JPH0480591B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a reproduction circuit for a videotape on which an audio signal is recorded using PCM.

Background technology and its problems In addition to integrating a video camera and a VTR,
A so-called 8mm video is being considered as a smaller version.

In this 8mm video, the output signal of the image pickup tube is processed to extract a brightness signal and red and blue color difference signals, and the brightness signal is converted to the FM signal Sf, and the color difference signal is converted to the FM signal Sf. It is assumed that the carrier color signal Sc is distributed on the lower side, and this carrier color signal Sc
and the FM signal Sf are added to form a summed signal Sm as shown in FIG. 1A. However, at this time,
The carrier color signal Sc is phase-controlled so as to interleave the frequency with each other in odd-numbered field periods and even-numbered field periods. For example, the phase remains unchanged during one field period Ta, and the phase remains unchanged during the other field period. The phase of Tb is inverted every horizontal period.

Further, as shown in FIG. 1, if the final 1/6 field period of one field period is defined as the period Tp, the audio signal is converted to a PCM signal, and
The time axis is compressed and the period Tp is shown in Figure 1B.
It is assumed that the PCM signal Sp located at

These signals Sm and Sp are converted into signals Sa and Sb as shown in FIG. 1C and D, that is, the signals
Sa has the signal Sp in the period Tp of the field period Tb and the signal Sm in the following field period Ta, and the signal Sb has the signal Sp in the period Tp of the field period Ta and the signal Sm in the subsequent field period Tb. have These signals Sa and Sb are then supplied to rotating magnetic heads 1A and 1B, respectively, as shown in FIG.

The heads 1A and 1B have different azimuth angles and are rotated at a frame frequency with an angular spacing of 180° from each other.
The magnetic tape 2 is run diagonally at a constant speed over an angular range of over 210 degrees with respect to the rotating peripheral surfaces of the heads 1A and 1B.

Therefore, as shown in FIG.
In addition, the signal Sa is recorded on every other track 3A of the tracks 3, and the signal Sb is recorded on every other remaining track 3B adjacent to each other.

At this time, the tape 2 is connected to the heads 1A, 1
It extends over an angular range of more than 210° with respect to the rotating surface of
is located in the previous 1/6 field period Tb, and the signal Sm
is located in the later one field period, so in track 3, there is no signal in the first 30° section.
Sp is recorded, and the signal Sm is recorded in the following 180° section. In addition, heads 1A, 1
Corresponding to the azimuth angle of track B, the azimuth angles of tracks 3A and 3B are different from each other.

In addition, in the above, as an example, the FM signal Sf
The frequency of is the sync chip level of the luminance signal.
4.2MHz, 5.4MHz at white peak level, carrier frequency of carrier color signal Sc is _47.25H (743kHz, _H is horizontal frequency). In addition, the transmission rate of the PCM signal Sp is 368 _H (5.79 Mbps), but the
It is modulated to a biphase mark signal as shown in the figure, and when the frequency is “1”,
5.8MHz, 2.9MHz when “0”.

The above is the recording process and recording format for 8mm video.

By the way, when playing tape 2 recorded with the 8mm video mentioned above, the
If the waveform of the PCM signal Sp is distorted, an error will occur when decoding (demodulating) the audio signal from the signal Sp, making it impossible to demodulate the audio signal correctly and generating noise.

Therefore, the reproduction signal system of the signal Sp must have flat frequency characteristics (amplitude characteristics) and group delay time characteristics at least in its occupied frequency band. Since the group delay time characteristic is obtained by differentiating the phase characteristic with respect to time, in order for the group delay time characteristic to be flat, the phase characteristic must be linear,
and a characteristic that passes through the origin, for example, is flat, and
It must have a characteristic in which the amount of phase is 0. That is, as shown in FIGS. 5A and 5B, the frequency characteristic (FIG. 5A) of the reproduced signal system of the signal Sp is flat, the phase characteristic (FIG. 5B) is flat, and
If the phase amount θ is 0, no waveform distortion occurs in the signal Sp, and therefore, the audio signal can be correctly demodulated without causing errors or noise.

Since the playback head (rotating head) of a VTR is a winding head, its playback characteristics have differential characteristics, the frequency characteristics are linear as shown by the broken line in Figure 6A, and the phase characteristics are linear as shown by the broken line in Figure 6A. As shown in Figure B, it is flat at θ=90°. Therefore, if the signal Sp obtained from the reproduction head is integrated, the frequency characteristic becomes flat as shown in Fig. 5, the phase characteristic also becomes flat, and θ=0 (θ=180° is θ = 0).

However, this is the case when the playback head is an ideal head, and in an actual playback head, due to core loss, gear loss, spacing loss, etc., the frequency characteristics will be 2 to 3M, as shown by the solid line in Figure 6A.
In the high frequency range above Hz, the response is lower than the ideal characteristic (shown by the broken line).

Therefore, in an actual reproduction system, high-frequency level compensation is required, but at this time, the phase characteristics must not be changed, so high-frequency compensation is generally performed using a transversal filter. .

