JPH0316004A - Audio noise suppressor - Google Patents

Abstract

PURPOSE:To generate no tone quality deterioration such as breathing phenomenon, etc., and to decrease an adjacent interference by prescribing a compressing/ expanding ratio of amplitude of a recorded audio signal in accordance with a level ratio of a desired FM audio signal and an interference FM audio signal. CONSTITUTION:An audio signal inputted from an input terminal 1 passes through a pre-emphasis circuit 2, and thereafter, passes through a switch 3 and inputted to a 2/3 compressing circuit 6 whose compression ratio is 1.5 at the time of using an SP mode, and on the other hand, passes through the switch 3 and inputted to a 1/2 compressing circuit 4 whose compression ratio is 2 at the time of using an LP mode. Also, a BPF 13 extracts only an FM sound signal from a reproducing signal, and as for a demodulated sound signal, by a holding circuit 16, noise which follows head switching is processed by holding the previous value. Subsequently, as for an output signal of the holding circuit, its dynamic range is returned to its original state by a 3/2 fold expanding circuit at the time of using an SP mode, and by a 2 fold expanding circuit 20 at the time of using an LP mode. In such a case, by determining the compressing/ expanding ratio at every system, an adjacent interference noise can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio noise suppression device for reducing noise due to crosstalk from adjacent tracks in a magnetic recording/reproducing device that records a frequency modulated (FM modulated) audio signal and a video signal in a superimposed manner. It is something.

Conventionally, audio signals are recorded in magnetic recording and reproducing devices (hereinafter referred to as VTRs) that frequency-modulate a luminance signal (FM modulation) and frequency-convert and record a chromaticity signal below the FM-modulated luminance signal. One method is to superimpose an FM-modulated audio signal and the video signal and record them on the same track of a magnetic tape using a rotating head (hereinafter referred to as audio F).
This is called the M superimposition method. )It has been known. By the way, the improvement in recording density in recent years has been remarkable, and the V
It achieves 17 times higher density recording than TR. As high-density recording technology advances, cassettes become smaller and the diameter of the rotating cylinder becomes smaller.
VTRs designed to be more compact are beginning to be developed. In order to reduce the size and weight of these small VTRs and to reduce the running speed of the magnetic tape, the conventional audio signal recording and reproducing method using a fixed head has been unable to improve the wow/flutter characteristics and playback S/N.
It is becoming increasingly difficult to obtain sufficient performance in terms of audio and reproduction frequency bands, and there is an increasing need to adopt new audio recording and reproducing methods such as the audio FM superimposition method described above. The characteristics of the audio FM superimposition method are as follows: (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 reproduction frequency band does not depend on the tape running speed, and it is possible to increase the reproduction frequency band.

etc.

Here, let us consider the recording frequency spectrum of a VTR that records and reproduces the above-mentioned audio signal using the audio FM superimposition method.

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

An example of the frequency spectrum during recording of the video signal and FM audio signal of FIGS. 1 and 2 is shown.

Figure 1 shows an example in which an FM modulated audio signal A is arranged between an FM modulated luminance signal Y and a frequency-converted chromaticity signal C;
The figure shows an example in which an FM modulated audio signal A is arranged below the frequency-converted chromaticity signal C1. However, a major problem with audio FM superimposition is the need to make the head width wider than the video track width in order to perform so-called variable speed playback, which is played back at a tape speed different from the tape speed during recording, and to obtain tracking margin. If the signal of the adjacent video track is also reproduced due to tracking deviation, etc., the influence of the FM audio signal of the adjacent video track (
Hereinafter, this will be referred to as adjacent interference. ), which causes noise. In particular, when increasing recording density, the video track width becomes narrower, and adjacent interference such as tracking deviation becomes very annoying and becomes a problem. FIG. 3 is a plan view schematically showing the video tracks T, , T, formed on the magnetic tape 21 and the position of the video head H.

