HLG Explained: Benefits of Metadata-Free HDR for Broadcast

English Definition

HLG (Hybrid Log-Gamma) is a high dynamic range (HDR) video encoding standard developed jointly by the BBC and NHK, designed to deliver enhanced contrast and brightness for both broadcast and streaming content. Unlike HDR10 or Dolby Vision, HLG uses a backward-compatible hybrid approach—it works on both HDR-enabled displays and standard dynamic range (SDR) screens without the need for separate metadata, making it ideal for live TV broadcasts and over-the-air content.

Core Technical Principles

HLG combines two transfer functions to balance HDR performance and SDR compatibility:

Gamma Curve (for SDR content):For lower brightness levels (0–约 100 nits), HLG uses a traditional gamma curve (similar to SDR) to ensure compatibility with standard displays. This part of the curve maps input signals to output brightness in a linear, SDR-friendly way.
Logarithmic Curve (for HDR content):For higher brightness levels (above 100 nits, up to 10,000+ nits), HLG switches to a logarithmic curve. This compresses the wide HDR brightness range into a manageable signal, preserving detail in bright highlights (e.g., sunlight, specular reflections) and dark shadows.

Key Difference from Other HDR Formats

HLG is metadata-free, while formats like HDR10 and Dolby Vision rely on static/dynamic metadata to instruct displays how to render HDR content. HLG’s hybrid curve allows SDR displays to ignore the logarithmic HDR portion (rendering only the gamma curve), and HDR displays to decode the full logarithmic range—no additional metadata is required.

Core Features & Specifications

Feature	Details
Dynamic Range	Supports peak brightness up to 10,000 nits (theoretical) and a contrast ratio of 1,000,000:1, far exceeding SDR (100 nits peak, ~1000:1 contrast).
Color Gamut	Compatible with Rec. 2020 (wide color gamut) and Rec. 709 (SDR color gamut), ensuring color accuracy across devices.
Compatibility	Backward-compatible with SDR displays (auto-converts to SDR) and forward-compatible with HDR displays (full HDR rendering).
Bit Depth	Supports 10-bit and 12-bit color depth, reducing banding and preserving color detail in gradients.
Use Cases	Optimized for live broadcast (e.g., sports, news), over-the-air TV, and streaming services (e.g., BBC iPlayer, YouTube).

Advantages of HLG

Backward CompatibilityThe biggest advantage of HLG is its “one-stream” approach: a single HLG signal works on all displays (SDR or HDR). Broadcasters do not need to encode separate SDR and HDR versions of content, reducing bandwidth and production costs.
No Metadata DependencyUnlike HDR10 (static metadata) or Dolby Vision (dynamic metadata), HLG does not require pre-defined metadata. This makes it ideal for live content (e.g., live sports), where metadata cannot be pre-calculated before transmission.
Simplified WorkflowContent creators can encode HLG once, and it will automatically adapt to the viewer’s display. This eliminates the need for complex color grading for separate SDR/HDR versions.
Broad Industry AdoptionSupported by major platforms (YouTube, Netflix, BBC iPlayer), TV manufacturers (Samsung, LG, Sony), and broadcast standards (ATSC 3.0, DVB).

Limitations of HLG

Less Precision Than Dolby VisionWithout dynamic metadata, HLG cannot optimize content frame-by-frame for specific displays (unlike Dolby Vision). This may result in slightly less accurate brightness rendering on some HDR screens.
Limited Peak Brightness UtilizationWhile HLG supports up to 10,000 nits, most consumer HDR displays max out at 1000–2000 nits. The logarithmic curve may not fully leverage the highest brightness levels on premium displays.
Less Common in Streaming vs. HDR10While growing, HLG is still less widely used for on-demand streaming than HDR10 (which is mandatory for UHD Blu-ray and most streaming HDR content).

Typical Application Scenarios

Live TV Broadcasts: Sports, news, and live events (e.g., Olympics, FIFA World Cup) where real-time HDR delivery is critical and SDR compatibility is required.
Over-the-Air (OTA) TV: ATSC 3.0 (U.S.) and DVB (Europe) standards adopt HLG for HDR broadcast, ensuring compatibility with legacy SDR TVs.
Streaming Services: YouTube, BBC iPlayer, and some Netflix content use HLG for HDR videos, especially live or user-uploaded content.
Professional Monitoring: Used in video production for real-time HDR preview, as it works on both SDR and HDR monitors.

HLG vs. Other HDR Formats

Feature	HLG	HDR10	Dolby Vision
Metadata	None (metadata-free)	Static metadata	Dynamic metadata
Compatibility	SDR/HDR (one stream)	HDR-only (SDR via conversion)	HDR-only (SDR via conversion)
Peak Brightness	Up to 10,000 nits	Up to 10,000 nits	Up to 4000 nits (Dolby Vision IQ: 10,000 nits)
Use Case	Live broadcast, OTA	On-demand streaming, UHD Blu-ray	Premium streaming, UHD Blu-ray
Cost	Royalty-free	Royalty-free	Proprietary (royalties)