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A stand-alone non harmonic generating speech clipper with tight bandwidth control using digital signal processing.
The Microchip DM330011 dsPIC development board is an ideal ready-built low-cost platform to implement audio processing applications. [Farnell part no 1621788, £36.70 + vat] The software is based on DSP techniques described by Peter Martinez, G3PLX in the January 2010 issue of RadCom. Actual dsPIC code is written by Peter Baston, GW0PJA. The advantages of DSP clipping versus RF clipping are many. Compared to the [excellent in its day] Datong clipper, the DSP clipper has a firmer clipping level as there are no diode characteristics to soften things up, a wider frequency response, and less distortion particularly at lower frequencies. Response is still good down to a couple of hundred Hertz, with an effectively unlimited upper response. The actual high frequency limit is controlled by a selectable filter within the DSP program which gives a steep roll-off such that no further bandwidth limiting is needed for many applications. The prototype is running at GW4GTE's QTH and can be heard on the 80m AM frequency 3615kHz where the high quality can be demonstrated. It has so far proved 100% reliable with no crashes or glitches, and is not susceptible to strong RF fields.
January 2010
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