14 Dec 2011

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!


LY2SS said...

Here's very interesting work




Anonymous said...

Hello Roger

The speech processor used In the PW Melstock 70 MHz (Sept 2005 Tony Nailer ) AM Transmitter could be used In an SSB rig. The one advantage of using a processor for AM is you can acheive 100% modulation depth for a large part of the voice syllables with any over modulation being restricted. There was a simple design In PW early 1980's
using 4 BC109 transistors with diode clipping. I built the later which worked well with my FT221.