Rec. 709

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.

Technical details

Pixel count

Rec. 709 refers to HDTV systems having roughly two million luma samples per frame. Rec. 709 has two parts: 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 The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled (“square-pixel”) systems.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.

Part 2 codifies current and prospective 1080i and 1080p systems with square sampling. In attempt to unify 1080-line HDTV standards, part 2 defines a common image format (CIF) with picture parameters independent of the picture rate.

Frame rate

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 captures 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 higher than 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.

Digital representation

Rec. 709 coding uses “studio-swing” levels where reference black is defined as 8-bit interface code 16 and reference white is defined as 8-bit interface code 235. Interface codes 0 and 255 are used for synchronization, and are prohibited from video data. Eight-bit codes between 1 and 15 provide footroom, and can be used to accommodate transient signal content such as filter undershoots. Eight-bit interface codes 236 through 254 provide headroom, and can be used to accommodate transient signal content such as filter overshoots and specular highlights. Bit-depths deeper than 8 bits are obtained by appending least-significant bits. Ten-bit systems are commonplace in studios. (Desktop computer graphic systems ordinarily use “full-swing” encoding that places reference black at code 0 and reference white at code 255, and provide no footroom or headroom.) The 16..235 limits (for luma; 16..240 for chroma) originated with ITU Rec. 601.^[1]

Primary chromaticities

CIE 1931 xy chromaticity diagram showing the gamut of the Rec. 709 color space and location of the primaries. The white point is shown in the center.

Note that red and blue are the same as the EBU primaries while green is halfway between EBU and SMPTE C.

Standards Conversion

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, and SMPTE C primaries). In practice, this conversion is rarely performed and such a conversion would create a liability for post production facilities as they would need to ensure that the color bars on all the new masters are redone. Correcting for differences in the primaries would cause the resulting color bars on the converted tape to be inaccurate. Incorrect color bars will cause a (sub)master to be rejected by quality control checks.^[3]

Luma coefficients

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. ^[4] Although worldwide agreement on a single R’G’B’ system was achieved upon the adoption of Rec. 709, adoption of different luma coefficients created a second flavour of Y’C_BC_R. Whenever SDTV is upconverted to HDTV, or HDTV is downconverted to SDTV, at the studio or at the consumers’ premises, luma-chroma matrixing is required.

Transfer characteristics

Rec. 709 is written as if it specifies the capture and 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 [2.35 power function in EBU recommendations]. (Rec. 709 and sRGB share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred with a gamma of 2.2.) ^[5]

See also

• Rec. 601, a comparable standard for standard definition television

References

• ITU-R BT.709-5: Parameter values for the HDTV standards for production and international programme exchange. April, 2002. Note that the -5 is the current version, in May 2008; previous versions were -1 through -4.
• [3]: Poynton, Charles, Perceptual uniformity, picture rendering, image state, and Rec. 709. May, 2008.
• sRGB: IEC 61966-2-1:1999

1. ^ ITU-R Rec. BT.601-5, 1995. Section 3.5.3.
2. ^ ITU-R Rec. BT.709-5 page 18, items 1.3 and 1.4
3. ^ [1]: Chan, Glenn, "HD versus SD Color Space."
4. ^ [2]: Poynton, Charles, “Luminance, luma, and the migration to DTV” (Feb. 6, 1998)
5. ^ Poynton, Charles (2003). Digital Video and HDTV Algorithms and Interfaces. San Francisco: Morgan Kaufmann. p. 263. ISBN 1-55860-792-7. 

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