From: Dave McLain
Bob,
You are the man when it comes to audio. I was just wondering how the MP3 compression scheme works. I’ve tried it out on my computer and while it’s not perfect, it does seem to sound pretty good and the files are unbelievably smaller. Does MP3 alter the number of bits or the allocation of the bits program dependently? Or does it alter the sampling rate program dependently, or both?
Thanks for your help,
Dave McLain
Polaris
Dave:
Many thanks for your comments.
MP3 is part of a family of data compression algorithms that does alter the number of bits and uses masking theory to remove data that might not be heard in the presence of other data. For example, in a loud high frequency passage, softer high frequency information will be masked, and *in theory* can be safely removed. The “sample rate” can be any rate; the decoder actually produces an output at the standard sample rate, but it is a conversion to the standard sample rate and bit depth every time the file is played. The quality of the encoded MP3 depends on the encoder itself and the compression rate you select for the encoding process. Since there are no official standards for many of the relevant steps in the encoding procedure, the sound quality you get will vary in a very wide range – depending on the encoder itself (the algorhithms it uses for quantization, filtering, spectral processing, bitstream formatting…).
But MP3 (and all the other compression systems, e.g., MPEG, AC3) should never be used to record original material (masters). Always start with the highest resolution. My article “More Bits Please” makes that clear.
Best of luck,
Bob