M4A files (from YAMB) = slightly smaller filesize than original AACs?

[dinkypumpkin]

On 17/05/2011 17:21, Christopher Woods (CustomMade) wrote:
> I've been taking some previously downloaded raw AAC files (downloaded with
> get_iplayer) and wrapping them as M4As using YAMB. For e.g.:
>
> Randomfile.m4a = 111 MB (116,501,077 bytes)
> Randomfile.aac = 111 MB (117,371,538 bytes)
>
> I've observed that consistently the resulting M4As are slightly smaller than
> the raw AACs. Is this to be expected? Is it something to do with the way the
> stream is muxed? I'm certainly not intentionally altering any aspect of the
> file during the M4A creation, so how come the M4As (which one would
> logically expect to be slightly larger given the wrapper overhead) end up
> being the opposite?

I've noticed this with MP4Box as well.  I think it filters out ADTS 
frame headers when adding the AAC audio as a track to the MP4 file, 
which would account for most of the difference.  I would guess YAMB does 
likewise.