[icecast] Applying dynamic compression to live audio

Thomas Vander Stichele thomas at urgent.rug.ac.be
Wed Apr 10 02:44:48 PDT 2002



> On Thu, 4 Apr 2002, 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.
> 
> Quite possible, though I'm only using 56kbps 22khz stereo anyway so that's
> not such a big deal.  I really need it to even out levels, however, which
> is more important to me.

Here's my two cents on it.

First of all, like Jack says, it's mostly useful do to compression for FM 
radio.  The noise floor being pretty high and all that, it makes sense.

For digital music, I tend to avoid it as much as possible.  There is one 
thing you should do, though.  Most people have all of their music 
normalized, which basically means "make sure that the loudest sample is 
really close to the maximum".  It doesn't say *anything* at all about the 
average level or even the perceived level.

A very good start is to actually calculate the RMS in your songs, and 
scale your songs accordingly while playing them back.

For example, if a Destiny's Child song has an average RMS of 8000 and a 
Celine Dion song ends up at 3000 (examples are taken for broad 
applicability and are not necessarily related to my taste in music ;) ) 
you should scale down the DC song to match the RMS of the Celine Dion 
song.

In practice you take a low enough value and target all your songs to this 
value by scaling them down (don't scale up unless you KNOW you won't 
distort digitally).

I have written some small stuff that calculates this sort of thing and 
using these values it works out really well for me.

On another note, I have started to help out a lot on GStreamer 
(http://gstreamer.net/) and I think this is where the future of media on 
linux lies.  So I'll be applying my ideas there (GStreamer already has a 
libshout plugin) and start writing a decent mixer there, as well as 
probably adding support for icecast to rhythmbox 
(http://rhythmbox.sourceforge.net).

Any comments are welcomed ;)

Thomas

 -- 

The Dave/Dina Project : future TV today ! - http://davedina.apestaart.org/
<-*-                      -*->
nobody's interested
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