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Better solution using QuickTime Pro
Authored by: Mr. H on May 31, '07 02:17:13PM
Converting using iTunes is not a great idea. iTunes de-codes the AAC to uncompressed and then re-compresses it, resulting in additional quality loss. Granted, the loss should not be that great given that the starting point is a high-quality 256 kbps AAC, especially if the destination format is AAC at an equal or higher bitrate.

However, to avoid the quality loss issue, you can use QuickTime Pro instead:

  • Open the AAC in QT pro, and choose "export" from the "File" menu.
  • In the dialogue that appears, choose "Movie to MPEG-4" in the "Export" drop-down menu.
  • Click the "options…" button.
  • Choose "MP4 (ISMA)" from the "file format" drop-down menu at the top of the window.
  • Click on the "Audio" tab.
  • Choose "Pass through" from the "Audio format" drop-down menu.
  • Click on the "streaming" tab.
  • Click on the "enable streaming" check box. The settings can be left as default. This avoids any potential issues with streaming the track via iTunes sharing. It has no affect on quality, it just affects the MP4 wrapper that the audio is stored in.
  • Click "OK"
  • Now we're back at the "Save exported file as…" dialogue. Change the extension of the file name to be saved from .mp4 to .m4a
  • Choose a destination folder for the file to be saved to. (don't choose the same folder as the original, unless you've changed the name of the file or backed-up the original).
  • Click save
QuickTime Pro will now "re-wrap" your audio in a brand new ISMA-compliant .mp4 wrapper. This should solve any compatibility issues you are having without re-encoding the audio.

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I forgot to say…
Authored by: Mr. H on May 31, '07 02:29:49PM
…that the new file will not carry-over any of the meta-data from the original file. Once the new file is created, it can be added to iTunes, and one of Doug's excellent iTunes scripts: "copy tag info tracks to tracks" can be used to copy all the meta-data from the original track to the new one.

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