Compression is distorting the sound. If it were such good thing, it would already be done for us when we got the CD.

Actually, it is done for you on the CD. The problem is that different producers and mastering engineers use different amounts of it, depending on how they want the CD to sound.

A given instrument might have gone through five or more different compressors before it reached your ears. Let's take an electric guitar, for instance. It probably has a pre-effects compressor on the raw signal. Then, within its effects chain, there could be another compressor to even out the sound after the effects have been applied. If the guitarist takes a solo, he probably kicks in a whole other rack of effects just for the lead guitar sound, among which is most likely another compressor. As that guitar track is mixed into the rest of the tracks, it (like many of the other tracks) is probably bounced through another compressor. Then the final mixed tapes are handed off to the mastering facility, which applies another level of compression to the whole mix.

It's that last level, mastering, which determines the perceived volume of a given track. Everyone does it a little bit differently. A discussion of this process can be found here:

http://www.digido.com/compression.html

Waitamminit! If done right, amplitude compression is non-distortive. I've never built a compressor, but I imagine the trick is to have the time constant longer than half the period of the deepest bass (25 ms, for 20Hz) and shorter than you'd notice a gradual change in volume (several 100 ms).

Actually, in a real compressor, there are several adjustable parameters, including attack time and release time.

And although you can make the argument that compression is "non-distortive", it depends on your definition of distortion. A compressor alters the dynamics of a sound signal, and in many cases, changes its character. If you consider "alteration of the original signal" to be distortion, then yes, by definition, a compressor distorts the signal.

The trick is to do it in a nice way that achieves your goal without compromising the "flavor" of the original audio signal. But it depends on what your goal is. As a guitarist, sometimes I want the compressor on my lead guitar to significantly alter the character of the notes I'm playing. If you just want all your albums to sound like they're at the same volume, then more subtle compressor adjustments are necessary, and you have to decide where you want the trade-offs.

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Tony Fabris