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Resolving Sync Issues

Helpful tips aimed at resolving problems with synchronizing video, video playback and audio-video synchronization.

VIDEO SYNC

A video signal carries sync signals with it, which contain at least a horizontal and a vertical sync signal. Color images also contain color sync signals generated by builtin or standalone sync generators.

A “composite” video signal carries several separate sync signals. Horizontal sync tells the video image when to start scanning from the left side of the image, and vertical sync tells the image when to start scanning from the top of the image.

Some video playback machines accept sync signals that lock their outputs to those sync references. Some machines will take composite sync, while others will accept a “blackburst” video signal, which has a solid black image, but contains all of the sync and burst signals. The sync signals are fed into the players via connectors labeled “composite sync,” “external sync,” or “blackburst.”

However, although several playback machines have video outputs locked by these sync references, the segments of video played on them may not begin playing at precisely the same time.

SYNCHRONIZED PLAYBACK

Synchronized playback is calibrated in terms of video frames rather than in nanoseconds. It involves computer commands sent to playback machines to start and control the playback of segments of video content.

If a video installation has more than one channel of video, and you want those channels to start at the same time, it will require synchronized playback. The control signals come from a computer or a standalone “synchronizer”, and are fed into the player via a serial or proprietary control port.

When synchronized playback isn't accurate, you can see it with the naked eye. It depends somewhat on the content and placement of the images. If two images are displayed adjacent to one another, and objects move between the two, you can detect that they're off by as little as a single frame. Move the edges away from each other, and it becomes more difficult. Put them on opposite walls, and it's virtually impossible to detect.

AUDIO-VIDEO SYNC

Video system operators commonly experience audio-video synchronization at one time or another — especially with video that shows someone talking.

Why is sync lost?

Audio-video sync can be lost for several reasons:

  • The audio and video don't play back at the same rate because of computer system configuration problems.
  • The audio card clock and video card clock aren't synchronized.
  • The speed of the clock in the tape deck is unknown to the computer.
  • How to resolve audio-video sync issues
  • Optimize system configuration. Ensure that the computer system configuration is optimal. Synchronization will be lost if either the audio or video drive can't keep up with the playback of the media. Sync can also be lost if anything on the system interrupts it's performance.
  • Locked audio. Make sure the audio card can lock its clock to the video card's timing.
  • Compensating software. Special software is available to compensate for the audio-video synchronization offset.
  • Memory problems. Because a video editing system uses more RAM than most other applications, if a memory chip is bad, problems may only show up when using a video editor. If there are unrepeatable strange blue screens, crashes, or hangs, try changing memory chips.
  • Source: Dave Jones Design, NewTek



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