Genlock

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 [ [http://www.mivs.com/technical/appnotes/an005.html MicroImage Video Systems - An Overview of Genlock] ] [ [http://www.nvidia.com/object/IO_10793.html NVIDIA - Genlock] ] [ [http://www.atis.org/tg2k/_genlock.html ATIS Committee - genlock] ] .

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, also known as a production switcher in North America), or in a wider facility where all the video sources are locked to a single synchronizing pulse generator (e.g. a fast paced sporting event featuring multiple cameras and recording devices).

Genlock can also be used to ensure that multiple CRT monitors that appear in a movie are flicker-free, as was done in "Into The Void" for example [ [http://www.iconbar.com/gallery/into_the_void/ Icon Bar - Into The Void Photographs] ] .

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", "black and burst", "color black", or "black 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. Although some High Definition broadcast systems may use a standard definition reference signal as a genlock reference signal, the use of tri-level synchronising pulses directly related to the frame and line rate is increasing within HD systems. A tri-level synchronising pulse is a signal that initially goes from zero volts DC to a negative voltage, then a positive voltage, before returning to zero volts DC again. The voltage excursions are typically 300mV either side of zero volts, and the duration each of the two excursions is the same as a particular number of digital picture samples.

Most studio and broadcast cameras have dedicated genlock ports on the camera. If the camera is tethered with a triax cable, the analogue genlock signal is used to lock the Camera Control Unit which in turn locks the camera head by means of information carried within a data channel transmitted along the cable. If the camera is an ENG type camera, one without a triax connection or without a dockable head, the genlock signal is carried through a separate cable from the video.

"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.

References