JPH022201B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is directed to reducing noise due to crosstalk from adjacent recording tracks in a magnetic recording/reproducing device that records a frequency modulated (FM modulated) audio signal and a video signal in a superimposed manner. The present invention relates to an audio signal reproducing device that reduces audio signals.

[Conventional technology]

Conventionally, audio in magnetic recording and reproducing devices (hereinafter referred to as VTR) that frequency modulates (FM modulates) the luminance signal of the video signal and converts the frequency of the chromaticity signal to the lower side of the FM modulated luminance signal and records it. One of the signal recording methods is to record an FM-modulated audio signal by superimposing it on the video signal (hereinafter referred to as audio).
This is called the FM superimposition method. )It has been known. Incidentally, the improvement in recording density in recent years has been remarkable, achieving a recording density 17 times higher than that of VTRs from about 10 years ago. With the advancement of high-density recording technology, VTRs that are more compact have begun to be developed by reducing the size of the cassette and the diameter of the rotating cylinder. In these small VTRs, the inertial force of the magnetic tape running system has become smaller due to the reduction in size and weight and the lowering of the magnetic tape running speed. , it has become difficult to obtain sufficient performance in terms of playback S/N, playback frequency band, etc., and the above-mentioned audio FM
There is an increasing need to adopt new audio recording and playback methods such as superimposition methods. The features of the audio FM superimposition method are: (1) It is less susceptible to time axis fluctuations due to uneven tape running speed, so it has good wow and flutter characteristics.

(2) The playback frequency band does not depend on the tape running speed, and a wide band is possible.

etc.

Let us now consider the recording frequency spectrum of a VTR that records and reproduces the above-mentioned audio signal superimposed on a video signal.

The center frequency of the audio signal carrier must be determined so as to minimize its influence on the luminance and chromaticity signals. Also, in small VTRs, especially those with small rotating cylinder diameters, the relative speed between the tape and the head is low, so the recording frequency band is narrow.
The center frequency of the luminance signal carrier cannot be set too high. Therefore, the center frequency of the audio signal carrier wave superimposed on the luminance signal and chromaticity signal must be set to a frequency as low as possible below the FM modulated luminance signal.

FIGS. 1 and 2 show examples of frequency spectra of a video signal and an FM audio signal. Figure 1 shows the FM modulated luminance signal Y ₁ and the frequency converted chromaticity signal C ₁
This is an example in which an FM modulated audio signal _A1 is placed between the second
The figure shows an example in which the FM modulated audio signal _A2 is placed below the frequency-converted chromaticity signal _C1 . However, in order to perform so-called variable-speed playback, in which the tape is played back at a tape speed different from the tape speed during recording, or when the head width is made wider than the video track width in order to obtain tracking margin, tracking deviation may occur. Since the signal of the adjacent video track is also reproduced, noise is generated in the reproduced audio signal due to the influence of the FM audio signal of the adjacent video track (hereinafter referred to as "adjacent interference"), which is very unpleasant to the ears.
In particular, when aiming for high recording density, the video track width becomes narrower, so tracking deviations occur frequently, and adjacent interference becomes a problem in the audio FM superimposition method. FIG. 3 is a plan view schematically showing the positions of video tracks T ₁ and T ₂ formed on the magnetic tape 21 and the video head H.

