ITU-R Recommendation BT.709, more commonly known by the abbreviations Rec. 709 or BT.709, standardizes the format of high-definition television, having 16:9 (widescreen) aspect ratio. The first edition of the standard was approved in 1990.
Rec. 709 refers to HDTV systems having roughly two million luma samples per frame. Rec. 709 has two parts:
Part 2 codifies current and prospective 1080i and 1080p systems with square sampling. In an attempt to unify 1080-line HDTV standards, part 2 defines a common image format (CIF) with picture parameters independent of the picture rate.
Part 1 codifies what are now referred to as 1035i30 and 1152i25 HDTV systems. The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled (“square-pixel”) systems. The 1152i25 system was used for experimental equipment in Europe and was never commercially deployed.
Rec. 709 specifies the following picture rates: 60 Hz, 50 Hz, 30 Hz, 25 Hz and 24 Hz. “Fractional” rates having the above values divided by 1.001 are also permitted.
Initial acquisition is possible in either progressive or interlaced form. Video captured as progressive can be transported with either progressive transport or progressive segmented frame (PsF) transport. Video captured as interlaced can be transported with interlace transport. In cases where a progressive captured image is transported as a segmented frame, segment/field frequency must be twice the frame rate.
In practice, the above requirements result in the following frame rates (“fractional” rates are specified in commonly used “decimal” form): 25i, 25PsF, 25p, 50p for 50 Hz systems; 23.976p, 23.976PsF, 24p, 24PsF, 29.97i, 29.97p, 29.97PsF, 30PsF, 30p, 59.94p, 60p for 60 Hz systems.
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 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. Values 0 and 255 are reserved as timing references, and may not contain color data. Rec. 709’s 10-bit encoding uses nominal values four times those of the 8-bit encoding. Rec. 709’s nominal ranges are the same as those defined in ITU Rec. 601.
|Color space||White point||Primaries|
Note that red and blue are the same as the EBU Tech 3213 primaries while green is halfway between EBU Tech 3213 and SMPTE C (two types of Rec.601). 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%.
When converting between the various HD and SD formats, it would be correct to compensate for the differences in the primaries (e.g. between the Rec. 709, EBU Tech 3213, and SMPTE C primaries). In practice, this conversion is rarely performed because the difference is negligible in real-world scenes, except the ones with large patches of very saturated colors.
HDTV according to Rec. 709 forms luma (Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722. This means that unlike Rec. 601, the coefficients match the primaries and white points, so luma corresponds more closely to luminance. Some experts feel that the advantages of correct matrix coefficients do not justify the change from Rec. 601 coefficients.
Rec. 709 is written as if it specifies the transfer characteristics of HDTV encoding – that is, as if it were scene-referred. However, in practice it is output (display) referred with the convention of a 2.4-power function display according to EBU Tech 3320. (Rec. 709 and sRGB share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred with an average gamma of 2.2.)
The Rec. 709 transfer function from the linear signal (luminance) to the nonlinear (voltage) is, similar to sRGB’s transfer function, linear in the bottom part and then transfers to a power function for the rest of the [0..1]} [0..1] range:
The conversion to linear is as follows.
In typical production practice this function is not used. Instead the encoding function is adjusted so that the final picture has the desired look, as viewed on a reference monitor with a gamma of 2.4 in a dim reference viewing environment.
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