Genlock

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Genlock (for Generator Lock) is a common technique where the video output of one source, or a specific reference signal, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a combining or mixing or switching point. When sources are synchronized in this way, they are said to be genlocked.

Where two or more video signals are combined or being switched between, the horizontal and vertical timing of the picture sources should be coincident with each other. If they are not, the picture will appear to jump when switching between the sources whilst the display device (e.g. TV Set) re-adjusts the horizontal and/or vertical scan to correctly reframe the image.

Where composite video is in use, the phase of the chrominance subcarrier of each source being combined or switched should also be coincident. This is to avoid changes in colour hue and/or saturation during a transition between sources.

Genlock can be used to synchronize as few as two isolated sources (e.g. a television camera and a videotape machine feeding a vision mixer), or in a wider facility where all the video sources are locked to a single synchronizing pulse generator.

In broadcast systems, an analogue genlock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of colour burst. No picture information is usually carried to avoid disturbing the timing signals, and the name reference or black and burst is usually given to such a signal. A composite colour video signal inherently carries the same reference signals and can be used as a genlock signal, albeit at the risk of being disturbed by out of specification picture signals.

Natlock refers to a picture source synchronizing system using audio tone signals to describe the timing discrepancies between composite video signals, whilst Icelock uses digital information conveyed in the vertical blanking interval of a composite video signal.