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Rec. 709, also known as Rec.709, BT.709, and ITU 709, is a standard developed by
ITU-R The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU) and is responsible for radio communications. Its role is to manage the international radio-frequency sp ...
for image encoding and signal characteristics of
high-definition television High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the ...
. The most recent version is BT.709-6 released in 2015. BT.709-6 defines the Picture characteristics as having a (
widescreen Widescreen images are displayed within a set of aspect ratio (image), aspect ratios (relationship of image width to height) used in film, television and computer screens. In film, a widescreen film is any film image with a width-to-height aspect ...
)
aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
of 16:9, 1080 active lines per picture, 1920 samples per line, and a square pixel aspect ratio. The first version of the standard was approved by the CCIR as Rec.709 in 1990 (there was also CCIR Rec. XA/11 MOD F in 1989), with the stated goal of a worldwide HDTV standard. The ITU superseded the CCIR in 1992, and subsequently released BT.709-1 in November 1993. These early versions still left many unanswered questions, and the lack of consensus toward a worldwide HDTV standard was evident. So much so, some early HDTV systems such as 1125/60 and 1250/50 were still a part of the standard as late as 2002 in BT.709-5.


Technical details

The standard is freely available at th
ITU website
and that document should be used as the authoritative reference. The essentials are summarized below.


Image definition

Recommendation ITU-R BT.709-6 defines a common image format (CIF) where picture characteristics are independent of the frame rate. The image is 1920x1080 pixels, for a total pixel count of 2,073,600. Previous versions of BT.709 included legacy systems such as 1035i30 and 1152i25 HDTV systems. These are now obsolete, and replaced by the system defined in the 2015 ITU BT.709-6.


Frame rates

BT.709 offers over a variety of frame rates and scanning schemes, which along with separating the picture size from frame rate has provided the flexibility for BT.709 to become the worldwide standard for HDTV. This allows manufacturers to create a single television set or display for all markets world-wide. BT.709-6 specifies the following frame rates, where P indicates a progressively scanned frame, PsF indicates
progressive segmented frame Progressive segmented Frame (PsF, sF, SF) is a scheme designed to acquire, store, modify, and distribute progressive scan video using interlaced equipment. With PsF, a progressive frame is divided into two ''segments'', with the odd lines in one s ...
s, and I indicates
interlaced Interlaced video (also known as interlaced scan) is a technique for doubling the perceived frame rate Frame rate (expressed in or FPS) is the frequency (rate) at which consecutive images (Film frame, frames) are captured or displayed. The t ...
: ; 24/P, 24/PsF, 23.976/P, 23.976/PsF : match the frame rate used for theatrical motion pictures. The fractional rates are included for compatibility with the "pull-down" rates used with NTSC. ; 50/P, 25/P, 25/PsF, 50/I (25 fps) : regions that formerly used 50 Hz systems such as PAL or SECAM. There are no fractional rates as PAL and SECAM did not have the pull-down issue of NTSC. ; 60/P, 59.94/P, 30/P, 30/PsF, 29.97/P, 29.97/PsF, 60/I (30 fps), 59.94/I (29.97 fps) : regions that formerly used 60 Hz systems such as NTSC. Here again, the fractional rates are for compatibility with legacy NTSC pull-down rates.


Image capture, encoding, and distribution

Per BT.709, cameras may capture in either progressive or interlaced form. Video captured as progressive can be recorded, broadcast, or streamed as progressive or as progressive segmented frame (PsF). Video captured using an interlaced mode must be distributed as interlace unless a de-interlace process is applied in post production. In cases where a progressive captured image is distributed in segmented frame mode, segment/field frequency must be twice the frame rate. Thus 30/PsF has the same field rate as 60/I.


Primary chromaticities

Note that red and blue and yG are the same as the EBU Tech 3213 (PAL) primaries while the xG is halfway between EBU Tech 3213's xG and
SMPTE C The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
's xG (PAL and NTSC are two types of BT.601-6). In coverage of the
CIE 1931 color space The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that defin ...
the Rec. 709 color space (and the derivative
sRGB sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
color space) is almost identical to Rec. 601 and covers 35.9%. It also covers 33.24% of CIE 1976 u’v’ and 33.5% of CIE 1931 xy. White point is D65 as specified in 2° standard observer.


