An Overview of Genlock
? J. Kovacs, Oct 01, 2001
Genlock is a system which allows the synchronization of two or more video sources, such as cameras. Without this synchronization, switching between sources will result in a momentary loss of image stability while the monitor or other equipment tries to lock itself to the new signal. A familiar example of this is the momentary breakup of a TV image when switching from one station to another, as opposed to the smooth transitions between scenes on a TV show.
In cases where more than one source needs to be displayed within the same picture, the monitor or other device may be forced to switch back and forth from one to the other up to several thousand times per second. For video faders, the equipment is forced to resolve sync and color timing between two conflicting sets of signals. Without synchronization between all sources, the images may roll either vertically or horizontally, break up completely, or at a minimum suffer extreme color shifts.
Genlock performs four main functions: vertical, horizontal, frame, and color synchronization. These features are all normal parts of a standard composite video signal, which allows a TV or monitor to display an image properly. A fifth function, field 1 reference, is used mainly in broadcast video. A standardized signal which includes all of these, without any actual video image, is known as black burst. In the absence of a source of true black burst, many genlockable cameras will accept a standard composite video signal instead.
The H and V synchronization pulses on a black burst signal are needed so that all of the video sources begin at the top left corner of the video screen at the same time. As all of the video signals draw the video field in unison, any switch between signals simply picks up the new source at the same point on the same scan line as where the old one left off, without any visible interruption. Many B&W, and a few color cameras are designed to accept separate horizontal drive (HD) and vertical drive (VD) signals rather than a composite genlock signal such as black burst.
A standard NTSC video signal is divided into odd and even fields every 1/60th of a second, with the second field interlaced or vertically offset between the scan lines of the first, to produce a complete image every 1/30th of a second. The field ID tells the monitor, camera, or other video device whether the current field contains the odd or even scan lines. Most systems determine field ID directly from the H sync and V sync signal timing.
Color is determined by comparing the phase of high-frequency chroma information within the video signal against a known standard reference subcarrier wave, with the varying degrees of phase difference representing the colors of the spectrum. This standard is received as a short sinusoidal “color burst” at the beginning of each video scan line, and is held and regenerated for the duration of the entire line. If the subcarrier frequencies of two video sources do not match, the difference in frequency will appear as a continuous shift in phase, making one source appear to drift through “psychedelic” color changes.
Even if the frequency of the burst is identical between two cameras or signal sources, this is still no guarantee that they are in phase, or at the same point of the subcarrier wave as each other. For example, one may be at the high point of the wave while another is at the midpoint between the peak and valley. This would represent a 90 degree phase shift. Switching between the first video signal and the second would result in the colors of the new signal appearing completely wrong. A shift of only a few degrees is noticeable, while 60 degrees is enough to make all yellows appear as red, all reds appear violet, etc. This phase shift may also be affected by other factors, including delays caused by the cable lengths of both the video signals and the genlock signal. Standard video cable causes about 2 degrees of phase shift per foot. Most genlockable cameras have a subcarrier (SC) phase control to compensate for differences between sources. More primitive compensation methods include trimming cable lengths.
In order to genlock two video sources, at least ONE must have a genlock input. The OTHER signal source may be used as the master, from which the sync signal must be derived. Alternatively, a variety of sync generators are available which either produce black burst from an incoming video signal or generate their own internal black burst references for all connected genlockable cameras in the system. Some of these have independent phase adjustments for each black burst output.
A number of cameras have been produced without an SC phase adjustment, and may lock at random to any one of four color phases each time they are powered up. This makes any reliable form of color lock or phase compensation between cameras impossible.