Moonbase;409590 Wrote: > Some 'interesting discussion' > (http://www.hydrogenaudio.org/forums/index.php?showtopic=70548) also > going on over in the -hydrogenaudio- forums.
It sounds like the concept is kind similar to MP3pro. Standard players ignore the "pro" data, while pro players use _both_ streams together. In this case, it's a residual or "error signal" attached to an mp3 (stored in a large id3 tag). If you just decode the mp3 you get mp3 sound. If you decode the mp3 and then add the residual to it, you get the original PCM signal. This adheres to the basic principle of most lossless encoders: first attempt to approximate the signal with a simpler function, then record the difference between the original and the approximated, and store this as the residual. The residual is (hopefully) a low amplitude signal and so it can be further compressed with (eg) rice coding. I think people had tried this in the past with mp3, and found it to be a poor basis for a lossless codec. Since MP3 was not designed for that purpose, the psychoacoustic model allows for phase change, masking, and other effects which although minimally audible, can drastically alter the wave shape, making the encoder generate excessive residual content. So probably they've incorporated a special-purpose mp3 encoder which tries to give a small residual signal for the purpose of creating a lossless encoding. But there's a catch: this doesn't necessarily make for a good-sounding mp3. In fact, it's an additional constraint, so it necessarily compromises the quality of the lossy rendition compared to, say, lame at the same lossy bitrate. So as I understand it, the selling point of the format is the grotesque marriage of these two feature sets: 1. When you're using it as a lossless codec, it has worse encode/decode cost and larger file sizes than FLAC, and a royalty to pay. 2. When you're using it as a lossy codec, it has worse sound quality than lame and drastically larger file size, and a royalty to pay. Perhaps the larger filesize could be mitigated by automatically stripping out the residual when synching to a standard mp3 player. -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=61694 _______________________________________________ discuss mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/discuss
