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What do "sampling rate" and "bitrate" mean when describing MP3 files?
The sampling frequency is basically the number of times per second audio is sampled and stored as a number - CD audio is sampled at 44.1 KHz, which means 44,100 samples per second. CD audio uses 16 bit samples, so the bitrate of uncompressed CD audio = 44,100 x 16 bits per second (well x 2 actually, because it's stereo).
The "bitrate" on the other hand, when talking about MP3 files, refers to the transfer bitrate for which the files are encoded - i.e. an MP3 file encoded "at a bitrate of 128 Kbps" is compressed such that it could be streamed continuously through a link providing a transfer rate of 128 thousand bits per second. But most of us don't really use MP3 as a streaming medium (except for shoutcast, etc.) so really what the MP3 "bitrate" is a measure of is how severely the files is being compressed - the lower the bitrate, the more the file has been compressed... and the more you compress a file, the more of the original data is lost, and so the worse the playback sound quality will be. It's almost exactly analogous to compressing a JPG image with a higher compression ratio - you get a smaller file, but when you view it, it doesn't look as good.
