# Tri-level sync

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{{Short description|Analogue video synchronization}}
{{one source|date=January 2015}}

thumb|An oscilloscope trace of a tri-level sync pulse

'''Tri-level sync''' is an analogue video synchronization pulse primarily used for the locking of [high-definition video](/source/high-definition_video) signals ([genlock](/source/genlock)).

It is preferred in HD environments over [black and burst](/source/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.<ref>[http://www.tek.com/dl/20W_18580_0_0.pdf Tektronix – Timing and Synchronization].</ref>

==Synchronization==
Modern real-time multi-source HD facilities have many pieces of equipment that all output HD-[SDI](/source/Serial_digital_interface) video. If this baseband video is to be mixed, switched or [luma key](/source/luma_key)ed with any other sources, then they will need to be [synchronous](/source/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](/source/Vision_mixer) 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](/source/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.<ref>Digital Video and HDTV: Algorithms and Interfaces – Charles Poynton – 2003 – {{ISBN|1-55860-792-7}}.</ref>

==References==
{{reflist}}

Category:Synchronization
Category:Film and video technology
Category:Broadcast engineering
Category:Television terminology

{{tv-tech-stub}}

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Adapted from the Wikipedia article [Tri-level sync](https://en.wikipedia.org/wiki/Tri-level_sync) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Tri-level_sync?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.