UHD Blu-ray Ultra HD Blu-ray

What it is and how it works

Ultra HD Blu-ray is a physical optical disc format built to deliver 4K UHD video at 3840 x 2160 pixel resolution. Unlike standard Blu-ray, which uses AVC/H.264 encoding, UHD Blu-ray uses HEVC (H.265) video compression, a codec that achieves better compression efficiency than its predecessor. The format supports frame rates up to 60 progressive frames per second.

Discs are defined in three capacities: single-layer 50GB, and multi-layer 66GB and 100GB discs. The 50GB and 66GB discs use two layers, while the 100GB disc uses three layers.

Standard HDR content on UHD Blu-ray is encoded at 10-bit color depth, while Dolby Vision content uses a 12-bit color container. Discs use the Rec. 2020 color space, which covers a wider gamut than the color space used by standard Blu-ray and broadcast HD.

Bitrate, HDR, and audio specifics

The UHD Blu-ray spec defines several data-rate tiers by disc capacity: 50GB discs at 72 or 92 Mbit/s, and 66GB and 100GB discs at 92, 123, or 144 Mbit/s. Within that envelope, 4K Ultra HD Blu-ray discs can carry a maximum video bitrate of up to 128 Mbps.

HDR10 is the mandatory baseline HDR format for UHD Blu-ray. Every UHD Blu-ray disc and player supports it, making it the format viewers can count on regardless of hardware. Dolby Vision and HDR10+ are optional additions on top of that baseline, not guaranteed on every disc or player.

On discs using Dolby Vision Profile 7, video is packaged in two layers: a compatible base layer plus an enhancement layer. If the display doesn't support Dolby Vision, the player falls back to the base layer alone, still delivering standard HDR10 rather than failing to play.

HDR10+ was added to the UHD Blu-ray spec as an optional feature (BDA spec v3.2) in January 2018, but the first commercially released HDR10+ UHD Blu-ray discs did not arrive until 2019, such as Warner Bros' Godzilla: King of the Monsters release that August. Some studios, including Lionsgate, Warner, and Universal, have since included both Dolby Vision and HDR10+ on the same disc.

For audio, UHD Blu-ray carries lossless codecs that compressed streaming cannot match. Dolby TrueHD is 100 percent lossless. What you hear is bit-for-bit identical to the studio master recording. On Blu-ray, Dolby TrueHD supports up to eight full-range channels of 96kHz/24-bit audio, or six full-range channels of 192kHz/24-bit audio, exploiting the maximum audio fidelity the disc format allows.

Comparison to streaming and standard Blu-ray

UHD Blu-ray's bitrate ceiling sits well above what 4K streaming services target. Netflix requires a minimum of 17 Mbps internet connection speed for its Ultra HD 4K streaming tier, a required connection speed, not necessarily the actual encoded average bitrate of the stream itself. By contrast, UHD Blu-ray's maximum video bitrate of 144 Mbit/s is many times higher, and even its lower disc-capacity tiers (72 Mbit/s on 50GB discs) exceed typical streaming bandwidth requirements.

Compared to standard Blu-ray, which tops out around 40 Mbps of video bitrate, UHD Blu-ray's 144 Mbit/s maximum represents a substantial increase in the data budget available for encoding, on top of the higher resolution and wider color gamut HEVC and the format spec make possible.

Common confusions

Dolby Atmos as delivered on UHD Blu-ray is not identical to "Atmos" as delivered by most streaming services. UHD Blu-ray players are required by the format spec to pass through lossless Dolby TrueHD (including embedded Atmos object-based audio) and DTS:X / DTS-HD Master Audio over HDMI to a receiver. Most streaming services, by contrast, deliver Atmos via Dolby Digital Plus with Joint Object Coding, a lossy codec. Both carry Atmos object-based metadata, but the disc-sourced version is lossless while the streaming version is compressed.

Not all UHD Blu-ray discs and players support Dolby Vision and HDR10+. HDR10 is the only HDR format guaranteed across every UHD Blu-ray disc and player. Dolby Vision and HDR10+ support depends on both the specific disc's authoring and the playback hardware's capabilities, so a disc mastered in Dolby Vision will still play back correctly in standard HDR10 on hardware that lacks Dolby Vision support, rather than failing outright.