Here, the noise D(t) caused by the above-mentioned adjacent interference is the noise D(t) caused by the video track T which the video head H is trying to trace when the tracking deviates as shown in FIG.
The level of the first FM audio signal obtained from the part A of FIG.
FM audio signal (signal obtained from part B in Figure 3,
Hereinafter, this will be referred to as the interfering FM audio signal. ) level is b,
The difference frequency between the desired FM audio signal and the interfering FM audio signal is Δ
If ω, then b 1)(j,)oc−Δω(cosΔωt)・・・・・・
...(1) It is expressed as a. Here t represents time. That is, the adjacent interference noise D(t) is output as a sine wave with a difference frequency Δω (beat frequency) between the desired FM audio signal and the interfering FM audio signal, and its amplitude is equal to the difference between the interfering FM audio signal and the desired FM audio signal. It is considered that it is proportional to the amplitude ratio b/a and the difference frequency Δω. Therefore, as an effective method for reducing the adjacent interference in the VTR mentioned above, during recording, the amplitude is compressed according to the dynamic range of the audio signal or a specific band component of the audio signal, and the effective amount of frequency deviation is increased. After making the instantaneous difference frequency of the carrier waves between the interfering FM audio signal and the desired FM audio signal almost outside the audible frequency band, the signal is FM modulated, frequency-superimposed with the video signal, and azimuthally recorded on a magnetic tape. At the time of reproduction, it is conceivable to perform FM demodulation and then expand the compressed amplitude to restore it to its original value.

In this method, due to azimuth loss caused by azimuth recording, the interfering FM audio signal shown in equation (1) and the desired FM
By reducing the level ratio b/a with the audio signal to some extent, although not sufficiently because the FM audio carrier frequency is low, and by effectively increasing the amount of frequency deviation of the audio signal, the difference frequency in equation (1) can be reduced. The synergistic effect with the effect of reducing the noise level by moving the component Δω out of the audible band and inversely transforming it during playback suppresses adjacent interference noise to a practically sufficient level. Moreover, this method also has the following features: One, the video track width can be further narrowed by the adjacent interference low frequency range, allowing for high-density recording, two, noise other than adjacent interference noise can also be reduced, and three, the amount of frequency deviation of the actual audio signal. Compared to the method of simply increasing the amount of frequency deviation, because the frequency band required for recording can be small, and the frequency band used when recording the above-mentioned FM modulated audio signal can be small. Therefore, since the recording wavelength of the luminance signal can be made longer, the diameter of the rotating cylinder can be made smaller, and the device can be made more compact.

However, the effect of suppressing the interfering FM audio signal level due to the azimuth loss used in the above method of reducing adjacent interfering noise is
It varies greatly depending on the R system. For example, as shown in Table 1, FM audio carrier frequency fo=1
．． 3M ratio, azimuth angle φ=17 degrees, relative velocity Vh=4.
1 m/s is the same, and the recording track width Tp and video head width Tw are Tp = 18.7 μm and Tw = 28, respectively.
．． 0 μm system■ and Tp==9.3 μm, Tw
xl4μm system ■, the level ratio (hereinafter abbreviated as D/U) between the desired FM audio signal and the interfering FM audio signal mentioned above is approximately 22 dB for system ■ and approximately 1 for system ■.
7 dB, which is a large difference. Here, the system (2) has twice the recording density as the system (1), and the system (2) is in a so-called standard mode (hereinafter abbreviated as SP mode).

), system (2) corresponds to the so-called long-time mode (hereinafter abbreviated as LP mode). Therefore, L.P.
Compared to the SP mode, this mode generates more adjacent interference noise, which is very harsh to the ears, due to the difference in D/U. Table 1: Therefore, in the LP mode, the dynamic range change characteristic using the method of reducing adjacent interference noise, that is, the compression/expansion ratio, must be made larger than in the SP mode. However, if the compression/expansion ratio is increased unnecessarily, the S/N ratio is improved, but the so-called "breathing" phenomenon in which the noise level fluctuates depending on the signal level, and the distortion rate deteriorates, which is not good for audibility. Therefore, from the perspective of sound quality, it is necessary to define an optimal compression/expansion ratio according to D/U.

The purpose of the present invention is to achieve high recording density without deterioration of sound quality,
The object of the present invention is to obtain a voice noise suppression device that reduces adjacent interference in a voice FM superimposition method to a practically sufficient level without hindering the realization of mechanical and circuit miniaturization.

