HLG — Hybrid Log-Gamma — is the HDR format designed specifically for live broadcast. Developed jointly by the BBC and Japan's NHK in 2014, HLG solves a problem that the other HDR formats can't: how to broadcast HDR signals over channels that also have to feed standard-dynamic-range TVs without doing two separate encodes. The trick is in the curve itself, which is half gamma (the SDR-era standard) and half logarithmic (for the extra HDR brightness range).
The practical magic: the same HLG signal looks correct on both SDR and HDR TVs without any metadata or conversion. An old TV sees a normal SDR picture using the gamma portion of the curve; an HDR TV recognises HLG and uses the full curve to display the extended range. This is exactly what live broadcast needs, because broadcast streams are one-to-many and can't run different versions for each viewer. HLG also doesn't require per-scene metadata — important because metadata can get lost or corrupted in live transmission.
HLG is the HDR format of choice for BBC iPlayer, NHK, Freeview Play, DirecTV's live sports, YouTube HDR and most live HDR broadcasts worldwide. The trade-off compared to pq-based formats (HDR10, HDR10+, Dolby Vision): HLG is slightly less precise in extreme brightness control because it lacks per-scene metadata, so it's typically used for live and broadcast rather than premium VOD. For a streaming product, the practical division is clear: HLG for live HDR, PQ-based formats for VOD HDR. Many services support both, using whichever fits the content type.
