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Subject: Re: MPEG Audio format

Re: MPEG Audio format

From: Jens Arnold <arnold-j_at_t-online.de>
Date: 2006-01-15

On 15.01.2006, Jacob Erlbeck wrote:

> for so-called 'main data', but (now it's going voodoo) the
> main data that belongs to a given frame may begin up to 511
> bits of main data space (this is without headers and side
> info) before this frame. It's even possible that it starts in
> frame N-2 and/or that it ends in frame N-1 (given that we are
> looking for the main data that belongs to frame N). This is
> the 'bit reservoir' thing that is only being used for constant
> bitrate streams (it doesn't make much sense for VBR streams).

It can also be used for VBR, and it makes sense. MPEG audio
frames can only have certain fixed sizes, and the bit reservoir
can be used to further optimise the file size. Let's say we have
2 blocks of audio (2 frames) which would need 144 kbps frames
at the selected quality. 144 kbps frames don't exist, so the
first frame must be a 160 kbps one. Some bits of it are unused,
and they add to the bit reservoir, which can then be used for a
part of the data of the second frame. The rest then fits in a
128 kbps frame.

The MAS3587F does this, in rockbox it can optionally be
deactivated.
 

Regards, Jens
Received on Sun Jan 15 22:59:28 2006


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