Re: [SoX-users] compand for rock concerts

[robs <aq...@ya...>]

--- On Sat, 29/8/09, Wesley Chow <we...@se...> wrote:
> I've got some recordings of concerts to which I'd like to
> apply range  
> compression. I've found that if I just increase the volume
> on the  
> recording, drums and clapping tend to blow out the ear
> drums while  
> listening with headphones... until I found sox's handy
> compand command!
> 
> compand 0.3,1 6:-70,-60,-20 -5 -90 0.2
> 
> Can someone suggest a better set of values to try?

The numbers in the manual were chosen as a reasonable typical set, but
have not been extensively tested by me at least so if someone has worked out a better set then please post them and I'll update the manual.

Looks like the manual currently does an okay job of describing the parameters but here a few additional comments:

The compander basically just turns a volume control up and down automatically.  The first 2 numbers (0.3,1) control how fast the volume is turned down and up respectively -- the numbers are measured in seconds, so the smaller the number, the faster the volume is changed.  Changing the volume too quickly may become obvious audibly so there is a compromise to be had here.

The next section (6:-70,-60,-20) specifies what is known as the 'transfer function' and defines, for each possible volume in the input file, what you would like the output volume to be -- they're basically points on a graph and, if you have access to any of the gnuplot, Octave, or Matlab programs, you can see what this graph looks like using SoX's --plot option.  The best way to evaluate audibly whether the transfer function is optimal is to check the output level from sections of the input audio where the volume is fairly constant.

HTH,
Rob