However, transversal filters are expensive, and therefore it is difficult to commercialize playback VCRs as consumer devices.

OBJECT OF THE INVENTION This invention attempts to solve such problems.

Summary of the Invention Now, when the frequency spectrum of the PCM signal Sp is measured, the distribution is as shown in FIG. 7, for example, and most of it is concentrated around 4MHz. In reality, if the band of 2 to 6 MHz could be transmitted, it would not be affected by the waveform distortion of the PCM signal Sp.

On the other hand, the playback head of a VTR has a frequency characteristic as shown by the solid line in Figure 6A;
Narrower than the occupied band of Sm. For this reason, an eighth equalizer circuit is used in the VTR video signal reproduction system.
As shown in Figure A, the unimodal characteristic, i.e. the frequency _M
A resonant circuit with a frequency characteristic that peaks at (5MHz) is provided to perform high-frequency compensation.

For example, as shown in FIG. 9, this resonant circuit includes a coil L 1 , a capacitor C ₁ , a resistor R _{1 ,} and a collector of a transistor Q ₁ whose emitter is grounded.
A parallel resonant circuit with R ₂ is connected and configured. _Therefore , the phase characteristic of this resonant _{circuit is} expressed as θ=Tan ^-1 R ₂ /R ₁ +R ₂ K− _Tan ^-1 _K , as shown in FIG. 8B. Note that the center frequency _M is _M = √ _{1 1} /2π.

The present invention focuses on the above points and compensates for the characteristics of the reproducing head shown by the solid line in FIG. 6, thereby making it possible to reproduce the PCM signal Sp without waveform distortion.

Embodiment That is, in FIG. 10, 11A and 11B indicate rotating magnetic heads for reproduction, and these heads 1
1A and 11B have the same relationship with the tape 2 as the recording heads 1A and 1B shown in FIG. It is reproduced as shown in Figure 1C and D. And this head 11
The signals Sa and Sb from A and 11B are sent to head amplifiers 12A and 12B with flat frequency characteristics and phase characteristics.
It is supplied to the resonance circuits 13A and 13B through.

The resonant circuits 13A and 13B are configured as shown in FIG. 9 and have characteristics as shown in FIG. 8. Therefore, the signals Sa and Sb have high-frequency levels. Compensated and taken out.

The high-frequency compensated signals Sa and Sb are then
is supplied to the switch circuits 14A and 14B, and the switch circuits 14A and 14B are controlled by a predetermined control signal, and the signal Sm is continuously taken out from the switch circuit 14A as shown in FIG. 1A. As shown in FIG. 1B, the PCM signal Sp is taken out from 14B every period Tp.

Then, the signal Sm from the switch circuit 14A is supplied to the high-pass filter 21 to extract the FM signal Sf, and this signal Sf is passed through the limiter 22.
The luminance signal is supplied to the FM demodulation circuit 23 and demodulated, and this signal is supplied to the addition circuit 24. Further, the signal Sm from the switch circuit 14A is supplied to the low-pass filter 31 to take out the carrier color signal Sc, and this signal Sc is sent to the frequency converter 31.
2, and the AAFC/APC circuit 33
An alternating signal with a predetermined frequency and phase is supplied to the converter 32, and the signal Sc is converted into a signal Sc with a color subcarrier frequency and phase in the NTSC system, for example. Supplied. Therefore, terminal 2
For example, an NTSC color video signal is extracted at 5.

Furthermore, the PCM signal from the switch circuit 14B
Sp is sequentially supplied to a high pass filter 41 and a low pass filter 42. In this case, the filter 41 has a cutoff frequency _H equal to that of the equalizer circuit 1.
It is a first-order filter whose center frequency _M is slightly lower than the center frequency M of 3A and 13B, and has frequency characteristics and phase characteristics as shown by solid lines in FIGS. 11A and 11B. Further, the filter 42 is a second-order filter whose cutoff frequency _L is slightly higher than the frequency _M , and the filter 42 is a second-order filter whose cutoff frequency L is slightly higher than the frequency M.
It has frequency characteristics and phase characteristics as shown by solid lines in Figures A and B.

Therefore, the overall frequency characteristics and phase characteristics of the signal line from the equalizer circuits 13A, 13B to the filter 42 are as shown in FIG.

That is, the characteristics of the equalizer circuits 13A and 13B are as shown in FIG.
When superimposed on the diagram (characteristics of the filter 41), the overall frequency characteristics and phase characteristics from the equalizer circuits 13A and 13B to the filter 41 are as shown by the broken lines in the same diagram. In other words, the frequency characteristic is a high-frequency enhancement characteristic, and the phase characteristic becomes almost flat with a lead of 90° below the frequency _M.

As shown by the chain line in Figure 13B, a linear phase characteristic passing through the origin has a flat group delay time characteristic, but even if the phase characteristic of the chain line is rotated around the origin to the position of the horizontal axis, , the group delay time characteristic itself is flat simply because the amount of group delay time changes uniformly. Therefore, the phase characteristic of the filter 42 is as shown by the solid line in FIG. 13, but this is equivalent in terms of the group delay time characteristic even when rotated around the origin to the position shown by the broken line.

