ITU-R BT.601
ITU-R BT.601 defines digital component-video encoding for standard-definition television.
An archived SD recording is enlarged to 1080p, and its colored objects change slightly even though neutral gray still looks sensible. One possibility is a disagreement about the luma/color-difference matrix: an image represented using BT.601 values is being reconstructed as if those values used BT.709.
BT.601, or Rec. 601, defines digital component-video encoding for standard-definition television. Its matrix remains relevant after a picture is resized, because enlarging the sample grid does not automatically change what the samples mean.
BT.601 color after upscaling
Start with nonlinear RGB components R′, G′, and B′. The familiar BT.601 luma weighting is:
Y′ = 0.299R′ + 0.587G′ + 0.114B′
Two color-difference components describe the color around that luma value. A playback stage uses the matching inverse relationship to recover RGB. The weights operate on nonlinear signal components; this luma is different from a direct measurement of emitted light.
For a neutral signal with R′ = G′ = B′, any weighting whose coefficients sum to one gives the same luma value, and the color differences are neutral. That explains why a matrix mismatch can leave a gray patch plausible. Colored patches exercise the different weights and reveal the disagreement.
Resizing the luma/chroma grids can retain the BT.601 interpretation. Alternatively, a processing path can reconstruct color and perform an appropriate conversion to a destination representation, such as BT.709. The delivered values and their signaling must then describe that destination. These are actual processing choices, rather than consequences of the output resolution alone.
When comparing an Emby Direct Play and transcoded session, inspect the source and delivered matrix information. An SD-to-HD conversion can change more than geometry, so comparing only the resolution leaves the color conversion unexplained.
Useful reference details
| Detail | BT.601 role |
|---|---|
| Standard 4:2:2 family | Full horizontal luma sampling with half-rate color-difference sampling |
| Luma sampling rate | 13.5 MHz for the standard 525-line and 625-line 4:2:2 systems |
| Corresponding chroma sampling rate | 6.75 MHz for each color-difference component |
| Nominal 8-bit luma black/white | Code values 16 and 235 |
| Familiar nonlinear luma weights | 0.299 red, 0.587 green, 0.114 blue |
The numerical range must also be interpreted correctly. Nominal black at 16 and white at 235 are not the same code positions as full-range black and white. A range mismatch affects contrast, while a matrix mismatch mainly changes the reconstructed colors.
BT.601-era material can have differing primary and transfer descriptions depending on its television system and history. Read those fields individually when available. The matrix answers how component signals relate; it does not replace the rest of the source's color description.