[TheBang]

AAC and MP3 are both lossy compressed formats.

1. lossy source -> lossy format = quality loss (ex. AAC -> MP3, 128 Kbps MP3 -> 192 Kbps MP3)
2. lossy source -> lossless format = same quality, huge file size increase (ex. MP3 -> FLAC)
3. lossless source -> lossy format = quality loss, huge file size decrease (ex. CD-DA -> MP3)
4. lossless source -> lossless format = same quality, up to 50% smaller or larger file (ex. WAV -> FLAC, FLAC -> AIFF)