Chroma Subsampling (4:4:4 / 4:2:2 / 4:2:0)
Definition
Chroma subsampling is the practice of storing colour information (chroma) at a lower resolution than brightness (luma), written as a J:a:b ratio. In 4:4:4 every pixel keeps its own colour information, which suits computer text and fine graphics. In 4:2:2 chroma is halved horizontally; it is the studio format of ITU-R BT.601 and BT.709 and the format carried by SDI. In 4:2:0 chroma is halved horizontally and vertically; it is used by most delivery codecs (H.264, HEVC, Blu-ray, streaming) and by HDMI 2.0 to fit 4K at 50 or 60 Hz into a lower bandwidth, and it softens coloured edges. The method relies on the eye's lower sensitivity to colour detail than to brightness detail.
In the field
Coloured text on a white background shows fringes in 4:2:0 but stays sharp in 4:4:4. When a laptop feed shows blurred red text after capture, a link in the chain has usually fallen back to 4:2:0 or 4:2:2 because the cable, the extender or the EDID does not support 4:4:4 at that resolution and frame rate. Lowering the frame rate or the resolution, or using a link rated for the full bandwidth (18 Gbit/s for 4K60 in 4:4:4 at 8 bits over HDMI 2.0), restores 4:4:4.
Related lessons
Sources
- ITU-R BT.601-7 : codage de studio 4:2:2 en définition standard · itu.int
- ITU-R BT.709 : paramètres de studio HD, échantillonnage 4:2:2 · itu.int
- HDMI Licensing Administrator, HDMI Forum Releases Version 2.0 of the HDMI Specification : bande passante jusqu'à 18 Gbit/s et 4K à 50/60 Hz · hdmi.org
- Extron, HDMI 2.0 FAQ : 18 Gbit/s (horloge TMDS de 594 MHz) nécessaires pour la 4K60 en RVB ou 4:4:4 sur 8 bits, 4:2:0 pour les liaisons de moindre débit · extron.com
- Wikipédia, Chroma subsampling : notation J:a:b, 4:4:4, 4:2:2 et 4:2:0 (pointeur secondaire) · en.wikipedia.org