Tri-level sync is an analogue video synchronization pulse primarily used for the locking of

high-definition video High-definition video (HD video) is video of higher resolution and quality than standard-definition. While there is no standardized meaning for ''high-definition'', generally any video image with considerably more than 480 vertical scan lines (No ...

signals ( genlock). It is preferred in HD environments over black and burst, as timing jitter is reduced due to the nature of its higher frequency. It also benefits from having no DC content, as the pulses are in both polarities.

Synchronization

Modern real-time multi-source HD facilities have many pieces of equipment that all output HD- SDI video. If this baseband video is to be mixed, switched or

luma key Compositing is the process or technique of combining visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live-action shooting for compositing is variously ca ...

ed with any other sources, then they will need to be synchronous, i.e. the first pixel of the first line must be transmitted at the same time (within a few microseconds). This then allows the

switcher A switcher, shunter, yard pilot, switch engine, yard goat, or shifter is a small railroad locomotive used for manoeuvring railroad cars inside a rail yard in a process known as ''switching'' (US) or ''shunting'' (UK). Switchers are not inten ...

to cut, mix or key these sources together with a minimal amount of delay (~1 HD video line 1/(1125×25) seconds for 50i video). This synchronization is done by supplying each piece of equipment with either a tri-level sync, or black-and-burst input. There are video switchers that do not require synchronous sources, but these operate with a much bigger delay

Waveform

The main pulse definition is as follows: a negative-going pulse of 300 mV lasting 40 sample clocks followed by a positive-going pulse of 300 mV lasting 40 sample clocks. The allowed rise/fall time for each of the transitions is 4 sample clocks. This is with a clock rate of 74.25 MHz.Digital Video and HDTV: Algorithms and Interfaces – Charles Poynton – 2003 – .

References

Synchronization Film and video technology Broadcast engineering Television terminology {{tv-tech-stub