[TenTec] Digital Speech Processing & Pegasus

[Paul Christensen]

> Langevin used to make a "distributed" clipper where the audio went
> through a many element low pass filter and at each shunt element of the
> filter there were clipping diodes. This rounded the corners. VOA liked
> to use it, probably still does.

Professional broadcast engineers have been using similar techniques for more than twenty years, beginning with devices like the
Dorrough DAP310, Inovonics 230/231-MAP-II, and later with Orban's line of multiband Optimods...and particularly with the Gregg Labs
multiband processors.

The idea is that the audio spectrum is split via a multiband cross-over network,  The resultant, distributed output from each
cross-over is then directed to an independent compressor/limiter/clipper circuit.  Typical processors contain from two to six audio
bands. As the audio is deliberately clipped in the audio domain, it is immediately proceeded by a low-pass filter whose sole
function is to filter all the harmonics which are generated by the clipping.  This process can be repeated at the final, combined
output of all spectral bands and the audio is low-pass filtered one final time (e.g., 15 kHz for FM stereo to protect the stereo
pilot and stereo baseband components).  Why do all this?  Loudness, of course.  The trick in the broadcasting world is to compress,
limit and clip to the greatest extent possible without introducing overly objectionable artifacts and maintain some reasonable
degree of fidelity.  Many AM/FM broadcast and shortwave stations have abused these audio processing systems for years in an attempt
to garner more audience share.  But are they attracting more listeners or turning them away from the artifacts?  That has always
been the $64K question.

-Paul, W9AC