Television Signals and Colour Systems
Television Signals and Colour Systems explains how colour became compatible with black-and-white receivers and how digital television now represents colour. Expect questions on luma weighting, the three composite systems, and which artefacts belong to composite decoding.
Colour fundamentals
- Additive mixing of red, green and blue light creates white; subtractive mixing of cyan, magenta and yellow pigments removes wavelengths.
- Colour is specified by hue, saturation and luminance.
- Luma Y = 0.299 R + 0.587 G + 0.114 B (NTSC/Rec.601), reflecting the eye's greater sensitivity to green.
- Colour-difference signals such as R−Y and B−Y are added to Y so that one signal carries brightness and the others carry chroma.
Composite analogue systems
- NTSC: 525 lines, 29.97 fps, YIQ coding, subcarrier about 3.579545 MHz; known for hue shifts ("never the same colour").
- PAL: 625 lines, 25 fps, YUV coding, subcarrier about 4.43361875 MHz; the alternating phase line-by-line cancels hue errors, fixing most differential phase problems.
- SECAM: 625 lines, 25 fps, sequential FM transmission of the two colour-difference signals.
- Composite artefacts include cross-colour (chroma detail showing as colour), cross-luma (colour subcarrier showing as dot crawl in luma) and moiré.
Digital colour
- Digital systems use Y'CbCr with subsampling 4:4:4, 4:2:2 or 4:2:0.
- Rec.709 defines HD primaries; Rec.2020 defines UHD and wide colour gamut; HDR adds PQ or HLG transfer functions.
Worked example. For R = 0.5, G = 0, B = 0: Y = 0.299 × 0.5 = 0.1495, about 0.15. A full red is only 0.299 luma, which is why red detail looks soft in mono.
Traps
- Writing NTSC as exactly 30 fps instead of 29.97.
- Swapping PAL and SECAM subcarrier techniques (PAL quadrature AM, SECAM sequential FM).
- Using hue and saturation as synonyms.
- Treating YUV (analogue) and YCbCr (digital) as identical.