A Guide to Video Standards: Rec. 709, 2020, and 2100

Rec. 709, Rec. 2020 (Rec. ITU-R BT.2020), Rec. 2100 (Rec. ITU-R BT.2100)

These are key standards defined by the International Telecommunication Union (ITU-R) for color gamut, dynamic range, and video signal characteristics, targeting different display and content creation scenarios.

1. Rec. 709 (ITU-R BT.709)

Basic Definition

Released in 1990, Rec. 709 (also known as HDTV standard) is the core color and video standard for high-definition television (HDTV) and most digital content (e.g., Blu-ray, streaming HD video, computer monitors). It was designed for traditional SDR (Standard Dynamic Range) content and remains the most widely used standard for HD content production and display.

Key Specifications

Parameter	Details
Resolution	Primarily supports 720p (1280×720) and 1080p (1920×1080) HD resolutions.
Color Gamut	Covers ~35.9% of the CIE 1931 color space (CIE xy chromaticity diagram) and ~53.6% of the CIE 1976 (CIE Lab*) color space.- Primary colors (xy coordinates):Red: (0.64, 0.33)Green: (0.30, 0.60)Blue: (0.15, 0.06)- White point: D65 (6500K, daylight white).
Dynamic Range	SDR (Standard Dynamic Range):- Luminance range: 0–100 cd/m² (peak brightness).- Gamma: 2.4 (for display) / 1.95 (for encoding).
Bit Depth	8 bits per channel (RGB/YCbCr), supporting 16.7 million colors.
Application Scenarios	HDTV broadcasts, Blu-ray discs, streaming HD content (e.g., Netflix HD), computer monitors, DSLR video recording, and most consumer-level video production.

2. Rec. 2020 (ITU-R BT.2020 / UHDTV Standard)

Basic Definition

Released in 2012, Rec. 2020 (also called UHDTV standard) is designed for ultra-high-definition television (UHD/4K/8K) and next-generation video content. It expands the color gamut and resolution beyond Rec. 709 to support immersive, lifelike visuals for UHD displays.

Key Specifications

Parameter	Details
Resolution	Supports 4K (3840×2160), 8K (7680×4320), and higher UHD resolutions.
Color Gamut	Covers ~76.6% of the CIE 1931 color space and ~99.9% of the CIE 1976 color space—far larger than Rec. 709.- Primary colors (xy coordinates):Red: (0.708, 0.292)Green: (0.170, 0.797)Blue: (0.131, 0.046)- White point: D65 (6500K).
Dynamic Range	Initially defined for SDR (0–1000 cd/m² peak brightness), but later adapted for HDR (High Dynamic Range) with extensions.
Bit Depth	10 or 12 bits per channel, supporting up to 1.07 billion colors (12-bit) for smoother gradients and less banding.
Application Scenarios	4K/8K UHDTV broadcasts, UHD Blu-ray, high-end UHD displays (e.g., OLED 4K TVs), professional video production for UHD content, and HDR10 (uses Rec. 2020 color gamut).

Critical Note

Most consumer UHD displays (e.g., 4K TVs) use a Rec. 2020 “container” but only cover a subset of the gamut (e.g., DCI-P3 or Adobe RGB) due to technical limitations of display panels. True Rec. 2020 gamut coverage is rare in consumer devices (limited to high-end professional monitors).

3. Rec. 2100 (ITU-R BT.2100 / HDR Standard)

Basic Definition

Released in 2016, Rec. 2100 is the global standard for High Dynamic Range (HDR) video, designed to replace Rec. 709 for HDR content and complement Rec. 2020 for UHD/HDR. It defines two core HDR formats: HLG (Hybrid Log-Gamma) and PQ (Perceptual Quantizer).

Key Specifications

Parameter	Details
Resolution	Supports all resolutions (HD, 4K, 8K) as it focuses on dynamic range/color, not resolution.
Color Gamut	Based on Rec. 2020 color gamut (same primary colors and white point), ensuring compatibility with UHD content.
Dynamic Range	HDR (High Dynamic Range):- PQ (Perceptual Quantizer): For premium HDR (e.g., HDR10, Dolby Vision). Luminance range: 0–10,000 cd/m² (peak brightness), with support for up to 40,000 cd/m² in future.- HLG (Hybrid Log-Gamma): For broadcast HDR (backward-compatible with SDR). Luminance range: 0–4000 cd/m² (peak brightness).
Transfer Functions	– PQ: Non-linear transfer function optimized for human visual perception, encoding brightness levels uniformly across the visible range.- HLG: Combines log and gamma curves, enabling SDR displays to show HLG content (without tone mapping) and HDR displays to render full HDR.
Bit Depth	10 or 12 bits per channel (same as Rec. 2020) for high color precision.
Application Scenarios	HDR streaming content (Netflix HDR, Amazon Prime HDR), HDR10/Dolby Vision movies, HLG broadcast TV, 4K/8K HDR displays, and professional HDR content production.

4. Side-by-Side Comparison

Feature	Rec. 709	Rec. 2020	Rec. 2100
Primary Use	HD SDR video/content	UHD SDR/HDR container	HDR video (4K/8K/HD)
Color Gamut (CIE 1931)	~35.9%	~76.6%	~76.6% (Rec. 2020 gamut)
Peak Luminance	100 cd/m² (SDR)	1000 cd/m² (SDR)	Up to 10,000 cd/m² (HDR)
Dynamic Range	SDR	SDR (with HDR extensions)	HDR (PQ/HLG)
Transfer Function	Gamma 2.4	Gamma 2.4 (SDR)	PQ/HLG
Target Displays	HD TVs, computer monitors	UHD 4K/8K TVs	HDR TVs/monitors, UHD displays

5. Practical Implications

Backward Compatibility: Rec. 2100 HLG is backward-compatible with SDR displays (Rec. 709), while PQ requires HDR-capable displays.

Content Creation: Rec. 709 is still used for HD content; Rec. 2020/2100 for UHD/HDR content (e.g., Netflix HDR uses Rec. 2100 PQ with Rec. 2020 gamut).

Display Compatibility: Most consumer 4K TVs support Rec. 2020 gamut (partial coverage) and Rec. 2100 HDR (PQ/HLG); HD TVs only support Rec. 709.