While somewhat old technology today, I developed and patented a multiband
audio processor for broadcast applications many years ago.
The basic processing system consisted of a method to divide the audio
spectrum (20Hz - 25KHz) into 5 bands. Each different band was passed
through a separate compressor/limiter. Each used different attack and
release times for each band as the compliment of audio was different. Then
the 5 "processed" signals were combined back into one signal. For the 2
channel stereo version the same method was applied for each the Left and the
Right channels. Then a mono sum signal was derived ( L+R) and a mono
difference signal (L - R) was derived. These 2 signals were sent to the
transmitter where the (L+R) was added to the (L-R) netting a Left channel
and a Right channel signal.
The system allowed each station to "tailor" their sound and their "loudness"
by adjusting the respective levels of the 5 bands. Made for a loud station
or a sweet sound or one with heavy bottom as one might imagine. Just what
the competitive market demanded.
Today, on the ham rig, I use a 5 band EQ followed by a Behringer Compressor
Pro on the Paragon. Most agree, it sounds great.
----- Original Message -----
From: "Paul Christensen" <firstname.lastname@example.org>
Sent: Tuesday, February 19, 2002 2:09 PM
Subject: Re: [TenTec] Digital Speech Processing & Pegasus
> > 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
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