JPS5916476B2 - Noise reduction method in FM multiplex signal recording and reproducing system - Google Patents

Description

[Detailed Description of the Invention] 15 The present invention relates to a noise reduction method in an FM multiplex signal recording/reproducing system, and the present invention relates to a noise reduction method in an FM multiplex signal recording/reproducing system, and the above-mentioned number of channels obtained by frequency modulating (FM) each separate carrier wave with an audio signal having an arbitrary number of channels. One modulated wave signal obtained by multiplexing the two modulated wave signals so that their bands do not overlap, and frequency-modulating one carrier wave again with this multiplexed signal 20, or the above multiplexed signal directly into a disk shape. In a recording/reproduction system that records on a recording medium and reproduces it, the jitter of the demodulated multiplexed signal is
It is an object of the present invention to provide a method that can significantly reduce noise due to jitter by modulating this demodulated multiplexed signal in a direction opposite to the direction of noise shift due to 5-Tutter.

FIG. 1 shows a block diagram of an example of a general FM-FM multiplex signal recording/reproducing 30 system.

In recent years, various studies have been conducted in audio signal recording and reproducing technology with the aim of achieving higher density and higher fidelity reproduction. on the other hand,
In the field of video, a video disc recording technique is attracting attention in which video signals are recorded at high density on a disc-shaped recording medium as geometrically uneven shapes. This video disc is a technology for recording and reproducing video signals on a disc-shaped recording medium that rotates at high speed, and is capable of recording and reproducing signals over a wide band. Therefore, a video disc can be said to be a suitable recording medium for recording multi-channel audio signals. Incidentally, there are two possible methods for multiplexing audio signals: frequency division multiplexing and time division multiplexing, but frequency division multiplexing is currently advantageous due to the trade-off between performance and cost. Furthermore, it is conceivable to use various modulation methods such as FM, AM, and SSB within the divided band using frequency division multiplexing, but it is considered promising to use the FM modulation method to obtain high-quality audio signals. . Note that when recording and playing back multiplexed signals on a disk using video disk technology, the multiplexed signal that mixes the FM waves of each channel may be recorded as is, but current video disk characteristics such as linearity and level fluctuations Considering this, the FM-FM multiplex recording method, which records the modulated wave signal obtained by FM-modulating the carrier wave of an appropriate frequency once again using the multiplexed signal, has obtained better results. In Fig. 1, 11 to 1n are audio signal input terminals for each channel of the number n of channels to be recorded and reproduced (n is any positive integer), and the audio signals input from these are the number corresponding to the number of channels n. are supplied to frequency modulators (hereinafter referred to as subcarrier modulators) 2, to 2n, respectively, to frequency modulate mutually different carrier waves. At this time,
The subcarrier modulators 21 to 2n output FM signals (
The carrier frequency and modulation degree are selected so that the sidebands f1-Fn (hereinafter referred to as subcarriers) do not overlap in adjacent channels. Subcarrier F, ~F
n is supplied to the mixing circuit 3, where it is mixed and multiplexed. The output multiplexed signal of the mixing circuit 3 usually has a large number of channels, so it becomes a wideband signal.

In order to record such a wideband signal on a recording medium that involves level fluctuations, it is most advantageous to perform frequency modulation. Therefore, the multiplexed signal is supplied to a frequency modulator (hereinafter referred to as a main modulator) 4, which frequency modulates one carrier wave of a predetermined frequency. The FM signal (hereinafter referred to as main carrier) outputted from the main modulator 4 is amplified to an appropriate level by a recording amplifier 5, and then transferred to a disk-shaped recording medium (disk) 6 using well-known video disk technology to form a spiral shape. recorded in During playback, the recorded main carrier is detected by pick-up, for example, and
is supplied to an FM demodulator (hereinafter referred to as a main demodulator) 7,
Here, the signal is FM demodulated and then passed through a low-pass filter 8 to be multiplexed as a subcarrier.

The regenerated multiplexed subcarriers are then supplied to band filters 91 to 9n, where they are separated into subcarriers f1 to Fn of each channel, respectively, and then to an FM demodulator (
(hereinafter referred to as sub-demodulator) 101 to 10n low-pass filter 1
The signals are demodulated into audio signals of each channel through channels 1 and 11n, respectively, and guided to output terminals 121 to 12n, respectively.
In the recording and reproducing system as described above, when a recorded signal is extracted from the disk 6 and demodulated and reproduced, relatively low frequency periodic noise that is not actually recorded may occur.

