[icecast] Applying dynamic compression to live audio

Tim Pozar pozar at lns.com
Thu Apr 4 07:37:21 PST 2002



On Thu, Apr 04, 2002 at 07:13:45AM -0700, Jack Moffitt wrote:
> While dynamic compression makes the transmission 'smaller' on FM radio,
> it makes music harder to compress by psychoacoustic codecs, and therefor
> you might have somewhat of a quality hit, although I haven't tested
> this.

I have tested this to a limited extent.

Jack is correct that the addition of compression and limiting will
cause the codecs to work harder.  Any audio processing will generate
distortion in additional spectral energy as second and third harmonics
that the codecs will need to try to figure out how to compress.
You will start to hear more artifacts in the stream the more you
compress, limit and clip.

But this isn't to say you shouldn't do some processing.  There are
two reasons to audio process for streams; to prevent digital clipping
and to reduce the dynamic range so the audio can be hear in noisy
environments.  Sometimes folks will process the audio as they like
the "sound".

Ideally, you would want to use a soft knee limiter to prevent
clipping and perhaps slow automatic gain control (AGC) to help
reduce the dynamic range.  The Behringer (http://www.behringer.com)
Composer Pro MDX2200 is about the cheapest bang for the buck (~$150)
processor out there.  I found that it takes a lot of tweaking to
make it sound okay.  You can't use the use it too much as it sounds
pretty bad if you push it.

If folks have a couple extra bucks, I would suggest looking at the
Aphex (http://www.aphex.com) gear as it sounds much better but you
can expect to pay about $250 for a used "Compellor" (AGC) and
$400-$500 for a "Dominator" (limiter) on Ebay.

Tim

-- 
  Snail: Tim Pozar / LNS / 1978 45th Ave / San Francisco CA 94116 / USA
               POTS: +1 415 665 3790  Radio: KC6GNJ / KAE6247
"It's a damn poor mind that can only think of one way to spell a word."
                        - Andrew Jackson
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which is the exact opposite." - Bertrand Russell, "Skeptical_Essays"

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