[LAU] How bad is mp3/ogg

[Atte André Jensen]

Hi

I'm teaching a course in electronic music, and one of the subjects I'd 
like to cover is what compression does to the music. I googled a bit abd 
found this:

http://www.youtube.com/watch?v=u5gdwpPrv_8

Basically it's a matter of loading the original + the mp3 encoded 
version of the same track, inverting the phase in one of the two clips 
and listening to the artifacts.

I did some tests, and the results are scary. Lame (128 kbps)and oggenc 
(q=3) are different but both horrible, the artifacts are very ugly and 
distorted and are as loud as -19 dB!. My favorite mp3 encoder, gogo, has 
another strange result: The artifacts sound almost like the mp3, which 
should mean it changes the audio much more! However I don't really hear 
that much difference between lame and gogo encoded files...

This got me thinking if this is even a realistic test. Assume for 
instance the encoder introduces a simple, constant delay in the encoded 
audio. This will result in a lot of sound slipping through the 
invert-the-phase-of-one-of-the-signals test. Although it could be said 
it alters the audio dramatically, when aligning the files and comparing 
them sample for sample, it has no impact on the perceived quality of the 
encoded audio. I didn't fiddle with delaying the gogo encoded file, though.

My question is: is this really a fair way to judge the artifacts 
introduced by encoding?

-- 
Atte

http://atte.dk   http://modlys.dk