[FFmpeg-devel] AC3 Format Detection unreliable.

[Michael Niedermayer]

Hi

On Sun, Aug 05, 2007 at 04:22:19AM +0200, Baptiste Coudurier wrote:
> Michael Niedermayer wrote:
> > Hi
> > 
> > On Sat, Aug 04, 2007 at 08:33:03PM -0400, Justin Ruggles wrote:
> >> Justin Ruggles wrote:
> >>> Dan Sully wrote:
> >>>> * Dan Sully shaped the electrons to say...
> >>>>
> >>>>>> Yes, please make the stream(s) available.
> >>>> Also, I am on an amd64 system.
> >>> Doesn't look like that's an issue.  I'm getting the same thing on my 
> >>> system.  The file is valid.  The issue seems to be the frame size. 
> >>> 640kbps 48kHz is the largest AC3 frame size, and I think it's not 
> >>> fitting enough frames into the probe buffer to give a positive 
> >>> detection.  I'll look into it.
> >> The problem with this sample is two-fold.  The first issue is that the 
> >> probe buffer in this case is only 2048 bytes.  The frame size for this 
> >> sample (and any others which are 640kbps & 48kHz) is 2560.  Therefore, 
> >> only 1 frame is detected.  I changed the threshold for a score of 50 
> >> down to 2 frames, but this doesn't help here.  After I changed to 1 
> >> frame (at the start of the stream) it now gets a score of 50, but then a 
> >> 2nd issue occurs.
> > 
> > so you havent fixed the issue but you sucessfully broke the whole probe
> > system in lavf, great
> > 
> > the ff_ac3_parse_header function checks only something like 18bits
> > a single match at the start IS NOT enough for a score of 50%
> > mpeg-ps checks for somthing like 60bits and does misdetect it so
> > how many misdetections do you think you added by decreasing the
> > bits for ac3 to 18 at the start or 36 at a random pos?
> > 
> 
> Indeed half of my pdf's are now detected as ac3, nonetheless if it's not
> ac3, it is mp3. Mp3 would need more accurate detection also ;)

^_^;;

actually ...
the problem with pdf and other random files is that the lavf probe code is
designed to find the most likely format, its not designed to identify
"unknown" files ...

maybe we should print a warning if the final score is very low like
<10 ?

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I am the wisest man alive, for I know one thing, and that is that I know
nothing. -- Socrates
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