You are correct - you can't know for sure without the original audio to compare.  But there is a tool called TAU Analyzer that attempts to detect signs of bitrate (as opposed to dynamic range) compression in audio:

http://en.true-audio.com/Tau_Analyzer_-_CD_Authenticity_Detector

(Windows only, unfortunately)

Since most lossy audio codecs slice audio into frequency bands and look to see which of those can be eliminated based on human psychoacoustic models, bands of missing frequencies are often a tell-tale sign of compression.  But not always - it can also indicate heavy or sloppy DSP.


-Theron
^

On Sun, Jan 9, 2011 at 5:26 PM, Mathieu Bouchard <matju@artengine.ca> wrote:
On Mon, 10 Jan 2011, ~E. wrote:

I'm searching how i can detect the change in the compression of an audio signal. The purpose is to detect (and quantified) the compression changes between the music and the ads in a radioshow. Have any ideas ?

If you don't have the original uncompressed recordings, I don't see how you could be doing that. You'd have to guess how complex sounds are supposed to fade out normally, to find out how much the fade out has been messed with.

And then, in the compressor, you have both a measurement of input volume and a formula for turning that input volume into a gain to be applied, and both of those parts are subject to a lot of variation and tweaking.

 _______________________________________________________________________
| Mathieu Bouchard ---- tél: +1.514.383.3801 ---- Villeray, Montréal, QC

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