Dear Elan Dev-Team,

We are a group of linguists working on a language documentation project and are experiencing problems with Elan 3.7.2 when playing back MPEG videos. Elan does not playback them in sync with the annotation of the utterances in Elan's timeline which we import into Elan from a Toolbox file. Elan moves its red cursor in the timeline a tiny bit slower than it plays the MPEG file. This tiny difference adds up so that it progressively becomes several seconds out of sync after about 30 minutes of the movie. The MPEG file does not need to be played back in Elan continuously to get out of sync, though. If we scroll the timeline to about 30 minutes and play from there, the playback starts already out of sync with the annotations. So, as you see, our problem is not the often discussed video vs the audio of the MPEG which is out of sync (we use just one MPEG file with no additional wav, anyway), rather it is the MPEG and utterance annotations.

I must emphasize that we do not transcribe or annotate the utterances in Elan. Our workflow is the other way round. Creating Elan files is the last what we do. We first create the Toolbox file in Toolbox and for each record we enter values of seconds in the fields \ElanBegin and \ElanEnd. When doing this, we enter the time values in the Toolbox file according to what Praat or Adobe Audition show us when we listen to the audio track (a 96 kHz 32 bit (no floating point) PCM WAV file). After that we sync our recorded video with this audio track in Adobe Premiere and from there we export the movie as an MPEG. In this MPEG the audio track is no longer a 96 kHz 32 bit PCM wave but it's compressed as MPEG-1 Audio Layer 2, 2 channels, 48 kHz, 224 kbit/s. (The video is MPEG2 PAL.) At this point our time alignment of utterances as it is entered in the Toolbox file matches perfectly the exported MPEG, no matter in which media player we play it. (e.g. when we see in a Toolbox record that a particular utterance begins at 3125 (seconds) we scroll our media player to 52 minutes 05 seconds and there comes the utterance.) Now comes the time when we start Elan, import our annotation from the Toolbox file and import this MPEG. And what we get in Elan is a scenario where the annotation and playback progressively get out of sync the way I described above.

In Elan, the Toolbox annotation and playback are every time out of sync. In small files, the difference is about 1 or 2 seconds (already annoying enough) but it gets really bad and most noticable in long files where there is enough time for the difference to add up. We even have a case where the imported utterance annotations in Elan's timeline start 1 or 2 seconds ahaead of the video and because the red cursor in the timeline moves slower than the video is played back they eventually get in sync with the video for a moment but progress to end up several second behind the video playback.

The times in Elan's timeline match exactly the times which we have entered in our Toolbox file and hence they also match the times at which the coresponding utterances can be heard when the MPEG is played in any normal media player. But when Elan plays the MPEG, the red cursor in Elan's timeline moves a tiny bit slower that it should and at a slightly different timecode position -- which makes the video and the annotations in Elan's timeline virtually out of sync.

Maybe I should add that we work in Windows XP SP 3 on Mobile AMD Sempron 3500+, 1.79 GHz, 1.37 GB of RAM.

If you don't know yet what could be the source of our problem, I can provide you our actual source data: an MPEG file, a Toolboxfile and the *.typ file of it and you can see how the annotation and video get out of sync for yourself. I thought maybe the actual problem is not with Elan at all but rather there is something wrong with the MPEG-1 Audio Layer 2 codec and I tried to install some new codecs for it but I don't think I have done well. However, nothing has changed thereafter. Elan is still getting off sync the same way as before.

We would appreciate very much if you would have a close look at this and assist us with the troubleshooting.

Thanks a lot

Hmmm, it would be interesting to test this on other machines, with other codecs. Maybe you could make one set of eaf+mpeg available for download somewhere?

What ELAN normally does is telling an underlying media framework to play a file. So, especially if you start the player and just let it play for an hour, the only action from the ELAN side is to tell the player (on Windows a DirectShow player) to start playing. From that moment on it is actually the "native" player that is doing its work. ELAN doesn't mess with the player's clock or playback speed. So, I have no explanation for any difference with e.g. the Windows Media Player.

- Han (han.sloetjes AT mpi.nl)