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

[dinkypumpkin]

On 17/05/2011 19:01, Christopher Woods (CustomMade) wrote:
>> 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.
>
> It seems to be non-destructive though; when re-exporting the raw streams out
> of the MP4 file the resulting filesize is identical to the original file. I
> *haven't* checksummed them though (just thought about doing it) - will run
> QuickSFV when I get home and report the results, I'm about to leave the
> office.
>

Yep - I should pointed that out.  The ADTS frame headers are there for 
use in streaming, but when they get stripped you still have the raw AAC 
audio intact inside the MP4 file - it doesn't get changed in any way, at 
least not by MP4Box.  When I first noticed this, I checksummed ADTS AAC 
audio going in and out of MP4Box, and it's OK.