Here, the noise D(t) caused by the above adjacent interference is
If the tracking deviates as shown in FIG. 3, the first FM audio signal obtained from the video track _T1 that the video head H is trying to trace (the signal obtained from the part A in FIG. 3; hereinafter,
This is called the desired FM audio signal. ) level a, the second FM audio signal obtained from the adjacent video track _T2 (signal obtained from the part B in Figure 3, hereinafter:
This is called a damage prevention FM audio signal. ) level is b,
When the difference frequency between the desired FM audio signal and the interfering FM audio signal is △ω, it is expressed as D(t)∝b/a△ω(cos△ωt) (1). Here t represents time. That is, the adjacent interference noise D(t) is output as a sine wave of the difference frequency △ω (beat frequency) between the desired FM audio signal and the interfering FM audio signal, and its amplitude is equal to that of the interfering FM audio signal.
Amplitude ratio b/ of FM audio signal and desired FM audio signal
It is considered that it is proportional to a and the difference frequency Δω. Therefore, several methods have been considered to reduce the adjacent interference in the VTR mentioned above.
One method is to form a non-recorded portion (guard band) between a video track and its adjacent video track. However, with the method of forming a guard band, the utilization efficiency of the magnetic tape is extremely low, and it is impossible to achieve high-density recording. Another method is to record video signals using a rotating head with a different head gap inclination (azimuth angle) for each scan that draws one video track, and eliminate guard bands and adjacent interference noise by using azimuth loss (azimuth angle). recording method). Here, the azimuth loss La is given by the video track width W on the tape, the azimuth angle, and the recording wavelength λ, then La=20log ₁₀ [πW/λtan2/sin(πW/λtan2)]
(dB)...(2) Here, π represents pi. Therefore, in this azimuth recording method, the shorter the recording wavelength, the narrower the width of the video head tracing adjacent tracks, and the larger the azimuth angle, the greater the azimuth loss.
La becomes larger and adjacent interference can be reduced. here,
FIG. 4 shows an example of the azimuth angle and frequency versus azimuth loss characteristics showing that adjacent interference is reduced by the azimuth recording method. This is when the track width Tw is 58 μm and the relative speed v is 5.8 m/S.
This is an example of characteristics when the frequency of the recording signal is 629KHz and 3.4MHz. By the way, as mentioned above, the center frequency of the audio carrier wave in the audio FM superimposition method cannot be set to a very high frequency, and in order to achieve high recording density, the width of the video head tracing adjacent video tracks cannot be set too narrow. Therefore, in order to reduce adjacent interference to a level that does not cause any practical problems,
As mentioned above, the only option is to increase the azimuth angle. In the above numerical example, the FM audio carrier frequency is
When setting it to 1.3MHz, the azimuth angle needs to be 20 degrees or more. However, if the azimuth angle is made too large, the relative output between the magnetic tape and the head will be multiplied by cos, which will reduce the playback capability and cause problems in manufacturing the video head, such as yield. Furthermore, the tracking deviation causes discontinuity in the time axis of the reproduced signal at the time of head switching, or so-called skew.

Here, the skew amount t is the tracking deviation amount x, the azimuth angle, and the relative speed between the head and tape.
If Vh is expressed as t=x・2tan/Vh...(3), it changes greatly depending on the azimuth angle.
Also, especially when playing at a high tape speed,
During so-called search playback, many skews occur on the screen, and image quality deterioration becomes a major problem. Therefore, there is a limit to the ability of the azimuth recording method to reduce adjacent interference in the audio FM superimposition method, and it cannot be said that the level is sufficient for practical use.

As yet another method, Japanese Patent Application Laid-Open No. 53-80208 discloses an example in which a playback head with a head width narrower than the video track width is provided exclusively, and tracking control is performed so that this playback-only head does not cover adjacent tracks. ing.

[Problem to be solved by the invention]

The magnetic recording and reproducing apparatus disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 53-80208 requires a separate read-only head in order to prevent crosstalk interference, and this read-only head straddles two adjacent tracks. Since the position must be controlled so that the head does not move, extremely high-precision tracking control is required, and furthermore, the narrow head width makes variable speed adjustment difficult.

SUMMARY OF THE INVENTION An object of the present invention is to provide an audio signal reproducing circuit that can reduce audio crosstalk that occurs when two adjacent tracks are traced simultaneously.

[Means for solving problems]

In the present invention, a magnetic recording readout means for reading a frequency modulated audio signal from a magnetic track, a demodulator for frequency demodulating the read signal, and an expander for expanding the demodulated signal are provided.

[Effect]

The compressed audio signal is recorded as a frequency modulated audio signal on each track of the magnetic tape, and this recorded signal is read out from the magnetic recording/reading means.
The read signal is frequency demodulated by a demodulator, and a compressed audio signal is reproduced. This compressed audio signal is expanded by an expander, so that a desired audio signal is reproduced.

