[LAU] How bad is mp3/ogg

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