[Ffmpeg-devel] invalid new backstep: mpegaudio decoding either faulty or seeking is broken

[Thanos Kyritsis]

Good day,

I was using a ffmpeg CVS snapshot from before you guys switched to SVN. 
These days I decided to update my snapshot to the latest SVN and I've 
been noticing some things:

There is a lot of:

backstep:457, lastbuf:332
invalid new backstep 300
backstep:455, lastbuf:332
invalid new backstep 300

output when seeking and decoding mpeg audio from avi streams.
I'm both using own code and the ffmpeg tool for these tasks.

So far, I've came up to three occasions:

1) My own code and ffmpeg tool sometimes fail when seeking and cutting:

ffmpeg -ss 00:00:40.332 -t 2 -i movie.avi -acodec pcm_s16le clip.wav

This fails. clip.wav does not contain the decoded audio from within 
movie's 00:00:40.332 - 00:00:42.332

2) There is another avi movie for which both my own code and ffmpeg tool 
fail to decode the 1st 20 seconds, no seeking here:

ffmpeg -i movie.avi -acodec pcm_s16le clip.wav

after this full decoding, movie.avi is Y duration and clip.wav is Y-20 
seconds duration. Those 1st 20 seconds produced a lot of "invalid new 
backstep" output.

ffplay does the same: the video/audio sync gets screwed, it plays the 
video from the beginning and the audio with a +20 seconds offset. The 
old code (just before you change to SVN) works like a charm (both 
ffplay and ffmpeg).

3) Sometimes my own code succeeds to seek and decode, while at the same 
time the ffmpeg tool fails.

I'm using a code structure similar (but much more minimal) to the ffmpeg 
tool structure in my own project to call av_seek_frame and do the 
av_decode_audio loops, but there are some differences on the way I skip 
some frames in order to get from the I frame to an exact P frame (after 
calling av_seek_frame()).


However, the common factor to all these is the "invalid new backstep" 
output. That wasn't happening some months before.

Do you feel there are currently known issues with mpeg audio decoding or 
seeking ? Or would you like me to prepare some media clips and upload 
them for you to test ?


-- 
Thanos Kyritsis <djart at linux.gr>

- What's your ONE purpose in life ?
- To explode, of course! ;-)