Speech processors

Earlier today someone asked me to forward them some links I had on speech processors. This reminded me of a VERY effective speech clipper I built many years ago for a 2m AM QRP transmitter. The audio pre-amp had 2 stages of 6dB/octave pre-emphasis followed by very hard clipping and an active low pass filter to keep >2.5kHz audio levels to very low levels. The signal sounded remarkably punchy and allowed this little 100mW AM transmitter to punch well above its weight. It sounded like a rig running several watts. At some point, after we move, I'd like to replicate the design and give it a go again. It was based around the audio stages I used in the Pye PF8 PMR handheld but with value changes to increase the gain and add the second pre-emphasis stage. Double pre-emphasis meant that there was less clipping at lower audio frequencies and more at the higher end. Compressors are meant to be better, but honestly this little circuit took some beating.

Simple speech processing

Recently Peter Thornton G6NGR sent me a number of example circuits for speech processors, some dating back to the 1960s. Anyone who operates QRP SSB on HF or VHF will appreciate how important "punchy audio" is when running low power. One circuit I remember working extremely well was one based on an idea by John Hay G3TDZ. Back in the 1970s I was using 100mW AM on 2m and wanted to be heard across Cambridgeshire. My audio stage consisted of a couple of pre-emphasis stages followed by a hard audio clipper and a low pass active filter. The result was an amazingly punchy signal with barely any change in audio level when talking. The 12dB/octave pre-emphasis helped to ensure that clipping of LF signals did not result in many artifacts within the audio passband. The LPF was essential because the heavy clipping  results in many audio harmonics which would otherwise result in a broadened signal. With just 100mW and a dipole the QRP AM signal was excellent copy across the county. I keep meaning to try the same circuit (if I can remember it) with the FT817. It would produce a mighty punchy QRP signal!