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Concept
Rec. 709
Summary
Rec. 709, also known as Rec.709, BT.709, and ITU 709, is a standard developed by ITU-R for image encoding and signal characteristics of high-definition television.
The most recent version is BT.709-6 released in 2015. BT.709-6 defines the Picture characteristics as having a (widescreen) 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 The ITU superseded the CCIR in 1992, and subsequently released BT.709-1 in November 1993.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.
Previous versions of BT.709 included legacy systems such as 1035i30 and 1152i25 HDTV systems. These are now obsolete, and replaced by the system defined in the 2015 ITU BT.709-6.
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.
BT.709-6 specifies the following frame rates, where P indicates a progressively scanned frame, PsF indicates progressive segmented frames, and I indicates interlaced:
24/P, 24/PsF, 23.976/P, 23.976/PsF
match the frame rate used for theatrical motion pictures.
Official source
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