Tri-level sync is becoming a required part of HD system timing.
One reason is that tri-level sync can be created to exactly match any of the standard formats. Black burst only comes in two flavors 29.97 fps (525 line) and 25 fps (625 line).
Another reason is that black burst (gen-lock) is measured at the halfway point of the leading (falling) edge of the pulse. Tri-Level sync uses the halfway point of the trailing (rising) edge of the pulse. These points are used to time the digital video. They are determined by means of a sync separator and voltage comparator.