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

[bat guano]

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> From: christopher at custommade.org.uk
> To: get_iplayer at lists.infradead.org
> Subject: M4A files (from YAMB) = slightly smaller filesize than original AACs?
> Date: Tue, 17 May 2011 17:21:10 +0100
>
> 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?
>
>
> Also is there anything technically incorrect about creating audio-only MP4
> files as opposed to M4A? (I've noticed that YAMB is intelligent enough to
> 'do things' appropriate to creating M4A files if you specify the output file
> extension to be M4A instead of the default MP4, I'm not a big user of either
> MP4 or AAC though outside of the scope of archiving my get_iplayer downloads
> - predominantly x264 in MKVs for me).
>
> TIA (and yes, I did UTFG ;-)
> Chris
>

*** Also is there anything technically incorrect about creating audio-only MP4
files as opposed to M4A? ***

Hi
Nothing 'technically incorrect' about it.
The explanation is on Wikipedia.
Here:- http://en.wikipedia.org/wiki/Mp4#.MP4_versus_.M4A_filename_extensions

In Ubuntu all mp4 files are described as 'MPEG-4 video' in the file explorer with an icon of a cine film.
And m4a files are described as 'MPEG-4 audio' with a musical symbol for the icon.
>