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:
http://crazedmuleproductions.blogspot.com/2007/05/nudge-avidemux2-and-reminder-about.html

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.

MPEG SOURCE FILES to MPEG RENDERED OUTPUT
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.

QUICKTIME (QT) SOURCE FILES to QT RENDERED OUTPUT
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.

MPEG SOURCE FILES to QT RENDERED OUTPUT
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.

TEST YOUR OUTPUT
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

Reference
My Article on Cinelerra's "nudge" parameter
FFMPEG HowTo
http://man-wiki.net/index.php/1:mplex

Cinelerra on FC6 working well and more YouTube-ness

I spent all day yesterday editing and rendering video using the Cinelerra install on my Fedora Core 6 box. I am happy to report that after about 10 hours, 15 projects and two or three times as many renders, Cinelerra did not crash once! Now, the types of activities I performed did not vary that much:
- open hour long DVD resolution video
- select five or ten minute long sections of video
- compensate for audio sync problems using nudge
- render out to YouTube compatible format (ala my previous post)

My previous Cinelerra install on Core 4 had crash problems as well as strange audio synchronization problems. One major difference between the systems is that my FC4 Cinelerra system had OpenGL enabled. My new FC6 system does not have OpenGL enabled. On FC4, I noticed that enabling OpenGL seemed to cause the instabilities mentioned above, though I was never able to prove it. Also, the more unstable projects tended to be projects that I had previously created. So as a test, I am recompiling Cinelerra on FC6 using OpenGL and will perform some edits on both old and newly created projects to see if the OpenGL version of Cinelerra is as stable as the non-OpenGL, X11 XV version on FC6. I am hoping that the latest NVidia drivers for my 7600GS 512MB card in combination with various bug fixes in FC6 will allow me to run Cinelerra on FC6 without crashes.

I have uploaded the results of yesterday's rendering session to my new YouTube account here:
http://www.youtube.com/profile?user=snufflerstormpig

I'm learning more about YouTube. It is a really rich environment for social interaction. Everybody has seen a YouTube video and peoples comments beneath it. But depending on how the content producer has configured the video, people can comment upon it by using a video response. Pretty cool. Also, most people know that you can embed a YouTube video into any web page simply by selecting the Embed code that appears on every page that displays a YouTube video and integrate that code into your web site. It looks like this:
<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/cW8EL2RgAEg"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/cW8EL2RgAEg" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed></object>

Some of the more interesting stuff is not as evident. For example, if you are a content producer, you can create a "Channel", basically a home page of your profile like in the above link so that people can see your accumulated work. As a member (content producer or regular viewer), you can create playlists of your favorite videos and share them with your friends. When someone gets one of your playlists, they can play all the videos in that playlist simply by clicking once. Here is a playlist I created from the videos I rendered yesterday:
http://www.youtube.com/view_play_list?p=021BB1E61A3BFD19

Also, people can subscribe to your Channel (Subscribers) or you can make Friends who also like your videos. The difference between the two seems to be that Subscribers are more passive viewers of your content and Friends are content producers themselves, or at least have a more well defined profile created on YouTube. YouTube can automatically alert both of these two different types of people when you've uploaded new videos.

Anyway, granted I am a little late to the YouTube game, but I think a lot of other content producers are too. They should take advantage of this great experiment in social networking and get their hard work up on YouTube to show the rest of the world.

audio sync, nudge, avidemux2 and a reminder about archiving

I spent most of the night trying to debug an audio synchronization problem in Cinelerra. The video was trailing the audio by a little bit. So I used the Nudge feature to slide the audio .5 seconds behind to the right.

A word about nudge:
If the audio is running behind the video, use POSITIVE values for nudge.
If the video is running behind the audio, use NEGATIVE values for nudge.

Another way to put it:
If your audio is lagging and is behind the video (more to the RIGHT on the timeline than the video), push it FORWARD using positive values.
If your audio is running too fast and is ahead of the video (more to the LEFT on the timeline than the video), push it BACKWARD using negative values.

2/17/2008
Since nudge is good for fixing an entire track, one way to fix synchronization problems that occur on only part of a track is to use the Delay Video video effect or the Delay Audio audio effect:

• The Delay Video track works when audio lags behind the video.
• The Delay Audio track works when the video lags behind the audio.

The two effects take positive values for the delay in seconds. So they only work "one way", so to speak.
:)

This syncronization problem was after another discovery that Cinelerra on the Fedora Core 6 box I just built is not letting me drag and drop video. So I couldn't manually slide my video around. I'm not sure why this is not working for this particular project, as the Record button is enabled and my editing Preferences seem correct. The funny thing is that it does not happen when I create a new project. My gut feeling is that this version of Cinelerra I am using is SLIGHTLY newer than the one on my main box. So the XML is probably screwed up in some way. Ugh. Tiring.

But not tired enough to write a reminder to everybody that if they want the MOST PORTABLE video files, keep them below 4GB. That way they will fit on a DVD for archiving or transferring, fit on a FAT32 file system and be welcomed on any modern file system. Any larger, and it becomes a nightmare to transfer or archive. As I edit in HDV, file sizes get huge pretty quickly. But keep 'em below 4GB kids! It'll save you much pain in the long run.

Your friend in the battle to trim files down to size is AVIDEMUX2, a wonderful little program I've mentioned before that chops up large video files in no time flat. On my system, a Dell 400SC, 3.2Ghz, 2GB RAM, multiple drives, it took about four minutes total to chop up a 5GB HDV file into two 2.5 chunks. Not bad! One thing I do is to make sure that when you are saving out your file, use Copy for both the Audio and Video tracks and then save to your final format. The Copy feature will save out a new file to the exact same specs as your source file. Very nice feature!