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ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 (or its former name, CCIR 601,) is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector (formerly CCIR) for encoding interlaced analog video signals in digital video form. It includes methods of encoding 525-line 60 Hz and 625-line 50 Hz signals, both with 720 luminance samples and 360 chrominance samples per line. The color encoding system is known as YCbCr 4:2:2. For a pair of pixels, the data are stored in the order Y1:Cb:Y2:Cr, with the chrominance samples co-sited with the first luminance sample.
The Rec. 601 signal can be regarded as if it is a digitally encoded analog component video signal, and thus includes data for the horizontal and vertical sync and blanking intervals. Regardless of the frame rate, the luminance sampling frequency is 13.5 MHz. The luminance sample is at least 8 bits, and the chrominance samples are at least 4 bits each.
The first version of Rec. 601 defined only a parallel interface, but later versions introduced the bit-serial family of serial digital video interfaces that are now commonly used. The 8 bit serial protocol (216 Mbit/s) was once used in D1 digital video tape recording. Modern standards use an encoding table to expand the data to 9 or 10 bits for improved behavior over long transmission lines. The 9 bit serial version has a data rate of 243 Mbit/s. By far, the most common version of the interface is the 10-bit serial digital interface (which was later standardized as SMPTE 259M), which is now a ubiquitous interconnect standard for professional video equipment which operates on standard-definition digital video. This format, originally used in D5 digital tape recording, has a data rate of 270 Mbit/s.
There is an 8 bit version in which only data from the active video periods are transmitted, with a bit rate of only 165.9 Mbit/s.
In each 8 bit luminance sample, the range has clamped the value for black at 16 and clamped the value for white to 235, no signal is allowed out of this range although transients that cause overshoot or undershoot are allowed. The values 0 and 255 are used to encode the sync pulses. The Cb and Cr unsigned samples use the value 128 to encode the neutral color difference value, as used when encoding a white, grey or black area.
The Rec. 601 video raster format has been re-used in a number of later standards, including MPEG.
Slightly different primaries are specified for the 625 line and 525 line systems.
|Color space||White point||Primaries 625 line||Primaries 525 line|
- Chroma subsampling
- D-1 (Sony)
- Rec. 709, the equivalent standard for high-definition television (HDTV)
- Rec. 2020, ITU-R Recommendation for ultra-high-definition television (UHDTV)
- ITU-R BT.656
- Pixel aspect ratio