Sync Concepts

    Confused by Sync vs. Synchronization? You're not alone.

  INTRODUCTION

    The following tutorial tries to clear up some of the confusion that artists, producers, and installers have about the terms "Synchronized" and "Synchronization".

    There are two types of synchronization, and they use the same name to describe their process, so it is obvious why people are confused. One type is used in defining when a video image starts scanning an image at the top or left side of a monitor, projector, or camera. To be considered accurate, this type of synchronizing must be within about one pixel (about 70 nanoseconds). The other type is used to define the start of playback of several channels of video, usually on tape or disc, and is considered accurate if they start during the same video frame (1/30th second for NTSC).

  SYNC SIGNALS

    A video signal carries sync (or synchronization signals) with it. These signals contain at least a horizontal and a vertical sync signal, and for color images also contain color sync signals. These sync signals are used, for example, by a monitor to define where the scan lines go, or to decode the color portion of a video signal. They can also be used by other video devices to make sure that several images start their scan lines at the same time. These sync signals are generated by a device called a Sync Generator. It can be a stand-alone device or can be part of a video camera or other studio equipment.

    A "composite" color video signal (the normal single wire video signal) is a combination of the black and white visual information and color information, plus a complex sync signal. The sync part of the signal is actually a composite of several separate sync signals. Horizontal sync tells the video image when to start scanning from the left side of the image. Vertical sync tells the image when to start scanning from the top of the image. The combination of these two is known as composite sync.

    For color video, a composite video signal also contains a signal known as "Burst". This is made by combining 2 other sync signals, subcarrier and Burst Flag. Subcarrier is a sinewave oscillation that is used as part of the color encoding process. A small section of it (about 8 cycles) is inserted onto the composite video signal after horizontal sync but before the visual information starts. This little bust of subcarrier is used as a reference to recreate the subcarrier in the machine receiving the video signal.

    S-Video signals, like Hi-8 and S-VHS, keep the black and white information in a different signal than the color information. They contain the same sync signals as composite video but with the black and white sync (horizontal and vertical) on the black and white video signal, and the color sync (burst) on the color signal.

    Some video playback machines accept sync signals so that their video outputs are locked to those sync references. Some players will take composite sync , while other players will accept a "Black Burst" signal. Black burst is basically a video signal that has a solid black image, but contains all the sync and burst signals. The sync signals are fed into the players via connectors labeled "Composite Sync", "External Sync", or "Black Burst".

    It is very important to understand that just because several playback machines have their video outputs locked together by these sync references does not mean that segments of video played on them will begin playing at the same time.

    If you place two monitors next to each other that have their video signals locked to the same sync reference it is impossible for the naked eye to tell.

  SYNCHRONIZED PLAYBACK

    The other kind of synchronization takes place in a different scale of time. Instead of being calibrated in nanoseconds, it is calibrated in terms of frames of video. It usually involves computer commands that are sent to playback machines to start and control the playback of segments of video discs or tape. Rather than telling a video image to start it's scan lines at the same time, synchronized playback involves telling a player to start playing a video segment at a specific frame.

    If a video installation has more than one channel of video, and you want those channels to appear to start at the same time, this requires synchronized playback. The control signals used to create this type of synchronization come from a computer or a stand-alone "Synchronizer". These control signals are usually fed into the player via a serial port or some proprietary control port.

    When synchronized playback is not accurate, you can often see it with the naked eye. It depends somewhat on the content of the video and the placement of the images. If two images are projected with their edges touching, and objects move between the two, you can detect them being 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 tell. When the images are related, but not closely linked (for example a person on each image talking to each other), it gets harder to tell when they are off by a frame or two.

    Dave Jones Design's LasrPlay and DVDplay series are controllers for synchronized playback. They control laserdisc and DVD players so that multi-channel video installations start playing all channels at the same time. They also have optional sync generators in them that can generate Composite Sync or Black Burst for those times when you need sync and synchronized playback. Having these sync reference outputs fed to your players will increase the accuracy of synchronized playback by as much as one frame.