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Originally Posted by Edward_Vinatea  Yeah, I guess the reason is you convert from to 24 to 16 first then you make the mp3 copy. I am not sure why this is the recommendation, I haven't paid attention to that as i usually rip from master CDs ---  |
Bob's point was referring to something that has already been mp3, being turned into mp3 again. Perceptual coders of this ilk, including things like AC-3 and ATRAC, are designed to throw out data for parts of the signal that will be masked and therefore won't be missed by the casual listener in the typical consumer environment. However, if you take the resulting uncompressed PCM stream and data-compress it again, the critical decisions the original coder made become painfully obvious.
Double-encoded mp3s sound bad as the fragile perceptual coding falls apart upon being presented previously coded material. Dolby E was made in part to address this problem for the digital television broadcast chain since multiple encode/decode passes may be unavoidable during the many links in the chain from source to local affiliates.