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Subject: Re: Iffy MP3 file

Re: Iffy MP3 file

From: Linus Nielsen Feltzing <linus_at_haxx.se>
Date: 2005-08-08

Geoff Smith wrote:
> I was wondering if anyone could tell me what's wrong with a specific MP3
> file (available here http://www.fogcat.co.uk/rockbox2/).
>
> It plays file in the iriver H149 firmware and plays fine in WinAmp - but
> in the RockBox firmware it plays much to slow and looking at the tag info
> in RockBox it reports:
> MP1 32000Hz 384k
>
> The iriver and WinAmp report it as:
> MP3 44kHz 192k
>
> The track was ripped with DB PowerAmp's Music Converter and other tracks
> ripped at the same time from the same CD play OK.

There are 22 bytes of garbage right after the ID3V2 tag before the MP3
stream starts, in which the Rockbox metadata parser finds a valid MP3
frame header. The ID3 tag ends at 0x38ca, where the garbage starts,
beginning with the data 80e3:

000038c0: 0000 0000 0000 0000 0000 80e3 4cdd d66e ............L..n
000038d0: 2e34 8d01 bc63 364f 9a14 ffff ffff ffff .4...c6O........
000038e0: ffca e277 2d9a 0b9c 83b9 3847 9071 9820 ...w-.....8G.q.

The real MP3 frame starts at 0x38e0, but rockbox finds an MP1 header at
0x38df: 0xffffcae2. This header says that the sample rate is 32000Hz,
and Rockbox sets the resampling rate accordingly. The MAD decoder will
of course detect the error and synchronize on the correct stream, but
the resampler is not updated.

Linus
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Received on Mon Aug 8 14:58:09 2005


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