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From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Optimod: how does it work ?
Keywords: radio modulation
Message-ID: <1310@unccvax.UUCP>
Date: 5 Jan 89 21:53:34 GMT
References: <1561@kulcs.kulcs.uucp>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 72

In article <1561@kulcs.kulcs.uucp>, marc@kulcs.uucp (Marc Van Barel) writes:
> Here in Europe, more and more radio and TV-stations are using a device
> called "Optimod" for improving sound modulation.  It's produced by an
> American company called "Orban".  Using this device, the audio signal
> seems to have more loudness.  Does anybody on the net know how this
> works ?  Is it just some smart compression/limiting of the dynamic
> range of the audio signal or is there more to it ?

	Optimod[tm] is a trade name for a variety of signal processing
widely used in broadcasting. I have worked with Optimod-AM quite extensively
and with products from competing companies, and can state that properly
set up, Optimod seems to do a decent job.

	The Optimod system consists of basically three sections:

	A wideband, gated AGC amplifier;
	A multi-band compressor
	Peak limiting and (in the AM case) bandpass filtering;
           negative and positive peaks are adjustable separately for
           Optimod-AM.

	The "gating" of the AGC amplifier  prevents the AGC from going
to maximum gain during pauses in the program or otherwise quiet passages.
The gating is adjustable to a specific RMS level; with the gating off,
it's a wideband compressor.

	The multiband compressor is purported to have the time constants
and bandwidths required for the most natural increase in loudness without
audible effects.  THe advantage of the multiband compressor is that, 
judiciously used, it has the ability to equalise program material to
a more "uniform" sound.  This isn't really a bad idea when you own an 
AM radio station, and have less-than-ideal sources, from CD's all the way
down to your news reporter's cassette deck and Traffic Patrol.

	Peak limiting is the time-honoured trick of increasing the RMS
of the signal by, er, deliberately clipping the peaks.  The Optimod does
this reasonably tastefully, and allows you to adjust the clipping threshold
from -3 dB to + 2 dB for negative peaks, and similarly for positive peaks.
An elliptical output filter allows this clipped audio to have reasonable
overshoot characteristics when it leaves the Optimod.

	The Optimod does have a phase scrambler (all pass filter) which
takes the assymetry out of normal speech and music programmes.  This helps
prevent certain types of transmitters from grossly overmodulating during
"difficult" recordings.  The phase scrambler might be in Optimod-FM as
well.

	Finally, there is parametric EQ after the wideband AGC, with both
bandwidth, center, and gain adjustable for bass and treble frequencies.
The Optimod-AM also has some adjustable single order filters for tilt and
overshoot compensation of older transmitters.

	Orban do make other products, such as Optimod-FM (the Optimod 8000
is mostly a stereo generator with some compression, the 9000 being sort of
the Optimod-AM with the stereo generator). When Optimod-FM was introduced,
it was the benchmark of stereo generators, and it is still damn good.  They
also make a TV stereo sound system for BTSC.  Orban also manufactures a line
of studio toys, such as parametric EQ, de-essers, compressors, etc.

	Eric Small left Orban and (I guess) formed Modulation Sciences, Inc.,
another manufacturer of this kind of stuff.

	I personally like the Optimod AM, but the chief engineer at our Boston
radio station has pasted over the front panel so that it reads IM  -AM (I guess
for intermodulation...). Compression and EQ is a matter of taste and virtually
a black art.  In the US, there is more anxiety over audio processing and its
supposed relationship to cumulative audiences than probably any other single thing
in a major market station.

Hope this helps...
York David Anthony
WKTD Wilmington, NC