When the characteristics of the filter 42 in FIG. 13 are superimposed on FIG. 12, they are as shown by the broken lines in the same figure, and therefore, the characteristics of the filter 42 in FIG.
The overall characteristics from B to filter 42 are as follows:
The result will be as shown in the figure. In other words, the frequency characteristics are
As shown in Fig. 14A, the high frequency band of the PCM signal Sp is enhanced and the band other than the substantially occupied band is attenuated.The phase characteristic is as shown in Fig. 14B. It is almost flat in the band and has a 90° lead characteristic.

Since the heads 11A and 11B have the characteristics shown in FIG. 6, the high frequency characteristics of the heads 11A and 11B are compensated for by the frequency characteristics shown in FIG. The frequency characteristics are almost flat at
Due to the phase characteristics shown in Figure 4B, the heads 11A, 11
The phase characteristic of B is advanced by approximately 90 degrees, is approximately flat over the entire range of the signal Sp, and has a phase characteristic of θ=180 degrees.

Therefore, a PCM signal Sp with almost no waveform distortion is extracted from the filter 42.

This signal Sp is shaped in a waveform shaping circuit 43 and then supplied to a PCM decoder 44 where error correction and error correction are performed, and the signal after this correction and correction is sent to a D/A converter 44.
5L, 45R and is converted into the original left and right channel audio signals, and these signals are passed through low-pass filters 46L, 46R to terminals 47L, 4.
It is taken out in 7R.

Thus, according to the present invention, the PCM signal Sp is regenerated, but in this case, especially according to the present invention, the resonant circuits 13A, 13B and the filter 4
1 and 42 perform reproduction equalization of the PCM signal Sp, and since amplifiers 12A and 12B and resonance circuits 13A and 13B share the video signal system, a transversal filter is used. The structure is much simpler and the cost is lower than that of the conventional method. Moreover, this makes it possible to easily commercialize the device as a consumer device.

Furthermore, since the resonance circuits 13A and 13B share the video signal system, the heads 11A and 11
The resonance circuit 13 is used to adjust for variations in the characteristics of B.
Only A and 13B are required, and it can be adjusted at the same time as the video signal system. Furthermore, as shown in Fig. 3, in the case of so-called azimuth recording, the signal Sp
Since the azimuth loss of the heads 11A and 11B is small in the low range of improves.

FIG. 15 shows a specific example of the filters 41 and 42. However, in this example, the filter 41,
The connection position of 42 has been switched from that in FIG. 10.

In FIG. 15, the filter 42
is configured as a feedback type, _L 7MHz, _H 2MHz, _M
It is 5MHz.

Moreover, FIG. 16 shows the results of actually measuring the frequency characteristics at the input end of the shaping circuit 43 by recording a sweep signal on the tape 2 instead of the signal Sp and reproducing it. Also from this figure, it can be seen that the frequency characteristics are compensated for the necessary band of the PCM signal Sp.

In the above description, the type and order of the filter 42 can be changed depending on the characteristics of the heads 11A and 11B, and can be composed of a coil, a capacitor, and a resistor, for example.

Effects of the Invention Reproduction equalization can be performed on a PCM signal reproduced together with a video signal with a simple and inexpensive configuration. Further, adjustment can be performed in the same way as in the video signal system. Furthermore, low-frequency inter-track crosstalk that cannot be removed by azimuth loss can be removed, and error characteristics can be improved.

[Brief explanation of the drawing]

Figures 1 to 9, Figures 11 to 14, and 16
10 is a system diagram of an example of the invention, and FIG. 15 is a connection diagram of an example of the invention. 13A and 13B are resonance circuits, 41 is a high-pass filter, and 42 is a low-pass filter.

Claims (1)

[Claims] 1. In a VTR in which a PCM signal and an FM luminance signal are reproduced by a common reproduction head, a head amplifier that amplifies the reproduction output of the reproduction head, a resonant circuit that performs high-frequency compensation of the amplified output of this head amplifier, and this resonant circuit. a video signal reproducing system that extracts the FM luminance signal from the output of the resonant circuit and demodulates the luminance signal; an audio signal reproducing system that extracts the PCM signal from the output of the resonant circuit and demodulates the audio signal; and the resonant circuit and the audio. A low-pass filter and a high-pass filter are inserted in series in the signal line of the PCM signal between the signal reproduction system, and the cut-off frequency of the low-pass filter is set higher than the center frequency of the resonant circuit, and the cut-off frequency of the low-pass filter is higher than the center frequency of the resonant circuit. A reproducing equalizer for a PCM signal, wherein the cut-off frequency of the filter is set lower than the center frequency of the resonant circuit, so that the frequency characteristics and phase characteristics of the PCM signal are compensated by the resonant circuit, the low-pass filter, and the high-pass filter. circuit.