Noise suppressor of a modern stereo complex. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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In any sound program, in addition to the useful signal, there are always extraneous noises. To combat them, effective compandering noise reduction devices are widely used. However, as is known, they do not suppress the noise of the original program, but only protect it from additional noise introduced by the information carrier in sound recordings or by the communication channel in radio broadcasting.
Dynamic noise reduction systems (DSNR) have such an important advantage as the possibility of significant noise attenuation not only of the signal processing channel, but also of the sound program itself. Therefore, it is advisable to combine the use of DSWB with compander noise suppressors.
The noise reduction device based on the K157KhPZ IC is capable of suppressing the noise of an audio program with a dynamic range of 40...50 dB, practically without introducing distortions into the processed signal. A similar noise reduction device (Fig. 1) is designed for use in a high-quality stereo complex.
Specifications:
- Bandwidth, Hz.......20...25 000
- Rated input voltage, V ....... 0,5
- Transmission ratio......1
- Harmonic coefficient, % (not more than) ....... 0,1
- Noise suppression, dB (not less than): in the "-50 dB" mode....... 8
- in the "-40dB" mode ....... 12
- in the "-30dB" mode ....... 15
The input divider R4, R5 attenuates the signal to 100 mV. The transfer coefficient equal to 1 is set by selecting the resistor R4.
On transistors VT1, VT2, a composite emitter follower is assembled, it is also a phase shifter with closed contacts of switch S2. The use of a phase shifter is desirable when processing a tape recorder signal, because due to the differentiating action, the magnetic head introduces phase distortions during playback. This is most clearly seen when playing a rectangular signal. Figures 2 and 3 show oscillograms of a signal with a frequency of 1 and 4 kHz at the output of the reproduction amplifier, respectively, without a phase shifter and with a phase shifter.
At high frequencies (6 kHz or more), the efficiency of the phase shifter is manifested in an increase in output up to 2 dB.
The most effective use of phase shifters during rewriting, because. reduces the accumulation of phase distortions. With phase distortion, it is advisable to make recordings intended for playback on tape recorders without a phase shifter.
For operational control of the noise suppressor and its effective use, the device has three fixed noise reduction thresholds.
The effectiveness of noise reduction is largely related to the initial lower cutoff frequency, which depends on the resistor in the pin 7 circuit and can be set in the range of 0,8...1,6 kHz. To indicate the operating mode, a two-color VD2 LED was used (-40 dB - green, -30 dB - red). The K157KhPZ chip is included according to a typical circuit and has no special features.
Set up the device in the following order. By selecting the resistor R4, the transmission coefficient of the SHP is equal to one. Temporarily connect pin. 20DA1 with a common wire. The K1 button turns on the "-30 dB" mode. A 50 mV signal with a frequency of 1 kHz is applied to the input. By selecting the resistor R15, the signal at the output of the SHP is attenuated by 3 dB. Restore installation.
Then you need to set the signal level to 16 mV at a frequency of 6 kHz. Trimmer R19 reduce the gain by 3 dB. Enable "-40 dB" mode. Reduce the signal level by 10 dB (up to 5 mV). Trimmer resistor R16 set the gain equal to 0,7 (attenuation 3 dB).
Next - temporarily connect pin 20 to a common wire. Enable "-50 dB" mode. Apply a 50 mV signal with a frequency of 1,5 kHz to the input of the SHB. By selecting the resistor R14, reduce the transfer coefficient by 3 dB. Restore installation.
Finally, you need to set the signal level to 1,6 mV at a frequency of 6 kHz. By selecting the resistor R17, achieve a transfer coefficient of 0,7.
References:
- V. Adrianov and others. "All about the K157KhPZ microcircuit". "Radio", No. 11/85, pp. 33-36.
Author: M.Voskoboynikov
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