Saturday, January 16, 2010

best practices to alleviate audio sync problems in Cinelerra

Audio synchronization problems are the bane of video editors everywhere. Here's what I do in Cinelerra to try to prevent them.

The first thing to understand is that playback in Cinelerra is a separate process from rendering (exporting) your project to a final format. So it is a good idea to calibrate the audio-video synchronization of your files when they are played back in the Cinelerra compositor as well as the a/v sync of rendered output. More specifically:
1) You need to calibrate the audio you see being played back in the compositor. Do this by setting Audio Offset in the Preferences -> Playback -> Audio Out so that your playback audio is in sync with the video. I generally use ALSA as my audio driver under Preferences -> Playback. (You might also want to try to use OSS or ESound to see if those audio drivers help your playback sync issues.) For reference, my offset is .1 seconds.

2) You'll need to calibrate the audio and video once it is rendered out of Cinelerra. This doesn't have to be the absolute final format of the file. Just an intermediate file format to determine that audio is not drifting and in sync. You'll do this by setting Nudge on your audio tracks so that your rendered audio and video are in sync. Here's an article on nudge:

Let me give you an example.

My projects usually run about an hour on the timeline. When I start a project, I will concatenate a bunch of clips or raw videos on the timeline. I will then render a short segment of video, about 15 seconds long, from about 45 minutes into the timeline. The segment I take should have key features like someone speaking or someone hitting a drum that I can use to determine audio synchronization. I then render the file to a format that works well in the various Linux media players:
-Quicktime for Linux using MPEG4 video compression and MP4 or twos complement (pcm) audio compression
-Quicktime for Linux container using JPEG video compression and MP4 or twos complement

The side benefit of using MPEG4 video compression is that it renders damn fast.

The good thing about these steps is that if you work with the same format of video over and over, you won't need to change #1, the Audio Offset.

Here are notes regarding specific source formats and output formats.

After much study and pain, I've found that when working with MPEG-TS HDV files as my source (and MPEG formats in general), the best pipe to render from Cinelerra in order to alleviate sync headaches is mpeg2enc. I'm not five channel yet, so I just export my 48.1Khz stereo audio as MPEG Layer II audio MP3s @ 384kbps. I then use mplex to combine my audio and video streams into a program stream. Further on, I use VLC to convert that PS into MPEG-TS (HDV) and then use FFMPEG to convert the HDV into DVD and iPod/iPhone compatible formats.

If my source video format is Quicktime for Linux (like from a screen capture), then I have no sync problems rendering out to QT for Linux.

If I render my MPEG-TS, HDV projects using Quicktime for Linux as an output format (MPEG4 or JPEG video w/twos complement or MP4 audio compression), then I will get sync problems in the rendered output. I then follow the above best practices to alleviate the problem.

In Fedora, I've found that the best media players to test final output are mplayer, vlc and ffmpeg.

Finally, there are options to mplex and ffmpeg to offset audio and video sync problems:
mplex -O ; eg, mplex -O 350
ffmpeg -itsoffset ; eg, ffmpeg -itsoffset 0.35

Hope this helps,
the mule

My Article on Cinelerra's "nudge" parameter

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