Tri-Level Sync

Tri-level sync is the analogue reference signal used to genlock high-definition video equipment. Each sync pulse goes from 0 to −300 mV, then to +300 mV, then back to 0, and the timing point is the zero crossing in the middle of the pulse, which does not depend on signal amplitude. The signal has no DC component and is defined with the HD formats in SMPTE ST 274 (1080 lines) and ST 296 (720 lines). It replaces black burst, the SD reference, which gives more timing jitter in HD, although many HD devices still accept black burst. It is distributed from a sync pulse generator to cameras, switchers and capture devices, and its format has to match the video format (1080i50 sync for a 1080i50 show).