The present invention provides a magnetic head device for reproducing a frequency modulated audio signal recorded together with a video signal as an audio recording trajectory inclined at a predetermined angle with respect to the longitudinal direction of the magnetic tape, and a frequency modulated audio signal reproduced by the magnetic head device. A demodulation circuit that modulates the frequency ut of the audio signal, an expansion circuit that expands the frequency demodulated audio signal, and a de-emphasis circuit that de-emphasizes the expanded audio signal are provided, and two adjacent recording tracks can be played back simultaneously. The audio signal before frequency modulation is compressed in advance according to predetermined compression characteristics so that the difference frequency between the instantaneous frequencies of the two reproduced signals has a frequency deviation exceeding the audible frequency, and the decompression circuit Each characteristic is set to have a stretching characteristic opposite to the characteristic. Further, according to a preferred embodiment of the present invention, the compression/expansion ratio of the amplitude of the audio signal to be recorded is set to an optimum value according to the level ratio (D/U) between the desired FM audio signal and the interfering FM audio signal. As a result, there is no deterioration in sound quality such as the so-called breathing phenomenon in which the noise level changes depending on the signal level, and the interference in the audio FM superimposition method is reduced to a practically sufficient level. The system can be made smaller at the same time.

As described above, in order to reduce adjacent interference noise, it is necessary to change the compression/expansion ratio of the amplitude of the audio signal depending on the level ratio (D/U) between the desired FM audio signal and the interfering FM audio signal. Here, the larger the compression/expansion ratio is, the more noise such as adjacent interference noise is suppressed and the S/N is improved, but the so-called "breathing" phenomenon in which the noise level changes depending on the signal level, distortion rate deterioration, etc. become more noticeable. This results in auditory discomfort. Therefore, it is necessary to define an optimal compression/expansion ratio according to the D/U, which does not cause deterioration in sound quality such as breath-breathing phenomenon and can reduce adjacent interference noise to a practically sufficient level.

Figure 4 shows the apparent improvement effect of compression/expansion when changing the compression/expansion ratio according to each D/U value.
This is an experimental value showing whether it corresponds to the value of /U. Here, the compression/expansion ratio is approximately in the range of 1:2 to 1:1. The reason for this is that when the compression/expansion ratio greatly exceeds 1:2, the so-called "breathing" phenomenon in which the above-mentioned noise changes depending on the signal level and distortion rate deterioration become significant, resulting in a significant deterioration of sound quality. Furthermore, if the D/tJ of the system is approximately 15 dB or less, it is difficult to reduce the adjacent interference noise to an audibly permissible range using the method of reducing adjacent interference noise described above.
/tJ range is approximately 15 dB or more.

If a compression/expansion ratio of 1:2 is applied to a system with a D/U of 17 dB, the adjacent interference noise level will be the same as that of a system with a D/U of 26 dB, and if a compression/expansion ratio of 2:3 is applied, the D/U2
This shows that the adjacent interference noise level is equivalent to that of a 1 dB equivalent system. Also, from the experiment, D/U22dB
The level of adjacent interference noise is within the audible perceptible limit. Note that when high-quality audio such as stereo audio is required, D/U 2 6 dB or more is desirable.

Therefore, once the required D/U value for sound quality is determined,
The optimum compression/expansion ratio can be determined from FIG. In other words, first find the value corresponding to the D/U of the system on the horizontal axis of Figure 4, then find the value equivalent to the D/U required for sound quality on the vertical axis of Figure 4, and find the intersection point of these. The value of the compression/expansion ratio straight line near to is approximately the optimum compression/expansion ratio to be sought.

