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DOUBLE-SIDEBAND LIMITING RF SPEECH PROCESSOR In this simple speech processor the audio signal from the first stage microphone amplifier Q202 (PC-200) is coupled to integrated oscillator and mixer circuit NE612A (SA612), which converts it to a 6.4 MHz signal. The heterodyning action of the mixer produces a double sideband signal (DSB), which is coupled to the second integrated clipper and mixer circuit. The DSB signal is then clipped in the TBA120A (SN76660) and is further converted back to audio for input to MIC. GAIN potentiometer. The resulting harmonics (of 6.4 MHz) are filtered out with simple RC filtering when it was demodulated back to audio. The result is a 'cleaner' clipped signal being finally transmitted. This increases the average output level of an audio signal from a microphone by clipping off the excessive signal peaks. By lowering the peaks in proportion to the average level, a higher average output level can be attained with an associated increase in intelligibility under difficult conditions. It is set up easily without special equipment because no RF filters are used. This effective design offers microphone level input and output adjustments. The clip level depends on the amount of microphone gain. The modification is simple; nothing has to be changed on any of the original printboards. Just disconnect a wire on the top of potentiometer R5 (MIC. GAIN). That point easily can be spotted because resistor R6 is connected to it. The wire must soldered on the top of the potentiometer (P) at the input of the processor. The audio from the output of the processor is then coupled to the top of R5. That is all! Preferably use screened wire for the assembly to or from the processor. The clipping level can be adjusted with potentiometer P. Do not run the gain to far, or you will decrease the intelligibility of your audio. So the gain must be carefully adjusted. If you always use the same microphone, the adjustment can be done once. Adjust R5 as follows: whistle in the microphone for maximum output of the transmitter, set P so that the voice peaks are just below the maximum level. Eventually check with a scoop if the voice peaks will not be reaching the maximum level. It is possible to set the controls so that the peak envelope power reaches the desired level without activating the ALC at all. The level then is only a few tenths of a dB below the level that would have been set by the ALC, but the transmission is a lot cleaner while maintaining very high average modulation.