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Speech processor

2018-05-02 06:01  
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To obtain the best modulation result of a transmitter you can use 2 ways: clipping and dynamic compression. Both methods have disadvantages: clipping – there is no constant signal carrier and so the transmitter has a tendency to be overmodulated or undermodulated and in dynamic compression degradation also happens because of the uncontrolled signal bursts due the time constant of the compressor’s regulator.

If you want to avoid these problems you can use this speech processor which combine both modulation tehniques, first, the signal is compressed to achieve a constant signal level and then clipped to remove unwanted amplitude peaks. The circuit first amplifies the audio signal, the gain of the amplifier stage depends upon the impedance of the microphone. Diodes D1 and D2 function together as voltage controlled signal processor.

When the voltage at the D3 anode exceeds the threshold voltage by 0.5V, the incoming signal will be reduced through C5 D1. Very fast signal bursts which escape the compressor will be clipped by diodes D6 D7, the amount of clipping being determined by R8 R9.

You can use this speech processor as modulator stage for SSB, AM, FM transmitter, the low pass characteristics is designed for 80 meter band operation but if one desired a low pass characteristic with a different cut-off frequency, multiply C1 up to C14 by factor A, A = 3/f, where f is cut-off frequency in kHz. For example if the desired cut-off frequency is 6 khz, the C11 and C14 must be replaced with half or the original values.

Speech processor circuit diagram


Clipper, dynamic compressor components

D1…D7 = 1N4148T1 = 2SC3112, 2SC2675T2 = 2SC3622T3, T4, T5 = 2SC1876, 2SD406, 2SD614, 2SD688, 2SD892A, 2SD1153, 2SD2116, 2SD2117, 2SD2213

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