〔Example〕

Examples of the present invention will be described below. Fifth
FIG. 6 is a block diagram showing an embodiment of the audio signal recording circuit of the magnetic recording/reproducing apparatus of the present invention, and FIG. 6 is a block diagram showing an embodiment of the audio signal reproducing circuit of the magnetic recording/reproducing apparatus of the present invention. In Fig. 5, the audio signal input from the input terminal 1 is processed by the 1/2 compression circuit 2, and according to the compression-expansion characteristics shown in Fig. 7, the large amplitude part is reduced to a small level, and the small amplitude part is reduced to a level lower than the noise level. The level is increased so that the value becomes larger, and the dynamic range is increased to 1/
It is compressed to 2. In FIG. 7, Rd indicates the dynamic range of the audio signal, and Ln indicates the noise level.
The compression circuit 2 can be composed of a level detector that detects the signal level of the audio signal input thereto, and a variable gain amplifier whose gain changes depending on the output signal of the level detector. And the variable gain amplifier is the eighth
According to the compression characteristics shown in the figure, if the signal level of the audio signal supplied to this is a predetermined level, the audio signal is output at the same signal level, and when the signal level of the input audio signal is below the predetermined level. The gain of the variable gain amplifier is increased to increase the signal level of the output audio signal. Further, when the signal level of the input audio signal exceeds a predetermined level, the gain of the variable gain amplifier is lowered, that is, the variable gain amplifier is operated as an attenuator to reduce the signal level of the output audio signal. In this way, the dynamic range of the audio signal is compressed.

The output signal of the 1/2 compression circuit 2 is subjected to FM modulation by the FM modulator 3. Here, the FM modulator 3 is
Carrier wave center frequency approximately 1.3MHz, frequency deviation approximately ±
Although it operates at 50 KHz, the frequency deviation amount of the small amplitude audio signal becomes larger than the signal before passing through the 1/2 compression circuit 2 by the amount that the level is increased by the 1/2 compression circuit 2. The output signal of the FM modulator 3 is added to the video signal input from the input terminal 7 by an adder 4, and then recorded by a rotating magnetic head 5 onto a magnetic tape 6 diagonally with respect to its longitudinal direction. . For example, a rotating magnetic head has an azimuth angle of 17°,
The head width is 25 .mu.m, and a track with a width of 18.5 .mu.m is formed.

The reason why adjacent interference noise occurs during playback is that FM audio signals recorded on adjacent tracks crosstalk during playback, and there is a difference between the instantaneous carrier wave signal of the FM audio signal on the playback track and the instantaneous carrier wave signal of the crosstalked FM audio signal. This is because the frequency of the instantaneous carrier signal is in the audible frequency band, and if the frequency of the instantaneous carrier signal is high, the difference frequency becomes high, and if this becomes a frequency higher than the audible frequency band, the adjacent interference noise can no longer be heard. 1/2 compression circuit 2 compresses the dynamic range of the audio signal to 1/2,
When frequency modulating the carrier wave with this compressed audio signal, the frequency shift of the carrier wave signal of the FM audio signal corresponding to the small amplitude audio signal becomes large, and most of the sideband signals of the carrier wave signal exist in a high frequency band. Become.

Also, since there is no correlation between the phase and frequency of the instantaneous carrier wave signal of the recorded FM audio signal between adjacent tracks, the FM audio signal recorded on the adjacent track during playback
Even if the audio signals crosstalk, the frequency of the difference between the instantaneous frequency of the carrier wave signal of the FM audio signal recorded on the playback track and the instantaneous frequency of the carrier wave signal of the FM audio signal that has undergone crosstalk will be a frequency higher than the audible frequency. In many cases, the interference noise becomes inaudible.

