What the heck is genlock … (and where can I buy one)?
I actually overheard that very question at a recent training session and thought that I might be able to shed some light on the mystery. For us propeller-heads in the video world, we’ve grown up with the knowledge of “genlock,” so it’s not a foreign concept to us. But for those in the lighting and projection realm who are now dealing with video equipment on an ever-increasing basis, the mysterious genlock connector might need some clarification.
You can find several definitions of genlock on the net, but quite often, an electrical engineering degree is required to understand ‘em. Here’s the setup. Your event is a simple three-camera shoot, with the switched output routed to both a projector and a recorder. You connect your cameras and character generator to the video switcher, the switcher’s outputs to each destination device, and then you begin cutting and mixing cameras. The result is jumps and glitches — with picture breakup occurring at each switch.
Why? Because the system isn’t “genlocked” — that is, all of the video devices are not locked to a common reference signal. When video devices are genlocked together, you can cut and mix signals without timing errors. Each cut will be a clean vertical interval switch, and all dissolves will be stable and smooth — both visually and electronically.
Every piece of video gear (cameras, switchers, media servers, character generators, etc.) has an internal sync generator that keeps the output video signal stable — both horizontally and vertically. We’ve all seen video images rolling around on screen, and this typically indicates that the device’s internal sync generator is toast. In each case, a standalone video device is synchronous to itself, but asynchronous with respect to other devices in a video system.
For example, even though two cameras are running perfectly at 59.94 Hz (the NTSC video frame rate), they’re still asynchronous to each other. If you put these cameras’ output signals together on a scope and look at the waveforms, each signal would be slowly moving with respect to the other. The signals would meet and cross — but never lock.
Here’s the reason that the picture jumps when asynchronous signals are connected to a switcher: Electronically, the video frames coming into the switcher are not all starting at the same time. If Camera One is on air and you cut to Camera Two, the switcher can’t make a clean, invisible vertical interval cut — because the cameras and switcher aren’t all locked together. The switcher has to wait a millisecond or two to lock to the next incoming frame, and you typically see half of the outgoing frame and half of the incoming frame. Glitch city!
Well, this is all well n’ good, but how do you fix it so that switches are clean? The answer is genlock (short for Generator Lock), a means of syn-chronizing multiple video devices to a common reference signal. In addition to coaxial cable, two key components are required: the genlock connector on each video device that you want to lock and an external gadget called a sync generator or black burst generator. The sync generator puts out a simple video signal called black burst (or “reference” video), which is simply a video signal that has sync, burst (subcarrier), but no luminance or chrominance values. Depending on their cost (and complexity), some sync generators also output a variety of other signals, including composite sync and test patterns. Some video devices will lock to black burst, while others will lock to composite sync. My own (extremely low-cost) sync generator has multiple black and composite sync outputs.
When black burst is connected to the genlock input of a camera, for instance, the camera disregards its internal sync generator and locks to the in-coming reference signal instead. Same deal with a media server, character generator or video switcher. When a black burst signal is connected, the internal sync is disregarded — and the reference signal is obeyed. By extension, when black burst is connected to all devices in your system (especially the switcher), each device now shares a common reference. The system is now officially genlocked. And because all signals share a common reference, switches are clean and dissolves are spotless. A good analogy would be to regard the genlock signal as the metronome or the “click track” for all video devices in your system.
All professional components that are required in a video switching system have genlock “input” connectors, and many have genlock “loop” con-nectors. This includes cameras, video tape recorders, video servers, and most important, the switcher itself. Most consumer and pro-sumer components do not have genlock inputs, and therefore, it’s not a good idea to connect DVD and VHS players directly to a video switcher — unless an intermediate genlockable device, such as a professional scan converter or time base corrector, is used.
There are typically two methods of distributing a reference signal to all of your gear, as shown in the diagrams. The first method (Fig. 1) is to “DA” the reference signal to all devices, using one or more distribution amplifiers. The second (and riskier) method (Fig.2) is to use the genlock loop connector on a device (provided it has one), to daisy-chain the black burst signal from one device to the next, and so on, until all devices are connected.
Whichever method is employed, genlock is an easy recipe for guaranteeing a stable, glitch-free switching environment, in which all video devices are synchronized to a common reference signal.
Paul Berliner is president of Berliner Productions in Davis, Calif. He can be reached at pberliner@plsn.com.
