[mad-user] Bad sync frame detection

[Hugues Lismonde]

Hi,

I've got a little problem with libmad. On some mp3 I have, libmad find
a first frame somewhere it probably shouldn't.
The problem appeared with mythtv where some files were played in mono
instead of stereo and at half the speed.

I made some tests with madlld and I get some strange output like this:
[...]
madlld: recoverable frame level error (forbidden bitrate value)
madlld: recoverable frame level error (forbidden bitrate value)
madlld: recoverable frame level error (forbidden bit allocation value)
madlld: recoverable frame level error (forbidden bit allocation value)
madlld: recoverable frame level error (forbidden bit allocation value)
madlld: 25000 kb/s audio MPEG layer I stream without CRC, single
channel with no emphasis at 44100 Hz sample rate
madlld: recoverable frame level error (lost synchronization)
madlld: recoverable frame level error (reserved header layer value)
[...]
I understand that "lost sync" can occur (and it does on good files too
so I guess it's ok) but I can't find 25000 kb/s in bitrate_table
(frame.c) so I think the "forbidden bit allocation value" errors
offsets something by less than a byte.
I wrote a small test program which search for sync word and try to
decode the frame header without using libmad. Using it I get the
following output:

Frame at 504 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 508 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 512 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 516 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 520 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 532 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 536 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 540 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 544 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 548 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 560 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 564 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 568 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 572 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 576 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 588 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 592 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 596 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 600 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 604 - MPEG v1 Layer I No CRC Bad frame! (bitrate)
Frame at 48330 - MPEG v1 Layer III No CRC Bitrate 192 Samplerate 44
Padding 1 Stereo No emphasis
[...]

It was written fast so it might be wrong, but it seems correct (the
last line is consistent with iTunes or mp3blaster).
I haven't found how to get the current location in the buffer from
libmad so I can't tell where it get this strange frame.
I'll be glad to give any precision or do any tests one might want but
I'm a bit lost right now.