by selecting a cutoff frequency of _2c ±10%.

That is, the characteristic 43 in FIG. 8 is a low pass filter with a cutoff of 2 _c -10%, and this characteristic is a necessary condition for not impairing the transmission band of high frequency (hereinafter referred to as RF). If the cutoff frequency is lowered further, the RF transmission band near 7MHz, that is, on the demodulation side, will limit the characteristics of 3MHz, impairing clarity. Further, the characteristic 42 in FIG. 8 is a low-pass filter with a cutoff frequency of 2 _c +10%. If the cutoff frequency is increased beyond this, 2
On the demodulation side, noise components with _c < _N and noise components with a frequency of 3 MHz or less are also generated, making the noise components visually noticeable.

Regarding the cutoff of 2 _c -10 [%], in order to improve image quality in the future, it will be necessary to extend the bandwidth by using alloy powder tape or thin film tape, for example.
In that case, setting it below 2 _c -10 [%] will limit the band.

In this way, the applicant has analyzed the relationship between the sliding noise in magnetic recording and reproduction and the carrier signal [frequency _c ] in a magnetic recording and reproduction device that records signals by frequency modulation or phase modulation.
The cutoff frequency of the low-pass filter 34 is set to 2.
It was found that it is better to set the frequency to _c ±10 [%].

As described above, according to the present invention, in a magnetic recording and reproducing apparatus that records signals by frequency modulating or phase modulating, a regenerative amplifier and a limiter are used to prevent high frequency components from entering the demodulated signal band. A low-pass filter having a cutoff frequency within a frequency range of ±10% from twice the frequency of the carrier wave frequency of the frequency modulation or phase modulation is interposed between the two, so that it is possible to reproduce images with good image quality and to reduce the frequency of the carrier wave frequency. This makes it possible to use even magnetic heads with large sliding noises of twice the frequency or more.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the reproduction and demodulation system of a conventional small VTR, Figure 2 is a spectrum diagram of various noises, Figure 3 is a C/N characteristic diagram of the equipment in Figure 1, Figures 4 A to F and 5A to 5H are diagrams explaining the principle of the present invention, FIGS. 6 to 8 show examples, FIG. 6 is a configuration diagram of a magnetic recording and reproducing device, and FIG. 7 is a diagram explaining the same device.
FIG. 8 is a characteristic diagram of the low-pass filter of the same device. 3...recording head, 4...magnetic tape, 5...
Reproducing head, 7... Reproducing amplifier, 9... Limiter, 11... Demodulation circuit, 34... Low pass filter.

Claims (1)

[Claims] 1. In a magnetic recording/reproducing device that records signals by frequency modulation or phase modulation, there is a signal between the regenerative amplifier and the limiter within a frequency range of ±10% from twice the carrier frequency of the frequency modulation or phase modulation. A magnetic recording/reproducing device characterized in that a low-pass filter having a cut-off frequency is interposed.