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 g ...
.
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 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 ratio greater than t ...
)
aspect ratio 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 The ITU superseded the CCIR in 1992, and subsequently released BT.709-1 in November 1993.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 of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured consecutively. This ...
:
; 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 y
G are the same as the EBU Tech 3213 (PAL) primaries while the x
G is halfway between EBU Tech 3213's x
G 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 supplement ...
's x
G (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 (red, green, blue) color space that HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the World Wide Web. It was subsequently standardized by the International Electrotechnical Commission ( ...
color space) is almost identical to
Rec. 601
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 (or its former name CCIR 601) is a standard originally issued in 1982 by the CCIR (an organization, which has since been renamed as the Internatio ...
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 proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
OETF (
opto-electrical transfer function) which is known as the "camera
gamma
Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...
" and which describes how
HDTV
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 g ...
camera encodes the
linear
Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
scene light into a non-linear electrical signal value. Rec. 709 doesn't specify the display EOTF (
electro-optical transfer function This article is about the transfer functions used in pictures and videos and describing the relationship between electrical signal, scene light and displayed light.
Definition
The opto-electronic transfer function (OETF) is the transfer function ...
) which describes how
HDTV
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 g ...
displays should convert the non-linear electrical signal into linear displayed light, that was done in
ITU-R BT.1886
ITU-R BT.1886 is the reference EOTF of Standard-dynamic-range video, SDR-TV. It is a Gamma correction, gamma 2.4 transfer function (a power law with a 2.4 exponent) considered as a satisfactory approximation of the response characteristic of Cat ...
. 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:
where
*
is the non-linear electrical signal value, in the range