In other words, the FM modulator 3 has an audio input signal of 0 dB.
It operates so that a frequency deviation of ±50KHz occurs when
If it is not compressed by the compression circuit 2 and is directly FM modulated by the FM modulator 3, it will be ±10KHz (= ±50×
A frequency shift of 10 ^-14/20 KHz) occurs. When these FM modulated signals are recorded as adjacent video tracks T1 and T2 and simultaneously played back by video head H, the two signals read out from video tracks T1 and T2 are
The difference frequency between the instantaneous frequencies of the two reproduced signals is from 0 to
20KHz range and all adjacent interference noise is
It falls within the audible frequency band below 20KHz. but,
−14dB audio input signal is processed by 1/2 compression circuit 2 −
When it is compressed to a 7dB signal and subjected to FM modulation, its frequency deviation is ±22KHz, and the frequency of adjacent interference noise (the difference frequency between the instantaneous frequencies of the two reproduced signals) is distributed in the range of 0 to 44KHz. This allows most of the adjacent interference noise to be at frequencies outside the audible frequency band of 20KHz or higher. In other words, the adjacent interference noise distributed from 0 to 20KHz is spread in frequency to the adjacent interference noise distributed from 0 to 44KHz, so the noise energy within the audible frequency band is reduced and the adjacent interference noise is almost eliminated. becomes undetectable. In this case, if the difference frequency changes sinusoidally, approximately 70% of the total period will be equal to or higher than the audible frequency. Expressing this in general terms, FM has a frequency deviation of ±θKHz when the input signal is 0dB.
All frequency deviations are ±10KHz for modulator 3
The upper limit input level within (difference frequency is 20KHz or less) is 20log10/θdB, but this upper limit input level is compressed to 10log10/θdB by the 1/2 compression circuit 2 and input to the FM modulator 3. Then, the frequency deviation will be within ±θ×10 ^1/2log10/ 〓KHz (at the difference frequency, it will be less than 2×θ×10 ^1/2log10/ 〓KHz). Therefore, even if the input signal level is 20log10/θdB, where all the difference frequencies are within the audible frequency band when they are not compressed, when they are compressed and input to the FM modulator 3, they are simultaneously transmitted from adjacent video tracks T1 and T2. A frequency shift occurs in the FM modulation signal such that the difference frequency between the instantaneous frequencies of the two reproduced signals becomes an audible frequency of 20 KHz or more, and the effect of the present invention is that adjacent interference can be reduced.

In addition, the recorded audio signal is recorded by widening the frequency shift of the carrier wave signal when the signal level is below a predetermined level, and conversely by narrowing the frequency shift when the signal level is above a predetermined level. frequency band can be narrowed.

The above methods simply compress the dynamic range of the audio signal and do not change the amplitude frequency characteristics of the audio signal, but there are other methods, such as methods that also change the amplitude frequency characteristics to remove noise. etc. is also fine. Alternatively, noise removal may be performed by changing the amplitude and amplitude frequency characteristics according to the signal level of a specific band of the audio signal. That is, when the frequency of the audio signal is low, the frequency of the adjacent interference signal is likely to be within the audible frequency band. Furthermore, when the frequency of the audio signal is high, the frequency of the adjacent interference signal tends to be higher than the audible frequency band. Therefore, by varying the amplitude frequency characteristic of the 1/2 compression circuit 2 and increasing the signal level of only the low frequency audio signal, it is possible to make most of the adjacent interference signals have frequencies outside the audible frequency band. Further, even if the signal level is increased only when the audio signal frequency is low and the signal level is small, most of the adjacent interference signals can be made to have frequencies outside the audible frequency band.

Next, in the audio signal reproducing circuit shown in FIG. 6, the signal reproduced from the magnetic tape 6 by the magnetic head 5 is input to a band pass filter (BPF) 8.
The BPF 8 extracts the FM audio signal from the reproduced signal. Here, the level ratio with the extracted FM audio is, for example, azimuth angle 17 degrees, frequency 1.3M
Hz, a track width of 18.5 μm, and a video head width of 25 μm, it is approximately 22 dB. Further, the signal reproduced by the magnetic head 5 is also outputted from an output terminal 12 to a video signal reproduction circuit (not shown). The extracted FM audio signal is demodulated into an audio signal by the FM demodulator 9. The demodulated audio signal still has its dynamic range compressed to 1/2, so
The compressed dynamic range is expanded in the double expansion circuit 10 to the original range according to FIG.

The reproduced audio signal returned to the original dynamic range is subjected to the same expansion operation as shown in Fig. 8, and is reduced to a low noise level, so that it is output from the output terminal 11 as a signal with suppressed noise. .

Here, an example of the characteristics of the noise frequency spectrum of adjacent interference noise when an FM audio signal is played back with the above-mentioned VTR is obtained by using the above-mentioned noise removal circuit consisting of the 1/2 compression circuit 2 and the 2-fold expansion circuit 10. Fig. 9 shows cases in which this is used and cases in which it is not used.

