Genlock (generator locking) is a common technique where the video output of one source (or a specific reference signal from a
signal generator
A signal generator is one of a class of electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typically used i ...
) is used to synchronize other picture sources together. The aim in video applications is to ensure the coincidence of signals in time at a combining or switching point. When video instruments are synchronized in this way, they are said to be ''generator-locked'', or ''genlocked''.
Possible problems
Video signals generated and output by generator-locked instruments are said to be
syntonized. Syntonized 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 often include a variable delay on each input to compensate for the phase differences and time all the input signals to precise phase coincidence.
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 re-adjusts the horizontal and/or vertical
scan
Scan may refer to:
Acronyms
* Schedules for Clinical Assessment in Neuropsychiatry (SCAN), a psychiatric diagnostic tool developed by WHO
* Shared Check Authorization Network (SCAN), a database of bad check writers and collection agency for ba ...
to correctly reframe the image.
Where composite video is in use, the
phase of the
chrominance
Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represen ...
subcarrier
A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...
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
Generator locking can be used to synchronize as few as two isolated sources (e.g., a
television camera
A professional video camera (often called a television camera even though its use has spread beyond television) is a high-end device for creating electronic moving images (as opposed to a movie camera, that earlier recorded the images on film). ...
and a
videotape
Videotape is magnetic tape used for storing video and usually sound in addition. Information stored can be in the form of either an analog or digital signal. Videotape is used in both video tape recorders (VTRs) and, more commonly, videoca ...
machine feeding a
vision mixer (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). Generator locking can also be used to ensure that multiple CRT monitors that appear in a movie are flicker-free. Generator locking is also used to synchronize two cameras for
Stereoscopic 3D
Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...
video recording.
In broadcast systems, an analog generator-lock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of
colorburst
Colorburst is an analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal. By synchronizing an oscillator with the colorburst at the back p ...
. 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 generator-locking 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 generator-locking 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 sync pulse is a signal that initially goes from 0 volts DC to a negative voltage, then a positive voltage, before returning to zero volts DC again. The voltage excursions are typically 300 mV 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
A television studio, also called a television production studio, is an installation room in which video productions take place, either for the production of live television and its recording onto video tape or other media such as SSDs, or for ...
and
professional video camera
A professional video camera (often called a television camera even though its use has spread beyond television) is a high-end device for creating electronic moving images (as opposed to a movie camera, that earlier recorded the images on film). ...
s have dedicated generator-locking ports on the camera. If the camera is tethered with a
triaxial cable or optical fibre cable, the analog generator-locking 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/fibre connection or without a dockable head, the generator-locking signal is carried through a separate cable from the video.
Variants
''Natlock'' is a picture-source synchronizing system using audio tone signals to describe the timing discrepancies between composite video signals, while ''Icelock'' uses digital information conveyed in the
vertical blanking interval
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...
of a composite video signal.
See also
*
Frame synchronizer (video)
Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media.
Without time base correction, a signal from a videotape recorder (VTR) or videocassett ...
*
Time base corrector
Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media.
Without time base correction, a signal from a videotape recorder (VTR) or videocassett ...
References
{{refs
Synchronization
Film and video technology
Broadcast engineering
Television terminology