Rec. 709

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ITU-R Recommendation BT.709, more commonly know by the abbreviations Rec. 709 or BT.709, standardizes the format of High-definition television, having 16:9 (widescreen) aspect ratio.

[edit] Technical details

[edit] 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 The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled (“square-pixel”) systems.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 1080i and 1080p systems (with square sampling) at various frame rates from 23.976 Hz to 60 Hz.

[edit] 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]

[edit] 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.

[edit] Luma coefficients

HDTV according to Rec. 709 forms luma (Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722. These coefficients are different from those of SDTV (for no good reason, according to some experts ^[3]). 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. See Poynton's book for details.

[edit] 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 respect to a 2.4-power function display. (Rec. 709 and sRGB share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred.) ^[4]

[edit] See also

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

[edit] 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.
• [2]: 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]: Poynton, Charles, “Luminance, luma, and the migration to DTV” (Feb. 6, 1998)
4. ^ Poynton, Charles (2003). Digital Video and HDTV Algorithms and Interfaces. San Francisco: Morgan Kaufmann, 263. ISBN 1-55860-792-7. 

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