Originally posted by matttichenor
Whatever MP3 format you are currently using will have some loss when converted to the AAC format. Indeed, a 320kbps MP3 converted to 128kbps AAC will sound better than a 160kbps MP3 converted to 128 kbps AAC, but it would not sound as good as a CD converted to 128 kbps AAC.
As well, if you originally ripped at 320 kbps MP3, quality must have been important to you, the point is that you are never going to get that same level of quality if you simply convert from MP3 to AAC. Re-Ripping from the original CD is always going to create the highest quality results. Whether or not those results can be heard or qualified is another question.
I used 160kbps MP3, which is basically as low as you want to go to maintain quality. Therefore, if I were to convert these files to AAC now, it would re-compress an already heavily compressed file and result in noticibely poor sound quality. Once you have compressed the audio, you can't get that information back without going back to your master (CD).
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