For example, if the system D/U is 17 dB and the required D/tJ is 26 dB, there is a 1:2 compression/expansion ratio straight line near the intersection of these from Figure 4, so this 1:2
is the optimal compression/expansion ratio. Note that in this example, a ratio of 1:2 is adopted, but a nearby value such as 1:1.6 or 1:2.4 may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The speech noise suppression circuit of the present invention will be explained below with reference to embodiments shown in the drawings. Figure 5 shows the recording density, that is, the recording time of 2
Audio FM in a rotary head type VTR that can be changed on the street
FIG. 1 is a circuit configuration diagram showing an embodiment in which the present invention is applied to an audio recording circuit that records audio signals using a superimposition method. FIG. 6 is a circuit diagram showing an embodiment of the audio reproduction circuit of the VTR to which the present invention is applied. Here, it is assumed that the specifications of the VTR whose recording density can be changed to two are the same as those listed in Table 1 above, and that the D/U that is actually required is 26 dB. Therefore, when the recording density (SP mode) corresponds to system ■ with a D/U of about 22 dB, the compression/expansion ratio is set to 2:3 from FIG. ), the compression/expansion ratio may be set to 1=2 from FIG. In Figure 5, input terminal 1
The audio signal input from
．． When in the LP mode, the signal is inputted to the 273 compression circuit 6 with a compression ratio of 2, and to the 1/2 compression circuit 4 with a compression ratio of 2 through the switch 3. Here, the switch 3 is controlled by an SP mode/LP mode switching signal inputted from an input terminal 28. In SP mode, the 2/3 compression circuit 6 compresses the dynamic range of the pre-emphasized audio signal to 273, and in the LP mode, the 1/2 compression circuit 4 reduces the dynamic range of the pre-emphasized audio signal to 1/2. Compress.

Here, the 273 compression circuit 6 and the 1/2 compression circuit 4 are detectors 5 and 7 each inputting a pre-emphasized audio signal.
The gain is controlled by the output signal of.

The output signal of the 2/3 compression circuit 6 or the 1/2 compression circuit 4 is input to the FM modulator 9 through the switch 8, and is subjected to FM modulation. Here, the switch 8 is controlled by an SP mode/LP mode switching signal inputted from the input terminal 28. FM
The modulator output is passed through a low pass filter (hereinafter referred to as LPF) 1
0 to remove unnecessary band components, the adder 29 adds the signal to the video signal input from the input terminal 30, and the magnetic head 11 records the signal on the magnetic tape 12 azimuthally.

Next, in the audio reproduction circuit shown in FIG. 6, the signal reproduced from the magnetic tape 12 by the magnetic head 11 is input to a band pass filter (hereinafter referred to as BPF) 13. BPF1
3 extracts only the FM audio signal from the reproduced signal. Also,
The signal reproduced by the magnetic head 11 is also output from the output terminal 25 to a video signal reproduction circuit (not shown). The extracted FM audio signal is demodulated into an audio signal by an FM demodulator l4. The demodulated audio signal is passed through the LPF 15 to F
After removing the M carrier wave leakage, etc., the hold circuit 16
The noise associated with head switching is processed by holding the previous value.

Here, the hold circuit 16 performs a previous value holding operation for a certain period of time using a control signal synchronized with the head switching signal inputted from the input terminal 26.

