Tri Level Syncs

I get loads of people asking me about Tri-level syncs - they are becoming more significant in this HD-world and so here is a quick note - the link is a good explanation on the Extron website.
The thing to remember is that unlike HD-SDi video tri-syncs can be distributed much like normal black & burst - the same cables, jackfields, and even DAs and routers can be used to carry them. Equipment that uses tri-syncs will often need it's reference input switched (one day it's a 24P job, the next it's a 1080i (50 field) one) and so small composite routers and multi-o/p SPGs are a good idea.
The horizontal timing reference point for a standard bi-level sync signal is defined as the 50% point of the falling edge of the horizontal sync pulse. In order to ascertain this point precisely, it is necessary to clamp both black-level and sync-tip level and derive a value half way between. If the signal is in any way distorted, this will give H timing inaccuracy. With a relatively pedestrian standard def signal (line rate in the province of 15 Khz) this isn't such a big deal, but with 1080 lines per frame this becomes more significant and with a tri-level sync signal, the timing reference point is the rising edge of the sync signal as it passes through the black-level. This point is much easier to determine accurately, and can be implemented relatively easily. It is also more immune to signal distortion.