A timing reference signal developed for HD. Typically this is originated by an sync-pulse generator and is distributed to most of the technical equipment, including cameras, mixer/switcher, and video processing equipment in a studio or truck so they all operate in sync.
With the opportunity to devise a new timing signal, the TLS signal combines a negative and positive pulse that is symmetrically above and below a baseline – nominally at zero volts. This means that accurate timing can be extracted from it even when the baseline voltage drifts.
In the early days of HD, the new equipment offered TLS and black and burst inputs for synchronization, but it was soon found that the old analog black and burst was preferred, already available and actually offered more accurate timing for 25p, 30p, 50i and 60i (but not suitable for other frame rates).