|First published||November 16, 1993|
June 17, 2015
|Base standards||Rec.709, BT.709, ITU—709|
|Domain||Digital image processing|
The most recent version is BT.709-6 released in 2015. BT.709-6 defines the Picture characteristics as having a (widescreen) 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 The ITU superseded the CCIR in 1992, and subsequently released BT.709-1 in November 1993.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.
The standard is freely available at the ITU website, and that document should be used as the authoritative reference. The essentials are summarized below.
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.
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.
- 24/P, 24/PsF, 23.976/P, 23.976/PsF
- match the frame rate used for theatrical motion pictures. The fractional rates for compatibility with the "pull-down" rates used with NTSC.
- 50/P, 25/P, 25/PsF, 50/I (25fps)
- 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.
|Color space||White point||Primaries|
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's xG (PAL and NTSC are two types of BT.601-6). In coverage of the CIE 1931 color space the Rec. 709 color space (and the derivative sRGB 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.
Rec. 709 specifies only the OECF/OETF (opto-electrical transfer function) of HDTV encoding in reference to the camera, known as camera gamma (sometimes indicated as "scene-referred" gamma). The Rec. 709 transfer function from the linear signal (luminance) to the nonlinear (voltage) is linear in the bottom part and then transfers to a power function for the rest of the range:
Here 1.099 number (called α) has the value 1 + 5.5 * β = 1.099296826809442... and β has the value 0.018053968510807..., while 0.099 is 1.099 - 1. Those values are coming from these simultaneous equations that are required to connect the two curve segments smoothly:
The conversion to linear is as follows.
The power function of the majority of the TRC (tone response curve) is 0.45, but because it is offset by the linear section the resulting equivalent gamma is more approximate to 0.50-0.53 (the inverse of which is approximately gamma 1.9-2.0 to convert back to linear).
While Rec. 709 does not specify the display referred gamma (EOCF/EOTF), display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as an equivalent gamma 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 2.0 the math shown above would indicate, because the television system has been deliberately designed with an end-to-end system gamma of about 1.2, to provide compensation for the ‘dim surround’ effect. Therefore, the monitor gamma is not the inverse of the camera gamma.
It is worth noting that Rec. 709 and sRGB 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).
In typical production practice the encoding function of image sources 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). Nevertheless the BT.1886 does have a problem in HDR and thus BT.2390 defines the Hermite spline (EETF) that adds a tapering factor (1 – E2)4 to BT.1886.
Rec. 709 defines an R’G’B’ encoding and a Y’CBCR 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 [16..235], and the CB and CR channels have a nominal range of [16..240] with 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’CBCR 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). 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.
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 programs and content presents further challenges. NTSC, PAL, and SECAM 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 process them to de-interlace and remove the inter-field motion, creating progressive frames.
Second is the issue of aspect ratios. 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.
In addition, the NTSC color primaries of red, green, and blue are different than those of BT.709. The red and blue primaries for PAL and SECAM are the same as BT.709, with only a minor change in the green primary. Converting NTSC properly means using a LUT (lookup table) to convert the colors to the new colorspace.
When encoding Y’CBCR 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’CBCR 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.
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 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 was created after the early development of Rec.709. The creators of sRGB chose to use the same primaries and whitepoint as Rec.709, but changed the tone response curve (sometimes referred to as gamma) to better suit the intended use in offices and brighter conditions than television viewing in a dark living room.
- Rec. 601, a comparable standard for standard-definition television (SDTV)
- Rec. 2020, a standard for ultra high definition television (UHDTV) with Wide Color Gamut (WCG)
- Rec. 2100, a standard for high dynamic range television (HDR-TV) with FHD and UHD resolution
- sRGB, a standard color space for web/computer graphics, based on the Rec. 709 primaries and white point
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- sRGB: IEC 61966-2-1:1999