JPH0327989B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for recording and reproducing an FM modulated signal by dividing the FM modulated signal into ²ⁿ signals (n=1, 2, 3...) each having edge information. Regarding a playback device.

The FM modulation signal S _p obtained by FM modulating a carrier signal of frequency _c with a modulation signal such as a video signal as shown in FIG. 1A, has a level inversion at each rising edge, as shown in FIG. 1B. The signal is divided into the FM modulation signal _S1 and the FM modulation signal _S2 , whose level is inverted at each falling edge, and recorded on the magnetic tape. As an example, as shown in FIG. 2A, a magnetic tape 2 is wound obliquely around a drum 1 at a winding angle slightly larger than 180°, and two rotating heads arranged at 180° intervals are used. FM modulated video signals are recorded by alternately sliding contact between 3A and 3B for each field. Rotating head 3
Tracks A and 3B are configured to record two FM modulated signals as separate tracks in one scan. In other words, the rotary head 3A has a track width equivalent to three tracks, with azimuth heads inclined in the same direction located on both sides, and a spacer located in the center. One field's worth of FM modulation signals S ₁ and S ₂ are sent to the heads on both sides.
are supplied to form tracks 4A and 4B in the track pattern shown in FIG. 2B. The other rotary head 3B similarly has a spacer in the center and two heads on both sides, and has an azimuth inclination direction opposite to that of the rotary head 3A. By scanning after 3A, 5A, 5 in FIG. 2B
A track marked B is formed. Therefore, as tracks 4A and 5A in Figure 2B, one
FM modulation signal S ₁ is recorded, tracks 4B, 5B
The other FM modulated signal _S2 is recorded as . Furthermore, the heights of the rotary heads 3A and 3B are shifted by one track, and in the next field where the tracks 5A and 5B are formed, new fields are created by the rotary head 3A so as not to overlap with the previous tracks. 2
A book track is formed.

The FM modulated signals _S1 and _S2 reproduced from the magnetic tape 2 are combined by an exclusive OR gate, returned to the original FM modulated signal _S0 , and then FM demodulated to obtain a reproduced video signal. .

The above-mentioned split FM modulation method can lower the carrier frequency of the FM modulation signal to be recorded and reproduced to _c / ²ⁿ , making it possible to effectively utilize the transmission frequency band of the recording and reproduction apparatus. Therefore, it is possible to record and reproduce high-definition video signals with a wider band than ordinary video signals.

By the way, when performing the above-mentioned recording and reproduction, the rotary heads 3A and 3B of the transmission system record and reproduce the FM modulated signals S1 and S2 at different timings, respectively, so that the recording device and the reproduction device are different from each other. For this reason, the rising edges and falling edges of these FM modulation signals S1 and S2 cause a phase shift with respect to the phase before recording, and the duty ratio of the FM modulation signal that combines both is 50%.
An asymmetry deviates from this occurs.

When the reproduced and synthesized FM modulated signal is expanded into a Fourier series, it can be seen that when there is no asymmetry, that is, when the duty ratio is 50%, spurious (harmonic components) that are odd multiples of _c are generated. When there is an asymmetry in which the duty ratio is not 50%, spurious not only an odd multiple of _c but also an even multiple of c occurs, and a spurious of 1/2 _c also occurs.

The occurrence of spurious of 1/2 _c can be explained not only by the formula of Fourier series expansion but also by the fact that a phase shift with a period of 2/ _c is included. For example, the FM modulation signal S ₁ reproduced by one rotary head 3A
When the rising edge of the FM signal shown in FIG. 1D causes a phase shift with respect to the correct one as indicated by the diagonal lines in FIG. 1C, and the duty factor changes, the FM modulation signal shown in FIG. This fluctuation component appears at a period of / _c . And it takes
When an FM modulated signal is demodulated, a spurious signal of 1/2 _c occurs in the demodulated output, and this spurious signal deteriorates the quality of the reproduced signal. For example, when a composite color video signal consisting of a luminance signal and a carrier color signal is used as a modulation signal, the beat components of the above-mentioned spurious and color subcarriers appear as a striped pattern on the reproduced screen. in this way,
The image quality of the reproduced image deteriorates due to the _c /2 spurious. Assuming that n is the number of divisions, this low-frequency spurious is a frequency component of _c / ²ⁿ . Of course, the asymmetry of the FM modulation signal synthesized during playback also occurs due to the phase shift and falling phase shift of the FM modulation signal S2 reproduced by the rotary head 3B, but the above qualitative explanation is for the sake of simplicity. However, these are not taken into account. In order to avoid the influence of spurious signals, by increasing the carrier frequency _c , high-frequency spurious signals of 2 _c or higher can be positioned higher than the signal band. However, _{the c} /
2 ⁿ spurious cannot be easily removed.