Transfer characteristics

Rec. 709 specifies a
non-linear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportionality (mathematics), proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, m ...
OETF ( opto-electrical transfer function) which is known as the "camera
gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician a ...
" and which describes how
HDTV High-definition television (HD or HDTV) describes a television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of Trans ...
camera encodes the
linear Linearity is the property of a mathematical relationship ('' function'') that can be graphically represented as a straight line Line most often refers to: * Line (geometry) In geometry, a line is an infinitely long object with no width, ...
scene light into a non-linear electrical signal value. Rec. 709 doesn't specify the display EOTF ( electro-optical transfer function) which describes how
HDTV High-definition television (HD or HDTV) describes a television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of Trans ...
displays should convert the non-linear electrical signal into linear displayed light, that was done in ITU-R BT.1886. Rec.709 is "scene-referred", which means that change of primaries should happen on scene linear light (by applying inverse OETF, changing primaries and applying OETF again, only after which you convert to display linear light using EOTF). Rec. 709 OETF is as follows: V=\begin 4.500L & L < 0.018\\ 1.099 L^ - 0.099 & L \ge 0.018 \end where * V is the non-linear electrical signal value, in the range \left , 1 \right/math>. * L is the linear scene luminance, in the range \left , 1 \right/math>. *1.099 is called α and is the approximate of α = 1 + 5.5 * β = 1.099296826809442... *0.018 is called β and is the approximate of the value 0.018053968510807... *0.099 is α - 1 *Those values are coming from these simultaneous equations that are required to connect the two curve segments smoothly: \begin 4.5\beta = \alpha\beta^ -\alpha +1 \\ 4.5 = 0.45 \alpha\beta^ \end Rec. 709 OETF is linear in the bottom part and then a
power function Exponentiation is a mathematics, mathematical operation (mathematics), operation, written as , involving two numbers, the ''Base (exponentiation), base'' and the ''exponent'' or ''power'' , and pronounced as " (raised) to the (power of) ". W ...
with a gamma 0.45 (about 1/2.2) for the rest of the range. The overall OETF approximate to a pure power function with a gamma 0.50-0.53 (about 1/1.9 - 1/2.0). Using any pure gamma as OETF is impossible, because compression into nonlinear values will remove a lot immediately near black shadows. Thus linear segment was invented and a gamma of 0.45 has been used for the power segment. Old CRTs had a EOTF of 2.35 pure gamma and thus the corresponding correction of 709 OETF to get EOTF linear image (if 1.2 end-to-end gamma is assumed) was a pure gamma of 1.2 / 2.35 = 0.51 = 1/1.9608. It was used in such way by Apple until Display P3 devices came into existence. In typical production practice the encoding function of image sources (OETF) is adjusted so that the final picture has the desired aesthetic look, as viewed on a reference monitor with a gamma of 2.4 (per ITU-R BT.1886) in a dim reference viewing environment (per ITU-R Rec. BT.2035 it is 10 lux of D65 or D93 in Japan). Rec. 709 inverse OETF describes the conversion of the non-linear electrical signal value into the linear scene luminance. It is as follows: L=\begin \dfrac & V < 0.081\\ \left ( \dfrac \right ) ^ & V \ge 0.081 \end The display EOTF of HDTV (sometimes referred as the "display gamma"), is not the inverse of the camera OETF. The EOTF is not specified in Rec. 709. It is discussed i
EBU Tech 3320
and specified in ITU-R BT.1886 as an equivalent
gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician a ...
of 2.4, that is deviating from it in black region depending on how deep the black is. This is a higher gamma than the approximately gamma 2.0 of Rec. 709 OETF. The resulting end-to-end system gamma ( OOTF) of HD television system is about 1.2 and it has been deliberately designed to provide compensation for the dim surround effect.EBU Tech 3320 page 11, https://tech.ebu.ch/docs/tech/tech3320.pdf Rec. 709 and
sRGB sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred with an equivalent gamma of 2.2 (the actual function is also piecewise to avoid near black issues).
Display P3 P3 is an RGB color space An RGB color space is any additive color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representat ...
uses sRGB EOTF with its linear segment, a change of that segment from 709 is needed by either using parametric curve encoding of ICC v4 or by using slope limit.


Digital representation

Rec. 709 defines an ''R’G’B’'' encoding and a ''Y’C''B''C''R encoding, each with either 8 bits or 10 bits per sample in each color channel. In the 8-bit encoding the ''R’'', ''B’'', ''G’'', and ''Y’'' channels have a nominal range of 6..235 and the ''C''B and ''C''R channels have a nominal range of 6..240with 128 as the neutral value. So in limited range ''R’G’B’'' reference black is (16, 16, 16) and reference white is (235, 235, 235), and in ''Y’C''B''C''R reference black is (16, 128, 128) and reference white is (235, 128, 128). Values outside the nominal ranges are allowed, but typically they would be clamped for broadcast or for display (except for Superwhite and
xvYCC xvYCC or extended-gamut YCbCr YCbCr, Y′CbCr, or Y Pb/Cb Pr/Cr, also written as YCBCR or Y′CBCR, is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the Luma (video), lu ...
). Values 0 and 255 are reserved as timing references (SAV and EAV), and may not contain color data (for 8 bits, for 10 bits more values are reserved and for 12 bits even more, no values are reserved in files or RGB mode or full range YCbCr digital modes like sYCC or opYCC). Rec. 709's 10-bit encoding uses nominal values four times those of the 8-bit encoding, to ease the conversion it uses simple padding for reference values, for example 240 is just padded by two trailing zeroes and gives 960 for 10 bit maximum chroma. Rec. 709's nominal ranges are the same as those defined in ITU Rec. 601.


Standards conversion

Conversion between different standards of video frame rates and color encoding has always been a challenge for content producers distributing through regions with different standards and requirements. While BT.709 has eased the compatibility issue in terms of the consumer and television set manufacturer, broadcast facilities still use a particular frame rate based on region, such as 29.97 in North America, or 25 in Europe meaning that broadcast content still requires at least frame rate conversion.


