[vorbis-dev] Difference between compressed & uncompressed audio?

Rajko.Jersic rajko.jersic at siol.net
Tue Jan 2 08:28:19 PST 2001

> But what is it that you are looking at to see the difference?
> I'm using Sound Forge (and its Sonogram - spectral view).

It's not the cut-off what you should look for. It's just that some
frequencies are masked out.
If high frequencies don't have enough energy, then psyhoacoustics and ATH
wipe them out.
So in spectral view there are just spikes where the energy was above the
treshold and there is nothing where energy was below the treshold.

Rip one song that you are sure was not compressed/decompressed.
Look at it at the spectral view.
Compresss it with various bitrates and decompress it back to wav.
Compare the spectral view of each decompressed files to the original.
You see spikes.
Compressed/decompressed files look like a comb.
At 256kbps you can still see a BIG difference. You can easily tell.

> Why I am asking this in the first place? Since there are
> a lot of pirated CDs where I live, I want to make sure
> that the one I get are not made of decompressed mp3s.
> If a CD was made out of mp3 with bitrates below 256kbps
> then there will be some frequency cut-off and I will see it
> easily, but at 256kbps there are no cut-offs and thus
> it's harder to "see" the difference so I'm wondering if
> there is some other way to be sure that no mp3
> compression was performed...

This doesn't happen only on pirated CDs unfortunately. You can see that on
some promo CDs from USA. Because they send the music over ISDN lines (of
course compressed) from studio directly to the distributers in Europe. But
this CDs are always marked as "NOT FOR SALE".



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