Bitrate settings for M4A (AAC) Export

[lucid79]

Hi, would it be possible to have bitrate settings in the AAC export options? The quality slider is quite useless as I don't have a clue what the actual bitrate the file will be. I'm always shooting in the dark and hoping for the best when exporting to AAC. Bitrate settings exist for MP3 export but I don't understand why they don't for M4A.

Thanks!

[kozikowski]

Someone will correct me, but I don't think M4A has a fixed bitrate. One of the advances over MP3 was intelligent, recursive compression making it a complete crap shoot what the bitrate was going to be at the end. It depends a great deal on what the source material is.

Koz

[steve]

AAC encoding for Audacity exporting is provided by the optional FFMpeg library. The exact bit-rate and other encoder options are determined by FFMpeg (which may vary between FFMpeg versions). For more control over AAC encoding options you could try converting with QuickTime or iTunes.

[waxcylinder]

Personally I find it easier to export WAV files and then use iTunes to convert these to AAC for me - in iTunes there is a relatively straightforward dialog to control the settings:

itunes aac encoder dialog.png (10.14 KiB) Viewed 17165 times

I think it's actually quicker to make the AAC in iTunes than it is in Audacity - plus I then get the WAV files for archive backup.

Have a look at this workflow I wrote for the manual: http://manual.audacityteam.org/o/man/sa ... tunes.html

WC

[Gale Andrews]

AAC is almost never implemented as a constant bit rate (and it isn't constant bit rate in iTunes). I wrote a couple of responses in the past about this which may be useful:

http://forum.audacityteam.org/viewtopic ... 260#p53260

http://forum.audacityteam.org/viewtopic ... 23#p165223 .

Basically the maximum Audacity quality of 500 will be "not too dissimilar" to the maximum iTunes bit rate.

I can add a vote for lucid79 that the Audacity AAC encoder should be expressed in target bit rates, but since FFmpeg exposes a quality control and not bit rate targets, it would need a lot of thought and possibly be misleading.

It would not be directly comparable to the iTunes bit rates or indeed to other AAC encoders because slightly different algorithms seem to be used from one encoder to another.

All these encoders though have the same idea, that the bit rate is increased as the music becomes more complex to encode, and decreased as it becomes simpler, in order to keep the same level of quality within the bit rate or quality target chosen.


Gale

[kozikowski]

Which leads to production decisions in both audio and video: "We should not use that particular content because it won't compress well."
Koz

[sinrise]

I've been experimenting with it and I found a quality setting of 200 gets you close to the 224 standard found in iTunes AAC files. Since it's variable, it won't be exactly the same for each file, if you're looking at the file properties in Explorer or Finder. Anyway, it should get you upwards of 200, which is a good target for file size/quality to avoid diminishing returns.