Re: decoding, how many chunks of "main info"?

From: Torsten Mohr <[email protected]>
Subject: [mp3encoder] decoding, how many chunks of "main info"?
Date: Sun, 4 Feb 2007 18:08:18 +0100

> reading in the ISO 11172-3 i see on page 43, Figure A.7.b. that
> the MP3 stream consists of frames (of course) and that a part of
> the "main data" can be located in a previous frame.
> 
> That is ok so far, but in the example i see also that the "main data"
> can span _several_ previous frames.
> 
> Is there a limitation to the number of previous frames that i
> need to consider for the "main data" of a frame?
> 
> For example, i'm decoding frame 345, it is ok that i need to look
> back to frame 344 in some cases.  As i understand Figure A.7.b i
> also may need to consider frame 343.  How far can this reach back?
> 
> Is this something that is just "on paper" but does not happen in
> reality?

Why don't you test by yourself :p ?
It happens every mp3 files.

The max number of buffered frames is 255, IIRC. It can be determined
by some easy calculation. The blow comments are cited from
'libmp3lame/encoder.h' in the LAME source.


    /* mpeg1: buffer=511 bytes  smallest frame: 96-38(sideinfo)=58
     * max number of frames in reservoir:  8 
     * mpeg2: buffer=255 bytes.  smallest frame: 24-23bytes=1
     * with VBR, if you are encoding all silence, it is possible to
     * have 8kbs/24khz frames with 1byte of data each, which means we need
     * to buffer up to 255 headers! */
    /* also, max_header_buf has to be a power of two */


> How does this interfere with resyncing?  If i need to resync to a frame
> and i find a valid frame, but its "main data" starts in the 4th previous
> frame, then i have to skip this valid frame, is this correct?

Right. In the worst case, you need to skip 255 frames.


> Can anybody suggest me a pseudo-algorithm that considers both problems
> (resyncing & back-referring of "main data")?

Sorry, I've ever wrote an mp3 decoder and I cannot show you.
-- 
Takehiro TOMINAGA // may the source be with you!