Genlock
Encyclopedia
Genlock is a common technique where the video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 output of one source, or a specific reference signal from a signal generator
Signal generator
Signal generators, also known variously as function generators, RF and microwave signal generators, pitch generators, arbitrary waveform generators, digital pattern generators or frequency generators are electronic devices that generate repeating or non-repeating electronic signals...

, is used to synchronize
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

 other television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 picture sources together. The aim in video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

 and digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...

 applications is to ensure the coincidence of signals in time at a combining or mixing or switching point. When video instruments are synchronized in this way, they are said to be genlocked.

Phase differences

When two video signals are generated or output by genlocked instruments the signals are said to be synchronized or synchronous. Synchronized video signals will be precisely frequency locked but because of delays caused by the unequal transmission path lengths the synchronized signals will exhibit differing phases at various points in the television system. Modern video equipment such as production switchers that have multiple video inputs will often include a variable delay on each input to compensate for the phase differences and time all the input signals to precise phase coincidence.

Jumping

Where two or more video signals are combined or being switched between, the horizontal
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...

 and vertical
Vertical direction
In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i.e., with the direction of the gravitational force at that point...

 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 (television set) re-adjusts the horizontal and/or vertical scan
Scan
Scan may refer to:Acronyms:*Schedules for Clinical Assessment in Neuropsychiatry , a psychiatric diagnostic tool developed by WHO*Shared Check Authorization Network , a database of bad check writers and collection agency for bad checks...

 to correctly reframe the image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...

.

Colour changes

Where composite video is in use, the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

 of the chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...

 subcarrier
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...

 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.

Scope

Genlock can be used to synchronize as few as two isolated sources (e.g. a television camera and a videotape
Videotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...

 machine feeding a vision mixer
Vision mixer
A vision mixer is a device used to select between several different video sources and in some cases Compositing video sources together to create special effects...

 (production switcher)), 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. Genlock is also used to synchronize two cameras for Stereoscopic 3D
Stereoscopy
Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth...

 video recording.

Broadcast systems

In broadcast systems, an analog genlock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of colorburst
Colorburst
Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...

. 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.

High-definition

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.

Connections

Most television studio
Television studio
A television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...

 and professional video camera
Professional video camera
A professional video camera is a high-end device for creating electronic moving images...

s have dedicated genlock ports on the camera. If the camera is tethered with a triax
Triaxial cable
Triaxial Cable, often referred to as triax for short, is a type of electrical cable similar to coaxial cable, but with the addition of an extra layer of insulation and a second conducting sheath. It provides greater bandwidth and rejection of interference than coax, but is more expensive...

 cable, the analog 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
Electronic news gathering
ENG is a broadcasting industry acronym which stands for electronic news gathering. It can mean anything from a lone broadcast journalist reporter taking a single professional video camera out to shoot a story, to an entire television crew taking a production truck or satellite truck on location...

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 and Icelock

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.
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