Problem exporting to AAC… Help!

[Gale_Andrews]

@poetofpiano

I tried exporting to WAV signed 32-bit PCM and converting that to AAC in itunes, and the resulting file was exactly the same size as an AAC file converted in itunes from a 16-bit WAV, and you were right — I don’t think I could tell the difference even with good headphones!

@Gale Andrews

You can however specify the constant bit rate if you choose “Custom FFmpeg Export” instead of “M4A (AAC) Files (FFmpeg)”. Click the Options… button, choose “mp4” in the formats list and “libvo_aacenc” in the codecs list, then set bit rate in bits per second. Up to 320 kbps is supported. For example, to export at 256 kbps, enter “256000” (without quotes). This does work - I’ve now tested it.

I also tried this, and though the quality was marginally better than just exporting using the standard M4A (AAC) Files (FFmpeg) option, and though itunes did say the resulting file was 319 kbps, the file still wasn’t nearly as good as a WAV converted to AAC with iTunes. So I think I’ll stick to converting to AAC in iTunes for now.

I should have mentioned that if you don’t mind using Audacity’s command-line export you can choose the native FFmpeg AAC encoder rather than the libvo-aacenc which you get if you export choosing “M4A (AAC) Files (FFmpeg)” or “Custom FFmpeg Export”. Using the native encoder also lets you encode more than stereo.

“-strict experimental -c:a aac” tells FFmpeg to use the native AAC encoder. This is the syntax for setting bit rate (AAC’s version of “constant bit rate”) with the native encoder at bit rate of 240 kbps:

ffmpeg -i - -strict experimental -c:a aac -b:a 240k "%f"

This is the syntax for setting quality (variable bit rate) to the maximum (10) with the native encoder:

ffmpeg -i -  -strict experimental -c:a aac -q:a 10 "%f"

The above should also apply to the FFmpeg we recommend for Mac OS X from the Buanzo site.

Gale