Tri level sync

An oscilloscope trace of a tri-level sync pulse.

Tri level sync is an analogue video sync pulse primarily used for the locking of HD video 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.^[1]

Synchronization

Modern real time multi source HD facilities will have many pieces of equipment which will 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. i.e. the first pixel of the first line will be transmitted at the same time (within a few micro secs). 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 synchronous sources, but these operate with a much bigger delay (~1 video frame).

Waveform

The main pulse definition is as follows, a negative going pulse of 300mv lasting 40 sample clocks followed by a positive going pulse of 300mv 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 74.25Mhz ^[2]

References