This invention utilizes the above-mentioned split FM modulation method.
In order to eliminate the adverse effects of _{the c} /2 ⁿ spurious, the above-mentioned spurious is prevented from entering the band of the demodulated reproduced signal.

Hereinafter, an embodiment suitable for converting the present invention into two FM modulation signals as described above will be explained with reference to the drawings. FIG. 3 shows the circuit configuration on the recording side, and FIG. shows the circuit configuration on the playback side.

In FIG. 3, a composite color video signal SV is supplied to an input terminal indicated by 6, and is supplied to an FM modulator 8 via a pre-emphasis circuit 7, and an FM modulated signal _S0 is generated at its output. Fifth
As shown in Figure A, the highest frequency of the color video signal SV. (for example, 4.5MHz), the frequency _c of the carrier signal used for FM modulation is ( _c >
2 _p ) is selected to a value (for example, 10 MHz) that satisfies the condition. This FM modulated signal S ₀ is supplied to flip-flops 9 and 10. flipflop 9
is triggered by the rising edge of the FM modulation signal S ₀ , and one FM modulation signal S ₁ is generated at its output,
The flip-flop 10 is triggered by the falling edge of the FM modulation signal _S0 and outputs the signal from the other FM.
A modulated signal S ₂ is generated. These FM modulated signals S ₁
and S ₂ are taken out to output terminals 12A and 12B via recording amplifiers 11A and 11B, respectively, and recorded on magnetic tape 2 as described above.

Also, from the magnetic tape 2 to the rotating heads 3A and 3B,
As shown in FIG. 4, the input terminals 13A and 13B are supplied with
The FM modulated signals S ₁ and S ₂ passed through A and 14B are combined by the exclusive OR gate 15, and the FM modulated signal S ₀ taken out as its output is
The signal is supplied to the FM demodulator 16. This FM demodulator 1
6 passes through a low-pass filter 17 and a de-emphasis circuit 18, whereby a reproduced color video signal SV is obtained at an output terminal 19. The cutoff frequency of this low-pass filter 17 is selected to be smaller than _c /2, for example, _p .

As shown in FIG. 5B, the FM modulated signal _S0 is modulated so that the pedestal part has a carrier frequency _c , and as shown by the broken line in the same figure, ( _p − _c ) ~
It has a band of ( _p + _c ). Then, the carrier frequency _c on the recording side is ( _c > _2p ),
Furthermore, since the cutoff frequency of the low-pass filter 17 is set to _p , the spurious of _c /2 is removed by the low-pass filter 17, as is clear from FIG. 5B. The cutoff frequency of the low-pass filter 17 is the highest frequency of the color video signal.
It does not necessarily have to be equal to _p .

As understood from the description of one embodiment above,
According to this invention, when performing split FM modulation, the carrier frequency _c is set to the highest frequency of the modulation signal.
By selecting ²ⁿ times or more and setting the cutoff frequency of the low-pass filter to which the FM modulation output is supplied to _c / ²ⁿ or less, it is possible to prevent spurious signals from entering the modulation signal band. Therefore, it is possible to prevent the spurious and color subcarrier beats from appearing as striped patterns on the playback screen.

[Brief explanation of drawings]

FIG. 1 is a waveform diagram used to explain the divided FM modulation method, FIG. 2 is a schematic diagram used to explain a rotating two-head magnetic recording and reproducing apparatus to which this invention can be applied, and
The figure is a block diagram showing the configuration of the recording side in one embodiment of the present invention, FIG. 4 is a block diagram showing the configuration of the reproduction side in one embodiment of the present invention, and FIG. 5 is an explanation of one embodiment of the present invention. FIG. 3A, 3B... Rotating head, 8... FM modulator, 9, 10... Flip-flop, 16...
FM demodulator, 17...low pass filter.

Claims (1)

[Claims] 1 An FM modulated signal that is obtained by FM modulating a carrier signal using a modulated signal and is recorded and reproduced by dividing the FM modulated signal into 2 ⁿ (n = 1, 2, 3...) signals each having edge information. In the recording and reproducing device, the frequency of the carrier signal is selected to be ²ⁿ times or more the highest frequency of the modulation signal, and the output of the FM demodulation circuit on the reproduction side is set to a cutoff frequency of 1/2n or less of the frequency of the carrier signal. An FM modulated signal recording and reproducing device that connects a low-pass filter with a FM modulation signal to remove spurious signals.