The output signal of the hold circuit 16 is passed through the switch l7 to S
In P mode, the expansion ratio is 1. 5 372 times expansion circuit 18
, in the LP mode, it is input to the 2x expansion circuit 20 with an expansion ratio of 2. Here, the switch 17 is controlled by an SP mode/LP mode switching signal inputted from an input terminal 27. The dynamic range of the hold circuit output signal is restored to its original state by a 3/2 expansion circuit 18 in the SP mode and by a 2X expansion circuit 20 in the LP mode. Here, the 3/2-fold expansion circuit 18 and the 2-fold expansion circuit 20 are gain-controlled by the output signal of the detection circuit 19.31 which receives the hold circuit output signal as input, and convert the dynamic range of the demodulated audio signal to the original one. Stretch on the stove. The expanded signal passes through a switch 22, is de-emphasized by a de-emphasis circuit 23, and then outputted from an output terminal 24. Here, the switch 22 is interlocked with the switch 17. Expansion circuit 1
The reproduced audio signal expanded in steps 8 and 20 undergoes the same expansion operation for its noise level, and has a low noise level, so that it is output as an audio signal with suppressed adjacent interference noise. That is, for example, the FM modulation 9 operates so that a frequency deviation of ±100 KHz occurs when the audio input signal is OdB,
Assuming that the pre-emphasized audio input signal is -20 dB, if this -20 dB audio input signal is directly modulated by the FM modulator 9 without being compressed by the compression circuit 4.6, it will have a frequency of ±10 KHz ( =±100XIO
A frequency shift of 20 KHz) occurs. When this FM modulated signal is recorded as adjacent video tracks T,,T, and simultaneously reproduced by the video head H, the difference frequency between the instantaneous frequencies of the two reproduced signals consecutively output from the video tracks T,,T, is as follows. 0 to 20 KHz, and all adjacent interference noise is within the audible frequency band below 20 KHz. However, the -20dB audio input signal is reduced to -10dB by the 1/2 compression circuit 4 (in LP mode).
Compressed into a signal of F? When modulated, the frequency deviation is ±31.5 KHz, and the frequency of adjacent interference noise (the difference frequency between the instantaneous frequencies of the two reproduced signals) is 0.
Therefore, most of the adjacent interference noise can be located at frequencies outside the audible frequency band of 20 KHz or more. In other words, 0 to 20KHz
Since the adjacent interference noise distributed in 0 to 63 KHz is spread in frequency to the adjacent interference noise distributed in the range from 0 to 63 KHz, the noise energy in the audible frequency band is reduced and the adjacent interference noise becomes almost undetectable. In this case, if the difference frequency changes sinusoidally, approximately 80% of the total period will be equal to or higher than the audible frequency. Expressing this generally, the input signal is O
For the FM modulator 9 whose frequency deviation is ±θKHz when dB, the upper limit input level at which all frequency deviations are within ±10KHz (difference frequency is 20Hz or less) is 20logO dB. On the other hand, when this upper limit n input level is compressed to 101ogθdB by the 1/2 compression circuit 4 and input to the FM modulator 9, the frequency deviation is ±
It is within θ Therefore, even if the input signal level is 201og θ dB, where all the difference 'IP numbers are within the audible frequency band when not compressed, if it is compressed and input to the FM modulator 9,
The difference frequency between the instantaneous frequencies of two signals simultaneously reproduced from adjacent video tracks T,, T, is the audible frequency 2.
A frequency shift of 0 KHz or more will occur in the FM modulated signal, and adjacent interference will be reduced. Needless to say, this output audio signal has been compressed and expanded at the optimal compression/expansion ratio, so there is little deterioration in sound quality such as the so-called breathing phenomenon in which the noise level changes depending on the signal level, and distortion rate deterioration. be.

In the embodiments shown in FIGS. 5 and 6, a switch circuit is used to change the compression/expansion ratio for each of two recording densities, but the configuration becomes a little complicated. Therefore, it is also possible to simplify the two compression/expansion ratios in the embodiment to only one, which is 1:2 in the LP mode. In this case, the reproduced audio signal in SP mode has an S/N of
However, the above-mentioned breathing phenomenon and distortion rate deteriorate to some extent. However, these amounts of deterioration are within a sufficiently tolerable range.

FIGS. 7 and 8 show an example of the circuit structure of the audio recording/reproducing circuit in the case where the compression/expansion ratio is simplified to only 1:2 in the LP mode in the above-mentioned VTR. Here, in Figures 7 and 8,
Circuits that are the same as those in FIG. 6 are given the same numbers. In the operation shown in FIG. 7, in both the SP mode and the LP mode, the audio signal input from the input terminal 1 passes through the pre-emphasis circuit 2, and then the audio signal is pre-emphasized in the 1/2 compression circuit 4. The dynamic range of is compressed to 1/2. Here, the gain of the 1/2 compression circuit is controlled by the output signal of the detector 5 which inputs the pre-emphasized audio signal, and the dynamic range is compressed to 1/2. 1/2
The output signal of the compression circuit 4 is FM modulated by an FM modulator 9. After removing unnecessary band components from the FM modulator output at the LPFIO, the adder 29 adds it to the video signal input from the input terminal 30, and the magnetic head 11 adds the video signal to the magnetic tape 12.
The azimuth is recorded.

In the circuit shown in FIG. 8, the signal reproduced from the magnetic tape 12 by the magnetic head 11 is input to the BPF 13, and only the FM audio signal is extracted. Further, the signal reproduced by the magnetic head 11 is also outputted from the output terminal 25 to a video signal reproduction circuit (not shown).

