Synchronizing signal for HD equipment.
Similar to black burst in standard definition equipment, tri-level sync is the signal which provides heartbeat to high definition equipment. The purpose of the signal is to synchronize various pieces of video and audio equipment so they may work in concert with each other.
Tri-level sync is generated by a sync generator and distributed to various pieces of equipment throughout the facility. It is necessary to provide proper tri-level sync for each of HD standards. For example a single signal can not be fed to both equipment working in 1080 59.94i and 720 59.94p standards.
In certain situations it is possible to use the old fashioned black burst in place of tri-level sync.