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

[Andy Bircumshaw]

On 17/5/2011, at 5:21pm, 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'm not going to swear to it, but as long as the conversion is only taking a few seconds, then there's probably nothing to worry about. Hopefully some others will chime in, but conversion to MP3 would take a lot longer (probably several minutes on a current PC? Try it to find out). If you're not transcoding, then you're (hopefully!) not performing a lossy operation on the audio stream.

aB.