The extracted FM audio signal is demodulated into an audio signal by the FM demodulator 14. The demodulated audio signal is passed through LPF15 to
After eliminating the leakage of the M carrier wave, the hold circuit 16 processes noise caused by head switching by holding the previous value. The hold circuit 16 operates based on a control signal input from an input terminal 26.

The output signal of the hold circuit 16 has its dynamic range doubled by a doubling expansion circuit 20, and is returned to its original range. Here, the gain of the double expansion circuit 20 is controlled by the output signal of the detector 31.

The output signal of the double expansion circuit 20 is sent to the de-emphasis circuit 2.
After passing through 3, it is output from the output terminal 24. Also,
In the recording circuit of the embodiment, the dynamic range of the pre-emphasized audio signal is compressed to prevent overmodulation from occurring. Furthermore, although compression and expansion are performed according to the amplitude of the entire band of the audio signal, the amplitude of a specific band component of the audio signal may be used.

As explained above, if the present invention is used, as shown below, (1). The optimum compression/expansion ratio can be easily defined according to the D/U of each system.

2. Since the compression/expansion ratio is determined for each system, the so-called breathing phenomenon in which the noise level changes depending on the signal level and distortion deterioration are minimized, and adjacent interference noise can be reduced to a practically sufficient level using a simple circuit. Can be reduced.

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

4. Since the required frequency bandwidth is small, the diameter of the rotating cylinder may be small.

It has many features such as miniaturization of VTR and audio FM.
This has a great effect on reducing adjacent interference noise in superimposition.

[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 the D/tJ of the system. Characteristic diagram showing the relationship between and the apparent D/U of the system improved by the compression/expansion effect, 5th
FIG. 6, FIG. 7, and FIG. 8 are circuit diagrams showing an embodiment of an audio recording/reproducing circuit using the present invention. 4, 6...Compression circuit, 18.20...Decompression circuit, 5, 7, 19. 31...Detector, 3, 8, 1
7,22...Switch. Figure 1 3rd flash 2nd closed T1 D2 Frequency structure 4 Senno S /7 l9 2l? 3 System D (d5)

Claims (5)

[Claims] 1. A magnetic recording system in which an audio signal is emphasized, compressed, frequency-modulated, and mixed with a video signal component, and the recorded signal is sequentially recorded on a magnetic tape as a recording track tilted at a certain angle with respect to the longitudinal direction of the magnetic tape. A reading means, a filter for extracting a frequency modulated audio signal from the reproduced signal read from the magnetic recording reading means, a demodulator for demodulating the frequency of the frequency modulated audio signal from the filter, and an expansion of the demodulated audio signal from the demodulator. It consists of an expander and a de-emphasis circuit that de-emphasizes the output signal of the expander, and compresses the audio signal by calculating the difference frequency between the instantaneous frequencies of two reproduced frequency modulated audio signals read simultaneously from two adjacent tracks. A voice noise suppression device characterized in that the frequency shift is caused by the frequency modulation such that the frequency is equal to or higher than the audible frequency, and the expansion by the expander is performed at a ratio equal to the compression ratio of the audio signal. 2. 2. The speech noise suppression device according to claim 1, wherein the expansion ratio of said expander is selected to be 2. 3. 2. The speech noise suppression device according to claim 1, wherein the amplitude frequency characteristic of said expander is changed. 4. 2. The audio noise suppression device according to claim 1, wherein said magnetic recording/reading means comprises a rotary head having a plurality of heads having different azimuth angles. 5. A magnetic recording system in which an audio signal is emphasized, compressed, frequency-modulated, and mixed with a video signal component, and the recorded signal is sequentially recorded on a magnetic tape as a recording track tilted at a certain angle with respect to the longitudinal direction of the magnetic tape. A reading means, a filter for extracting a frequency modulated audio signal from the reproduced signal read from the magnetic recording reading means, a demodulator for demodulating the frequency of the frequency modulated audio signal from the filter, and an expansion of the demodulated audio signal from the demodulator. 1. A speech noise suppression device comprising an expander and a de-emphasis circuit for de-emphasizing an output signal of the expander, wherein the expansion ratio of the expander is selected to be 2.