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What mp3 quality do you want?

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Clitheroe KidJul 22, 2009
 

 
The extension commonly used by iTunes files is .m4a (which stands for mp4
audio). This is simply an mp4 file which contains only an audio stream. MP4
is a video format, so usually has both a video stream and an audio stream.

The program ffmpeg has an option to save a raw .AAC file as an MP4 file
(with the extension .mp4 or .m4a), without re-encoding the file.

An audio-only MPEG-4 file can be named either .mp4 or .m4a since those types
of file are both encoded in exactly the same way, but the latter is only
encoding audio (instead of audio and video).

AAC is a type of MPEG-4 file. But a raw AAC file won't play in iTunes merely
by changing the file's extension from .aac to .mp4, because you have to do
more to the file than that! It has to be "muxed" in order to become an mp4
file. That doesn't involve re-encoding it! It has to do with creating a
frame header for each frame, one that iTunes can read.

MP4 is a video format, so it uses frames (normally 25 frames per second, the
standard video format). Each frame has a header (a few bytes, describing the
audio format of the data stream). FFMPEG can add those headers to the raw
AAC data, to create a fully valid MP4 file.

Almost *any* player can play an mp4 file (though iTunes might need the
file's extension to be renamed to .m4a).

Stephen

----- Original Message -----
From: "eriatarkae@..." <eriatarka1@...>
To: <just-a-minute@...>
Sent: Wednesday, July 22, 2009 1:56 PM
Subject: [just-a-minute] Re: What mp3 quality do you want?


> There are a few caveats to this. Firstly, the raw .aac files don't appear
to play in iTunes (Quicktime only), and changing the extension doesn't help
either. This is a huge problem for me, as iTunes is my central storage for
all things JAM. Secondly, I'm fairly sure that .aac is not a suitable format
for most people's usual use patterns - most mp3 players can't play it, and
car stereos don't usually play it either.
>
> As far as utilities go, they are not freeware. There's Quicktime Pro or,
for Macs, Fission. Quicktime Pro is useful, because if you trim to the
selection (which is not immediately obvious, but there's a selection ruler
across the bottom) and save as an .mp4, then that will play in iTunes.
Fission - I have no idea, I'm on XP.
>
> Robert

 
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