Converting standard definition

The vast legacy library of
standard-definition Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition Enhanced-definition television, or extended-d ...
programs and content presents further challenges.
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplementa ...
,
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
, and
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''color sequential with memory''), is an analog television, analog color television system that was used in France, some parts of Europe and Africa, and Russia ...
are all interlaced formats in a 4:3 aspect ratio, and at a relatively low resolution. Scaling them up to HD resolution with a 16:9 aspect ratio presents a number of challenges. First is the potential for distracting motion artifacts due to interlaced video content. The solution is to either up-convert only to an interlaced BT.709 format at the same field rate, and scale the fields independently, or use motion processing to remove the inter-field motion and
deinterlace Deinterlacing is the process of converting interlaced video Interlaced video (also known as interlaced scan) is a technique for doubling the perceived frame rate of a video display without consuming extra Bandwidth (signal processing), band ...
, creating progressive frames. In the latter case, motion processing can introduce artifacts and can be slow to process. Second is the issue of accommodating the SD 4:3 aspect ratio into the HD 16:9 frame. Cropping the top and/or bottom of the standard-definition frame may or may not work, depending on if the composition allows it and if there are graphics or titles that would be cut off. Alternately, pillar-boxing can show the entire 4:3 image by leaving black borders on the left and right. Sometimes this black is filled with a stretched and blurred form of the image. In addition, the SMPTE C RGB primaries used in North American standard definition are different than those of BT.709 (SMPTE C is commonly referred to as NTSC, however it is a different set of primaries and a different white point than the 1953 NTSC). The red and blue primaries for PAL and SECAM are the same as BT.709, with a change in the green primary. Converting the image precisely requires a LUT (lookup table) or a color managed workflow to convert the colors to the new colorspace. However in practice this is often ignored, except in mpv, because even if the player is color managed (most of them are not, including VLC), it can only see BT.709 or BT.2020 primaries only.


Luma coefficients

When encoding ''Y’C''B''C''R video, BT.709 creates gamma-encoded luma (''Y’'') using matrix coefficients 0.2126, 0.7152, and 0.0722 (together they add to 1). BT.709-1 used slightly different 0.2125, 0.7154, 0.0721 (changed to standard ones in BT.709-2). Although worldwide agreement on a single R’G’B’ system was achieved with Rec. 709, adoption of different luma coefficients (as those are derived from primaries and white point) for ''Y’C''B''C''R requires the use of different luma-chroma decoding for standard definition and high definition.


Conversion software and hardware

These problems can be handled with video processing software which can be slow, or hardware solutions which allow for realtime conversion, and often with quality improvements.


Film retransfer

A more ideal solution is to go back to original film elements for projects that originated on film. Due to the legacy issues of international distribution, many television programs that shot on film used a traditional negative cutting process, and then had a single film master that could be telecined for different formats. These projects can re-
telecine Telecine ( or ) is the process of transferring film into video and is performed in a color suite. The term is also used to refer to the equipment used in the post-production process. Telecine enables a motion picture, captured originally on fi ...
their cut negative masters to a BT.709 master at a reasonable cost, and gain the benefit of the full resolution of film. On the other hand for projects that originated on film, but completed their online master using video online methods would need to re-telecine the individual needed film takes and then re-assemble, a significantly greater amount of labor and machine time is required in this case, versus a telecine for a conformed negative. In this case, to enjoy the benefits of the film original would entail much higher costs to conform the film originals to a new HD master.


Relationship to sRGB

sRGB sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
was created after the early development of Rec.709. The creators of sRGB chose to use the same primaries and white point as Rec.709, but changed the tone response curve (sometimes referred to as
gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician a ...
) to better suit the intended use in offices and brighter conditions than television viewing in a dark living room.


See also

* Rec. 601, a comparable standard for
standard-definition television Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high-definition television, high or enhanced-definition television, enhanced ...
(SDTV) * Rec. 2020, a standard for
ultra-high-definition television Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) today includes 4K resolution#Resolutions, 4K UHD and 8K resolution#Resolutions, 8K UHD, which are two digital video formats with an ...
(UHDTV) with Wide Color Gamut (WCG) * Rec. 2100, a standard for
high-dynamic-range television High-dynamic-range television (HDR or HDR-TV) is a technology that improves the quality of display signals. It is contrasted with the retroactively-named standard-dynamic-range video, standard dynamic range (SDR). HDR changes the way the lumina ...
(HDR-TV) with FHD and UHD resolution *
sRGB sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
, a standard color space for web/computer graphics, based on the Rec. 709 primaries and white point


References


External links


ITU-R BT.709-6
''Parameter values for the HDTV standards for production and international programme exchange.'' June, 2015. Note that the -6 is the current version; previous versions were -1 through to -5. * Poynton, Charles,

'' May, 2008. *
sRGB sRGB is a standard RGB color space, RGB (red, green, blue) color space that Hewlett-Packard, HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the Internation ...
: IEC 61966-2-1:1999 {{Color space ATSC High-definition television Film and video technology Digital television ITU-R recommendations Color space 1990 in television