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 Genlock (generator locking) is a common technique where the video output of one source (or a specific reference signal from a signal generator) is used to synchronize other picture sources together. The aim in video applications is to ensure the ...

). It is preferred in HD environments over

black and burst Black and burst, also known as bi-level sync and black burst, is an analogue signal used in broadcasting. It is a composite video signal with a black picture. It is a reference signal used to synchronise video equipment, in order to have them ...

, 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 keyed with any other sources, then they will need to be

synchronous Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...

, 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 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

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