SU1598179A1 - Device for converting dynamic range of acoustic signals - Google Patents

Abstract

The invention relates to radio engineering. The purpose of the invention is to reduce the noise level. The device for converting the dynamic range of audio signals contains a quadrature phase shifter 1, quadratic detectors 2 and 3, an adder 4, filters of the lower 5 and upper 6 frequencies, an amplitude detector 7, an adjustable amplifier 8, a limiter 9 and a multiplier 10. To change the transmission coefficient of the device depending from the signal-to-noise ratio, the squares of the signal and noise envelopes are formed at the input and output of the adder 4, which are then separated using filters of the lower 5 and upper 6 frequencies. The signal voltage at the output of multiplier 10 changes under the action of the signal voltage, and the gain of the adjustable amplifier 8 and the limiter limiter level 9 under the action of the noise envelope voltage make it possible to increase the signal-to-noise ratio at the device output. 1 il.

Description

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The invention relates to a technique for transmitting signals and can be used in recording and reproducing sound signals in order to improve the sound quality of noisy magnetic photographs,

The aim of the invention is to reduce the noise level.

The drawing shows a structural electrical circuit device.

The device for converting the dynamic range of audio signals contains a quadrature phase shifter 1 first 2 and second 3 quadratic detectors, an adder A filters the lower 5 and 15 upper 6 frequencies, an amplitude detector 7. an adjustable amplifier 8. a limiter y and an analog multiplier 10.

The device works as follows: ° 20

The input of the device — the input of the quadrature phase shifter 1 — is fed to the reference one. SBx (t) signal. The quadrature phase shifter provides the phase shift of all spectral components in the range of predetermined frequencies by 90 ° and thus performs the Hilbert transform. As a result, two signals are generated at the output of the quadrature phase shifter 1: bW and the Gilbert conjugate signal 30 bd), which have the same spectra and their phases are shifted by 90 °. Thus, the signals at the output of the phase shifter can be recorded as follows.

S (t) - SWcOS 5P (t) -f-tf3b} -.- JC

S (t) -S (t),

where S (t) S2 (t) + S2 (t) is the envelope, t e

modulating function of instantaneous amplitude:

y5 (t) - current phase; 40

UH is the initial phase.

The signals S (t) and S (t} are fed to the inputs of vadratichnyh detectors 2 and 3. where 8 square is raised and then summed up in the adder 4. Thus, at the output of adder 4, we get a signal proportional to

From the output of the adder, the signal is received “and the inputs of filters 5 and 6.50

The output signal from the 5 low frequencies, which is proportional to the square of the Hilbert dying useful signal, arrives at

control amplifier stroke 8.

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A signal from the output of the high-pass filter 6, proportional to the square of the envelope noise, is fed to the input of the amplitude detector 7. From the output of which is fed to the control inputs of the adjustable amplifier 8 and the limiter 9

Depending on the level of the signal supplied to the control inputs of blocks 8 and 9. the gain of the adjustable amplifier and the limiter threshold level of the limiter vary.

With an increase in the control signal, which corresponds to an increase in the noise level at the input of the device, the transmission coefficient tends to a logical relationship, and to intermediate values of the control signal to a linear relationship. In general, the signal at the output of the adjustable amplifier can be represented as

 $ 2,

0

five

where K is the coefficient of proportionality p is the coefficient of approximation of the transfer function.

The amplitude-corrected signal is fed to the signal input of the limiter 9. The signal level of the corresponding threshold limiter varies and depends on the level of the control signal proportional to the average module of the square of the envelope of the noise: the higher the limit of the limiting threshold value determines the maximum noise level possible and is of the order of minus 30 dB relative to the nominal level. The limiting threshold value eliminates the effect of high-frequency components of a large level on the threshold value.

From the output of the limiter, the control signal (Sy) is applied to one of the inputs of the analog multiplier 10. The second input of which supplies a signal from one of the outputs of the quadrature phase shifter 1 The output of the multiplier is the output of the device.

At the output of the device to the threshold limit, i.e. for small input levels of x signals, we get a signal equal to 5outM (S) t (t) KMS (t)) after the threshold, and for large levels of the input signal we get a signal equal to 5out (t} s (t)) + SyS ( t) Warp Km) + pol where KM is the scale factor of the analog software multiplier.

Analyzing the above expressions, it can be concluded that at low signal levels close to or equal to the noise level, the output signals will be significantly attenuated due to the formation

control signal (Sy), proportional to the power value of the envelope. The use of a floating limitation threshold allows gaining not only in the signal-to-noise ratio, but also in the dynamic range, which improves the sound transmission quality in the low-level domain.

The transfer function of the variable amplifier depends on the noise level. With an increase in the noise level, the effect of noise reduction somewhat decreases, the amplitude characteristic of the device will have a smooth transition in the vicinity of the threshold, and thus the operation of the dynamic range conversion device on real signals becomes less noticeable during auditory perception.

When the noise level is minus (36-38) dB relative to the nominal signal level, the transfer function corresponds to the square root extraction function, i.e. control voltage is proportional to the Hubert envelope. When the noise level is minus (38-42) dB, the transfer function is linear, which corresponds to the control voltage proportional to the square-cube of the Hilbert envelope. When the noise level is less than 42 dB, the control voltage is proportional to the higher degrees of the Hilbert envelope. The cut-off frequency and the slope of the filters 5 and b are selected depending on the spectrum of the musical-speech signal. At low signal levels, the width of this spectrum does not exceed 3 kHz. The width of the spectrum of white noise occupies the entire band of sound

range from 20 Hz to 20 kHz. The width of the signal spectrum, proportional to the Hilbert envelope, is two or more times the spectrum of the signal itself and occupies a band from 0 to 1.5 kHz.

Claims (1)

Thus, the cut-off frequency of the low-pass filter should be no more than 1.5 Hz. The cut-off frequency of the high-pass filter is chosen to be equal to or slightly higher than the cut-off frequency of the low-pass filter. The invention The device for converting the dynamic range of audio signals containing a quadrature phase shifter, the first and second quadratic detectors, the outputs of the terminals connected to the inputs of the adder, as well as an adjustable amplifier and analog multiplier, one output of which is connected to one output of the quadrature phase shifter, about that, in order to reduce the noise level, a low-pass filter, a limiter and a series-connected high-pass filter and an amplitude detector are introduced, the output to This is connected to the control inputs of the adjustable amplifier and limiter, while the inputs of the first and second quadratic detectors are connected to the outputs of the quadrature phase shifter, the output of the adder is connected to the inputs of the lower and higher frequencies, the input of the adjustable amplifier is connected to the output of the low-pass filter, and the output is input limiter, the output of which is connected to another input of the analog multiplier.