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

[Cyrus A]

Thanos Kyritsis wrote:
> 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 ?
>
>
>   
I'm seeing this exact same behavior trying to re-encode MPEG-2 files 
from MPEG-4 avis. I would assume there is definitely a bug here.

Try using an older version of ffmpeg. It worked for me. (How old, I'm 
not sure. Mine was REALLY old.)

Cyrus