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List:       rockbox-dev
Subject:    Re: MPEG Audio format
From:       Jacob <jacob01 () gmx ! net>
Date:       2006-01-15 23:02:15
Message-ID: 20060115230215.GB18494 () localhost
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On Sun, Jan 15, 2006 at 10:51:56PM +0100, Jens Arnold wrote:
> On 15.01.2006, Jacob Erlbeck wrote:
> 
> > 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.

Thank you for correcting that nonsense above I wrote.

Jacob

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