This is because the grooves (lines) in which the signals are recorded are distorted during the manufacturing process of the disc, or due to eccentricity when the disc is installed in the player. (This is different from the change in the linear velocity at the detection position on the disk, such as the outer circumference or inner circumference, and is called jitter.)
This means that the frequency of the recorded signal may change, and noise proportional to the linear velocity will occur in the reproduced FM signal. Since the rotational speed of the disk 6 in the above method is much faster than that of a conventional audio record disk, the noise generated due to the above causes becomes noise within the audio band, and the S/N of the demodulated audio signal is It will drop. Therefore, conventional methods for reducing the above-mentioned noise include performing low-frequency emphasis on the audio signal during recording and low-frequency de-emphasis during demodulation, since most of the frequency components of the above noise are low frequency components even within the audio band. It was hot.

However, this conventional method has the disadvantage that the dynamic range in the low range is reduced because music signals generally have a high level signal in the low range. Another conventional noise reduction method uses a noise reduction circuit that compresses and expands the signal level, but this method only has a slight noise reduction effect and is said to cause changes in sound quality. There were flaws.

In addition to the noise reduction method using electrical processing as described above, there is a conventional mechanical method that moves the pick-up of the player back and forth in the direction of the signal recording groove to keep the relative linear velocity constant when detecting the recorded signal. The method was perfect.

Although this conventional method is quite effective in terms of noise reduction, it requires a complicated and expensive device, and requires a special signal to operate the pickup in order to be compatible with the device that plays the original video of the video disk. There were drawbacks such as the need to record and play back. The present invention eliminates the above-mentioned drawbacks, and each embodiment thereof will be explained with reference to FIGS. 2 and 3.

FIG. 2 shows a block diagram of a first embodiment of the system of the present invention.

In the figure, the same parts as in FIG. 1 are designated by the same reference numerals, and their explanations will be omitted. In FIG. 2, when a previously recorded signal (main carrier) on the disk 6 is reproduced as an audio signal, harsh low frequency noise may occur due to the reasons described above. Now, to explain this noise in more detail, if the linear velocity at which the main carrier is detected from the disk 6 is V1 and the recording wavelength is λ, then the main carrier frequency Fc is as follows. On the other hand, the linear velocity V is determined by the following formula, where r1 is the distance between the center of the disk 6 and the detection position, that is, the radius of rotation, and ω is the rotational angular velocity of the disk 6.

From these equations (1) and (2), the main carrier frequency Fc
is expressed by the following equation. From this equation, it can be seen that even if the main carrier frequency Fc during recording is constant, if the rotation radius r changes during reproduction, the main carrier frequency Fc will also vary.

However, the radius of rotation r naturally increases by 1 every time the disk 6 rotates once.
Although it changes by the pitch, it goes without saying that the recording wavelength λ changes during recording and the main carrier frequency Fc is recorded without any change in response to this change. In other words, the fluctuations here refer to those caused by eccentricity when the disc 6 is mounted on the player or distortion of the grooves (or lines) in which signals are recorded during the manufacturing process of the disc, and the basics of the fluctuations are as follows: The period is the same as the rotation period of the disk 6. Here, if the amount of variation in the rotation radius r is represented by Δr, then the main carrier frequency variation is mutually Fc.

rHowever, it must be noted that the frequency deviation here is different from the frequency deviation caused simply by FMing the main carrier with a certain signal, and is different from the frequency deviation caused by simply FMing the main carrier with a certain signal. The frequency components of each signal will change at a rate of Δr/r.

As an example, Δr=100 μM, . r=15 (V7!
Then, Shihei = ±0.067% (5. Since this is a peak-to-peak value, the effective value will further decrease (for example, if it is a sinusoidal fluctuation, it will be calculated as 0.0
47% Rms). This fluctuation is a sufficiently low value to be used as an audio signal wow and flutter. However, the problem here is that FM is used as a method to modulate the main carrier and subcarrier, so even if there is a fluctuation of the above degree, there is a frequency fluctuation in the carrier.
This results in a decrease in the S/N of the demodulated signal.

Fluctuation noise after main carrier demodulation is not a problem for subcarrier demodulation, but demodulation noise due to subcarrier frequency fluctuation is a problem because it degrades the S/N of the audio signal. If the maximum frequency deviation due to the frequency Fnl audio signal of each subcarrier is Δf, then the noise level N due to fluctuations in the radius of rotation r can be expressed by the following equation with reference to the signal at the time of the maximum frequency deviation. Therefore, as is clear from equation (4), if Δf is the same for each channel, the channel with a higher subcarrier frequency has a higher noise level.

