spod,
AAC, MP3, WMA, WAV and others are music compression formats. ID3 is a 'tagging' system for music files used by many compression formats. ID3 is not a music compression format (Google 'ID3' for more info).
Song file size is a factor of bit rate and song length. Audio quality is a factor of bit rate and encoding format. AAC and MP3 formats are considered Lossy, as they sample the target music file and reduce the total size with some reduction of audio quality. Lossless files are considered CD replicants as they contain all the digital data on the original audio CD. They can be fairly large in comparison to the traditional Lossy file.
Encoding a music file into a Lossy compression format will strip details from the file.
Transcoding from one Lossy compression format to another Lossy format will compound the loss of details from the file. (eg: transcoding a sound file from: AAC to MP3; or MP3 to AAC). The preferred method is to save all audio "masters" in a Lossless audio format such as Apple Lossless, WAV, AIFF or FLAC, and then transcode directly from the Lossless source file to your preferred Lossy format such as MP3 or AAC. This procedure preserves as much of the original audio signal as possible and prevents the
compound loss of audio details from the file.
The generally accepted theory is that AAC/128 sounds as good as, or better than MP3/160 (and possibly even MP3/192). Transcoding your AACs/MP3s will most likely result in noticeable audio quality degradation. But -- test it out for yourself. If you cannot hear the difference, then it may be acceptable. Bear in mind that any improvements &/or upgrades in equipment (iPods, headphones, your ears, etc.) may uncover the
additional audio limitations you created
at a later date.
Technically, AAC and Apple Lossless (ALAC) are separate codecs.
AAC is a newly developed codec by a conglomeration of several companies and is used with the MPEG4 ISO standards. It is not proprietary to Apple. See:
http://www.apple.com/quicktime/technologies/aac/
http://www.vialicensing.com/products/mpeg4aac/standard.html
ALAC is an Apple proprietary codec. See:
http://craz.net/programs/itunes/alac.html
You have to decide for yourself which best suits your needs. It's a trade off between what sounds best to you and how much space you are willing to use for a song file.
I use AAC/256/VBR, but I do not need all my songs on the iPod at one time. I do like to have better quality music. YMMV
