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As Genlock ( generator locking device ) the ability of video signal sources can be referred to can be synchronized in frequency and phase of the image change from the outside, so that for example uninterrupted two signal sources and interference mixed or may be switched between them. In studio environments, the synchronization is usually carried out by an externally generated studio clock, video signals from computers are synchronized with the external video signal to be processed.

A PLL circuit is often used for this purpose. From the mostly low-frequency clock signal (e.g. synchronous signal when watching television), this generates a higher-frequency signal (e.g. pixel clock), which is then used instead of its own free-running oscillator signal.

If the video signal at the output is still not clean enough, a Time Base Corrector (TBC) is often connected behind it to be on the safe side .

In EDP, genlocks often also contain small image mixers with which it is possible to combine the image generated by the computer with an image provided by an external source.

PC genlock

In the PC area , a genlock is used to synchronize the PC to a studio cycle. It should be noted that the entire video part of the PC is then synchronized to this external clock and is therefore dependent on its correctness. (In particular, precaution must be taken in the event that the external clock fails; then the PC should of course not stop.) At the same time (in the PAL range) the vertical refresh rate is set to 50 Hz in order to achieve a standard resolution of 720 × 576 points to enable smooth output of animation, video, ticker, etc. ( NTSC uses 720 × 480 dots at 59.94 Hz instead).

If the refresh rate were to continue to run at 60 or 70 Hz under PAL, for example, the animation would not be clean because the computer would then calculate the animation at 60 or 70 Hz. Inevitably, 10 or 20 frames are too many and the animation lags. This can also be observed with various scan converters that simply take a VGA signal with 60 or 70 Hz and convert it to 50 Hz: This works well with still images, but moving images jerk a lot.

With real genlock, the image is also not interpolated or resized. This fully maintains the quality of the PC image with 720 × 576 points.

In addition, genlocks often offer an overlay function to punch the PC graphics into a video signal using a key (mask, usually simply defined as transparent by choosing a certain color). This can e.g. B. for logo, ticker, PIP display and much more.

Amiga genlock

The Amiga computer from Commodore was the pioneer of this technology in the field of personal computers . Here not only the video part, but also the entire computer was synchronized to the external clock source, a tribute to the design of this platform, which was optimized for television resolutions. With this special feature, the Amiga became standard equipment in home and professional video studios for about 10 years. Some broadcasters used this technique for a long time to display their station logos in a corner of the current program.