For example, subcarrier frequency Fn=1MHz1 maximum frequency deviation △f-50KHz1△r=100μM. ．． r=1
If 5CrfL, then N=-37.5 dB. As mentioned above, it can be seen that even the slightest fluctuation can result in extremely large noise (hereinafter, to distinguish it from other noises,
Here, it is referred to as jitter noise because it is the noise generated by jitter due to eccentricity, etc.). The present invention focuses on the fact that jitter noise after main carrier demodulation and jitter noise after subcarrier demodulation have different levels but the same waveform, and uses jitter noise after main carrier demodulation or after demodulation of a newly inserted non-modulated channel. The jitter noise is reduced by modulating each of the multiple subcarriers at an appropriate level in a direction opposite to the noise deviation direction.

In FIG. 2, the output demodulated signal of the main demodulator 7 is converted into a demodulated multiplexed subcarrier through a low-pass filter 8, and then supplied to a multiplier 15. This multiple subcarrier f1~F
It is assumed that n has the following relationship. However, FO-
It is assumed that F2k and F2k are respectively modulated by the R and L channel signals of the stereo signal.

- On the other hand, the output demodulated signal of the main demodulator 7 is also supplied to the low-pass filter 13, where the jitter noise having the same waveform and different level as the jitter noise of the audio signal at the output terminals 201 and 202 is separated into a separation wave. The output oscillation frequency FL is frequency-modulated in the direction opposite to the jitter noise frequency deviation direction of the subchannel.
(hereinafter referred to as inverse modulation).

The output oscillation frequency FL of the local oscillator 17 is supplied to the multiplier 15, where it is multiplied by multiple subcarriers (frequency conversion). The multiplexed subcarriers multiplied and beaded up and taken out by the multiplier 15 are supplied to band filters 181 and 182 having a predetermined passband width, respectively, and the subcarriers F2k and f21 of the desired channel are filtered. Demodulated by demodulators 19, 192,
Audio signals of the L channel and the R channel are led to output terminals 20, 202, respectively.

Here, the center frequencies FBl, fB2 of the passband widths of the bandpass filters 181, 182 and the subcarriers F2k, f2k
The relationship between -1 and the local oscillation frequency FL is selected as follows.

Furthermore, the passband width of the bandpass filters 18, 182 is FBl±Δ, where Δf is the maximum frequency deviation of the subcarrier.
There is a relationship that F and fB2±Δf are selected.

As a result, since the inverse modulation of jitter noise by FL is performed at the same level for subcarriers F2k and F2k-1, it does not disappear completely, but the output terminal 2
1,202, an audio signal with sufficiently improved jitter noise is introduced.

For example, as a specific example, m=4, FO-150kHz1
Δf=50kHz,. To explain the jitter noise improvement effect of the subcarriers Fl, f2 and F7, f8 when f1=100kHz, the deviation amount δFn due to the jitter of the subcarrier when the rotation radius r - 150fIL1 and the amount of variation is Δr=100μm is expressed as follows. ,V↓
Ink -● V94Vguh赫υ becomes.

Now, if the frequency FL is inversely modulated with jitter noise of 7.15 x 10-1 kHz, the average value of the deviations δF7 and δF8, the jitter noise deviation of the bead-up subcarrier F7 is 7.15 x 10-1. −6.67×
101=4.8×10−2, and the noise level N7 for the maximum deviation amount at F7 is −60 dB. Similarly, the noise level N8 for the maximum deviation amount at F8
becomes -60dB. Similarly, for subcarriers Fl and f2, the noise level N with respect to the maximum deviation amount is
l and N2 are each -60 dB. As described above, although jitter noise cannot be completely removed, it can be reduced to a level that is sufficiently practical. FIG. 3 shows a block system diagram of a second embodiment of the system of the present invention.

In this figure, the same parts as in FIG. 2 are designated by the same reference numerals, and their explanations will be omitted. In this embodiment, the local oscillator 17 is operated at a level corresponding to the frequency deviation of each of the subcarriers F2k and f2b.
A feature is that the attenuators 161 and 162 are independently set so as to inversely modulate the output oscillation frequencies FL, , fL2 of 1,172. Here, the frequencies FBl and fB2 are 1
The jitter noise of the desired subcarriers obtained by passing the multiplexed subcarriers beaded up and taken out from the subcarriers 51 and 152 through the band filters 181 and 182 is almost completely removed. Furthermore, the method of the present invention is not limited to the first and second embodiments described above; for example, an unmodulated subcarrier is used as a reference carrier for jitter noise detection, multiplexed at a frequency higher than the subcarrier F2m, and then FM modulated and recorded on the disk 6. However, during reproduction, the local oscillation frequency, and thus the multiplexed signal, may be inversely modulated using a demodulated signal.

