Ultra HD Blu-ray

Ultra HD Blu-ray is an ultra-high-definition read-only disc format defined by the Blu-ray Disc Association.

Within the licensing system it is called Blu-ray Disc Read-Only Format Version 4, or ROM4. The association announced completion of the specification in May 2015, began licensing the same year, and the first commercial players and discs reached market in 2016. It continues the Blu-ray technology family but is not a content label for ordinary 1080p Blu-ray or a simple capacity expansion.

ROM4 supports 50 GB and 66 GB dual-layer read-only discs and 100 GB triple-layer read-only discs. The 66 GB and 100 GB structures increase per-layer capacity and read rate to accommodate UHD video and multiple high-bitrate audio tracks. Disc capacity alone does not determine picture quality: program length, video compression, audio track count, bonus content, and multiplexing overhead all change how much data can be allocated to main video.

Main video uses HEVC/H.265 and no longer carries UHD pictures with the MPEG-2, VC-1, or AVC/H.264 commonly used on standard Blu-ray. The specification supports progressive video up to 3840×2160, with common frame rates including 23.976, 24, 25, 29.97, 50, and 59.94 fps. Video typically uses 10-bit quantization and 4:2:0 chroma subsampling; 50p and 59.94p programs require greater processing and transport capability but do not automatically convert low-frame-rate film to high frame rate. Color signals use the BT.2020 system as a container and can carry a wider gamut than BT.709 on standard Blu-ray. A BT.2020 tag does not prove the program actually covers the full BT.2020 primary range; many commercial masters are graded within P3 and then packaged as BT.2020 signals. Ten-bit quantization provides 1,024 encoding levels per component, reducing banding in large brightness and color gradients. Ultra HD Blu-ray allows both standard dynamic range and high dynamic range programs. HDR10 forms the most common baseline HDR path, using the PQ transfer function and static metadata; some releases add Dolby Vision or HDR10+ dynamic metadata extensions. Discs, players, receivers, and displays must jointly support the relevant extension; otherwise playback usually falls back to baseline HDR10, depending on the disc's compatibility structure. An HDR label on packaging alone cannot specify which format was used.

Audio continues the main Blu-ray system and may include LPCM, Dolby Digital, Dolby Digital Plus, Dolby TrueHD, DTS, DTS-HD High Resolution Audio, and DTS-HD Master Audio. Dolby Atmos is often carried via Dolby TrueHD, and DTS:X is usually built on a DTS-HD Master Audio compatible structure. Object-based audio is not mandatory for Ultra HD Blu-ray and is not unique to the medium; a UHD disc may use only traditional stereo, 5.1, or 7.1 tracks. Programs still use BDMV and UDF 2.5 organization. Main audio, video, subtitles, and other elementary streams are multiplexed into M2TS files under BDMV/STREAM; playlists, clip information, graphic subtitles, menus, and BD-J programs are managed by other files. Directory names resemble standard Blu-ray, but video encoding, control information, and media layers follow ROM4 and cannot be read by the same device merely because the file tree looks similar.

Ultra HD Blu-ray uses AACS content protection and certification requirements oriented to ROM4. Unlike the A, B, and C regional system for standard movie Blu-ray, the Ultra HD Blu-ray specification does not define comparable regional playback codes; commercial UHD discs are usually not restricted by traditional Blu-ray region codes. A bundled standard Blu-ray in a set may still have its own region code; the two disc types must be judged separately.

The association requires new ROM4-licensed players to support backward playback of standard Blu-ray; products typically also support DVD and CD. Reverse compatibility does not exist: standard Blu-ray players lack the optical and signal-processing capability to read 66 GB or 100 GB media, as well as HEVC Main 10, ROM4 control, and content-protection support, and cannot become Ultra HD Blu-ray players through ordinary firmware updates alone. Complete external playback also involves HDMI transport, HDCP 2.2 or later protection versions, and display support for resolution, bit depth, and HDR.

"4K restoration," "4K scan," "4K digital intermediate," and Ultra HD Blu-ray describe different stages. A program scanned or restored at 4K can be downgraded for standard 1080p Blu-ray; Ultra HD Blu-ray may also use a 2K digital intermediate, HD video, or an upscaled master. The medium label describes the final disc, encoding, and playback specification, not the spatial resolution of original capture and post-production alone.