Standard-dynamic-range video

Standard-dynamic-range video (SDR video) is a video technology which represents light intensity based on the brightness, contrast and color characteristics and limitations of a cathode ray tube (CRT) display.^[1] SDR video is able to represent a video or picture's colors with a maximum luminance around 100 cd/m², a black level around 0.1 cd/m² and Rec.709 / sRGB color gamut.^[1]^[2] It uses the gamma curve as its electro-optical transfer function.^[1]^[3]

The first CRT television sets were manufactured in 1934 and the first color CRT television sets were manufactured in 1954.^[4]^[5] The term "standard-dynamic-range video" was adopted to distinguish SDR video from high-dynamic-range video (HDR video), a new technology that was developed in the 2010s to overcome SDR's limits.^[1]^[6]

Technical details

Transfer function

Conventional gamma curves:

Opto-electronic transfer function (OETF):
- Rec. 601 (analog video signals in SD-TV digital video form)^[7]^[8]
- Rec. 709 (HD-TV)^[8]
- Rec. 2020 (UHD-TV)
- sRGB
Electro-optical transfer function (EOTF):
- ITU-R BT.1886 (SDR-TV)^[3]
- sRGB (monitors, printers, World Wide Web)^[9]

The linear part of the conventional gamma curve was used to limit camera noise in low light video but is no longer needed with high dynamic range (HDR) cameras.^[8] An example of a conventional gamma curve would be Rec. 601:

$E={\begin{cases}4.500L&L<0.018\\1.099L^{0.45}-0.099&L\geq 0.018\end{cases}}$

ITU-R Recommendation BT.1886 describe the reference EOTF of SDR.^[3] It's a gamma curve representing the response of CRT to video signal.^[3] It has been published by ITU in 2011.^[3]

A transfer function that is closer to Weber's law allows for a larger dynamic range, at the same bit depth, than a conventional gamma curve.^[10] HDR standards such as hybrid log–gamma (HLG) and SMPTE ST 2084 allow for a larger dynamic range by using a different transfer function.^[10]^[11] HLG is compatible with SDR displays.^[12]

Color gamut

In some cases the term SDR is also used with a meaning including the standard color gamut (i.e. Rec.709 / sRGB color primaries).^[1] HDR uses wide color gamut (WCG) such as Rec. 2020 or DCI-P3 color primaries.^[1]^[13]

Dynamic range

The dynamic range that can be perceived by the human eye in a single image is around 14 stops.^[10] SDR video with a conventional gamma curve and a bit depth of 8-bits per sample has a dynamic range of about 6 stops, assuming a luminance quantisation threshold of 5% is used.^[10] A threshold of 5% is used in the paper (instead of the standard 2% threshold) to allow for the typical display being dimmer than ideal. Professional SDR video with a bit depth of 10-bits per sample has a dynamic range of about 10 stops.^[10]

Displaying SDR video on modern displays

While conventional gamma curves are useful for low light and are compatible with CRT displays, they can only represent a limited dynamic range.^[10]^[11] Standards require SDR to be viewed on a display with the same characteristics as a CRT (i.e. 100 nits peak brightness, gamma curve, Rec. 709 color primaries).^[1]^[3] However, current displays are often far more capable than CRT's limits.^[1] On such displays, higher brightness and wider color gamut can be displayed by adjusting and trying to enhance the SDR picture.^[1] HDR is however required for the creative intents to be preserved.^[1]