In this case, the jitter noise can be extracted by replacing the band filter 13 in FIGS. 2 and 3 with the band filter for detecting the reference carrier wave for detecting the jitter noise, and replacing the amplifier 14 with an FM demodulator. Otherwise, audio signals can be reproduced with the same configuration as in FIG. 2 or 3. As described above, the noise reduction method in the FM multiplex signal recording and reproducing system according to the present invention uses modulated waves of the number of channels obtained by frequency modulating carrier waves of the number of channels that are different from each other using audio signals of an arbitrary number of channels. One modulated wave signal obtained by multiplexing the signals so that their bands do not overlap, and frequency modulating one carrier wave again with this multiplexed signal, or directly recording the above multiplexed signal on a disk-shaped recording medium. In the recording and reproducing system that demodulates and reproduces the audio signal, there is provided a Fuchish wave means for extracting the noise component due to jitter from the reproduced multiplexed signal, and an output signal of the Fuchish wave means that determines the shift direction of the noise component. Since this is a means of modulating the reproduced multiplexed signal in the opposite direction, the degree of noise improvement due to jitter is 20 dB or more, which was at most 5 to 10 dB in conventional methods such as low-frequency emphasis, reinforcement, or noise reduction. The noise of the reproduced audio signal can be brought close to the noise of the recording and reproduction system itself, and the noise caused by jitter can be reduced and eliminated without any change or deterioration in sound quality. A means for multiplexing an unmodulated reference carrier wave onto the encoded signal so that the bands do not overlap, and demodulating the reference carrier wave during reproduction, and an output signal of the demodulation means to determine the direction of noise shift due to jitter in the reproduced multiplexed signal. Since it consists of a means for modulating the multiplexed signal in the opposite direction to the direction of the signal, the noise component due to the jitter can be detected more accurately, and the noise due to the jitter can be reduced and eliminated.

[Brief explanation of drawings]

Figure 1 shows a typical FM-FM multiplex signal recording and reproducing system.
FIGS. 2 and 3 are block diagrams of the first and second embodiments of the present invention, respectively. 7...FM demodulator (main demodulator), 8, 13
...Low pass filter, 15,151,152...
...Multiplier, 16,161,162...Variable attenuator circuit, 17,171,172...
-Local oscillator, 18, 182...Band filter, 19, 192...FM demodulator.

Claims (1)

[Claims] 1. Modulated wave signals of the above number of channels obtained by frequency modulating carrier waves of the above number of channels that are different from each other with audio signals of an arbitrary number of channels are multiplexed so that the bands do not overlap, and this multiplexing is performed. One modulated wave signal obtained by frequency modulating one carrier wave again with a signal is recorded on a disk-shaped recording medium, and the above-mentioned multiplexed signal is reproduced in a recording/reproducing system that demodulates and reproduces an audio signal from the disc-shaped recording medium. filtering means for extracting a noise component due to jitter from a signal, and means for modulating the reproduced multiplexed signal in a direction opposite to the direction of shift of the noise component with the output signal of the filtering means. Noise reduction method for multiple signal recording/reproduction systems. 2 Multiplex the modulated wave signals of the number of channels obtained by frequency modulating the carrier waves of the number of channels that are different from each other using the audio signals of the arbitrary number of channels so that the bands do not overlap, and use this multiplexed signal to regenerate one carrier wave. One modulated wave signal obtained by frequency modulating the
Alternatively, directly record the multiplexed signal on a disc-shaped recording medium,
In a recording and reproducing system for demodulating and reproducing an audio signal, a means for multiplexing an unmodulated reference carrier wave onto the multiplexed signal at the time of recording so that the bands do not overlap, a means for demodulating the reference carrier wave at the time of reproduction, and a means for demodulating the reference carrier wave at the time of reproduction; 1. A noise reduction method in an FM multiplex signal recording/reproducing system, comprising means for modulating the multiplexed signal in a direction opposite to the direction of noise shift due to jitter of the reproduced multiplexed signal using the output signal of the means.