HDR HLG Explained: How One Signal Serves HDR and SDR TVs

Hybrid Log-Gamma puts the compatibility trick inside its transfer function, not in instructions attached to each scene. The lower portion resembles the gamma curve used by SDR systems; higher up, a logarithmic curve squeezes a much wider brightness range into the signal.

That split does the work. Dark and middle-bright picture values remain where established broadcast equipment broadly expects them, while the logarithmic region makes room for sunlight, lamps, reflections and other intense highlights that SDR would clip or compress.

The name is literal: hybrid gamma and log. It also explains why an HLG program has no fixed mastering peak that every display must reproduce. A compatible television interprets the scene-referred values, considers its own peak brightness and viewing environment, then maps the result to the panel using behavior specified in ITU-R BT.2100↗, which defines both HLG and PQ systems for high dynamic range television.

There is still signalling. The television must learn that the incoming video uses HLG, whether that identifier arrives through a broadcast service, HDMI or a video container. This is not dynamic metadata directing brightness scene-by-scene. If the identifier disappears somewhere between a receiver and the screen, the TV may apply an SDR or PQ curve instead, and the picture can turn flat, dim or plainly wrong.

Many SDR televisions can treat an HLG signal much like ordinary gamma video and produce a watchable picture. An HLG-compatible display recognizes the format, applies its HDR processing and opens the upper signal region into brighter highlights.

Watchable is the key word. It does not mean identical.

An older SDR set ignores the intended HDR rendering, so highlight detail may look compressed, and color can shift if the receiver mishandles BT.2020 signalling. Broadcasters therefore monitor the standard dynamic range result during production rather than assuming the mathematical compatibility curve will rescue every old television, converter and set-top box in the chain.

A dedicated HLG-to-SDR conversion remains safer when the operator controls a separate SDR service. It can tone-map the extended brightness, reshape contrast and convert brightness and color deliberately. That costs equipment and attention. For live broadcast, one camera grade, one vision mixer output and one distribution feed are considerably easier to keep synchronized than parallel HDR and SDR productions.

The honest shorthand is that HLG is displayable on many SDR sets. It does not turn them into HDR displays. Deeper blacks still depend mainly on the panel, local dimming and room light; HLG's distinctive contribution is extra headroom above diffuse white for higher-brightness objects.

HLG represents relative scene light and leaves the final rendering to the television, while HDR10 uses PQ to encode absolute display brightness. HDR10 also commonly includes static mastering metadata; HLG needs no conventional mastering metadata to establish its brightness scale.

PQ specifications can represent brightness levels as high as 10,000 cd/m². Consumer displays do not reach that figure, and most programs are mastered for a much lower peak, so the set uses tone mapping whenever encoded highlights exceed what its panel can show. HDR10's static metadata describes the program or mastering display as a whole. Dolby Vision and HDR10+ can change their guidance scene-by-scene or frame-by-frame through dynamic metadata.

The failure modes differ sharply. If HDR10 loses its format signalling and a television treats PQ values as SDR gamma, the image looks severely washed out. HLG usually degrades more gracefully on suitable SDR equipment. That is the broadcast bargain.

For streaming and movie delivery, the tighter brightness intent of PQ can matter more than backward compatibility. The HDR10 and Dolby Vision comparison covers the static-versus-dynamic metadata choice in more detail.

A correctly detected HLG program should keep ordinary midtones sensible while allowing brighter highlights and more highlight detail than its SDR presentation. How striking that looks depends on the display's peak brightness, local dimming, ambient-light processing and the grade itself.

Every frame should not blaze. Good HDR leaves faces, grass and diffuse white at plausible levels, then spends its extended brightness range on a stadium lamp, polished metal, clouds or a flash of sun on water. Push the whole picture toward the panel's maximum and the viewing experience becomes tiring while the intended contrast range collapses.

Picture quality can still lose to SDR. Usually, the chain is at fault:

• A receiver reports HLG support, but the selected HDMI input is not using its enhanced-bandwidth mode.
• A set-top box performs a poor SDR conversion, then the television processes that converted image again.
• The TV forces an aggressive HDR preset that crushes blacks or clips high brightness detail.
• A broadcaster grades conservatively because difficult legacy receivers must remain usable.
• The panel accepts HLG yet lacks enough sustained brightness or local contrast to make the difference convincing.

Check the television's signal-information page instead of trusting a generic HDR badge inside an app. If the set explicitly identifies HLG but the image remains dull, disable energy saving first, because those controls can cap panel output independently of the HDR format and make correct tone mapping look broken.

HLG is a high dynamic range transfer system, not a resolution, so it can accompany 4K, HD or another raster size. Broadcasters choose it because a live production can supply HLG-capable screens and many existing SDR receivers from one feed, without authoring brightness metadata after every cut.

Resolution, frame rate, codec, bit depth and dynamic range are separate specifications. A 3840×2160 HLG channel is both 4K UHD and HDR. A 1920×1080 HLG production is HD HDR. Simple enough.

HEVC frequently carries HLG broadcasts because it can distribute UHD video at a practical bitrate, but HLG is not a codec. This distinction catches buyers: a television may understand the HLG curve yet fail to decode the service's HEVC profile, 10-bit video or frame rate. Conversely, a box can decode the pixels perfectly and still strip the HDR identifier before sending them over HDMI.

Live sport makes the production advantage obvious. Cameras swing from a roof's shadow to floodlights and direct sun, operators shade them continuously, and the director may cut again before metadata tied to the previous shot could be useful. Relative scene light suits that workflow. The BBC and NHK developed HLG around this constraint, and the BBC describes its work on HDR production and distribution↗ across new and legacy displays.

For a carefully mastered film sent only to known streaming clients, I would not pick HLG merely because it avoids metadata. PQ-based formats communicate intended brightness more precisely, while Dolby Vision or HDR10+ can guide tone mapping dynamically. Terrestrial television, satellite feeds and live events have a different priority: keep one signal moving, simultaneously, through a mixed population of receivers.

Streaming services can carry HLG, but adaptive delivery removes much of its one-signal broadcast advantage. YouTube can derive HDR renditions for compatible clients and serve an SDR conversion to devices that cannot present high dynamic range.

The old television is no longer interpreting HLG directly. The platform performs the conversion ahead of playback, which can improve compatibility but makes image quality depend on its tone mapping, codec ladder, processing rules and device detection.

Use the format that matches the master. Keep HLG when the cameras, scopes and grade were built around it, especially if the same program must also enter a broadcast chain. Deliver PQ when a service requests HDR10, Dolby Vision or another PQ-based package.

Blind conversion is the trap. Repeated trips among HLG, PQ and SDR can move midtones, alter saturation, crush darker values or clip a higher peak unless each transform knows the source color space, transfer function and destination display assumptions. Adding an HDR label after grading does not restore brightness information that the SDR master already discarded.