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. 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 The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled ("square-pixel") systems.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’C_BC_R 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 ${\displaystyle [16..235]}$, and the C_B and C_R channels have a nominal range of ${\displaystyle [16..240]}$ 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’C_BC_R, 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.

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