In FIG. 9, indicates the case where there is no noise removal circuit, and indicates the case where the noise removal circuit is used, respectively. As is clear from FIG. 9, when the above-mentioned noise removal circuit is used, adjacent interference noise can be improved by about 20 dB.

Note that the 2x expansion circuit 10 has completely opposite characteristics to the 1/2 compression circuit 2, and like the compression circuit 2, this expansion circuit 10 also includes a level detector that detects the signal level of the input audio signal. It can be configured with a variable gain amplifier whose gain changes depending on the output signal of the level detector. Note that the variable gain amplifier operates as an attenuator, and by attenuating the input audio signal according to the expansion characteristics shown in Figure 8, it is possible to expand the dynamic range of the audio signal and return it to the original dynamic range of the audio signal. can.

As described above, since the present invention effectively increases the amount of frequency deviation of the FM audio signal, it is possible to reduce adjacent interference to a practically sufficient level without widening the frequency band of the FM audio signal.

〔Effect of the invention〕

As explained above, by using the present invention, as shown below: 1. Adjacent interference can be reduced to a practically sufficient level with a simple circuit configuration.

2. Since the video track width can be further narrowed, high-density recording can be performed.

3. Noise other than adjacent interference noise can be reduced at the same time.

4. Since this method increases the apparent frequency deviation of the audio signal and makes the difference frequency outside the audible band, the frequency bandwidth required for recording may be small.

5. Since the required frequency bandwidth is small, the diameter of the rotating cylinder can be made small.

6. Variable speed playback with good sound quality can be performed without using complicated mechanisms or circuits.

7. Since the audio occupied band may be small, deterioration in sound quality due to the influence of video signals, so-called video buzz, is less likely to occur.

It has many features such as these, and is highly effective in reducing the size of VTRs.

[Brief explanation of drawings]

Figures 1 and 2 are frequency spectrum diagrams showing examples of signal frequency spectra in the audio FM superimposition method, Figure 3 is a plan view of a magnetic tape for explaining adjacent interference, and Figure 4 is an azimuth angle, recording A characteristic diagram showing the characteristics of wavelength vs. azimuth loss, FIGS. 5 and 6 are circuit configuration diagrams showing one embodiment of an audio signal recording circuit and a reproducing circuit using the present invention, and FIG.
A level explanatory diagram for explaining the operation of the 2x compression circuit and the 2x expansion circuit, FIG. 8 is also an input/output characteristic diagram,
FIG. 9 is a frequency characteristic diagram of adjacent interference noise amplitude showing the effect of reducing adjacent interference noise. 2...1/2 compression circuit, 10...2x expansion circuit,
5...Video head.

Claims (1)

[Claims] 1. A magnetic recording readout means for reading out an audio recording signal in which the audio signal is compressed and frequency modulated and sequentially recorded on a magnetic tape as an audio track tilted at a constant angle with respect to the longitudinal direction of the audio signal. , a demodulator for frequency demodulating the reproduced signal read from the magnetic recording/reading means, and an expander for expanding the demodulated audio signal from the demodulator, and the audio signal is compressed simultaneously from two adjacent tracks. 1. An audio signal reproducing device characterized in that the frequency modulation causes a frequency shift such that a difference frequency between instantaneous frequencies of two read reproduced signals becomes equal to or higher than an audible frequency. 2. The audio signal reproducing apparatus according to claim 1, wherein the expansion ratio of the expander is selected to be 2. 3. The audio signal reproducing device according to claim 1, wherein the amplitude frequency characteristic of the expander is changed. 4. The audio signal reproducing apparatus according to claim 1, wherein the magnetic recording readout means comprises a rotary head having a plurality of heads having different azimuth angles. 5. A magnetic recording readout means for reading out an audio recording signal in which the audio signal is compressed, frequency modulated, and sequentially recorded on a magnetic tape as an audio track tilted at a constant angle with respect to the longitudinal direction of the magnetic tape; and a magnetic recording readout means. A reproduction of an audio signal, comprising a demodulator for frequency demodulating a reproduced signal read from the demodulator, and an expander for expanding the demodulated audio signal from the demodulator, the expansion ratio of the expander being selected to be 2. Device.