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{{Short description|Simple digital video protocol for standard-definition television}}
'''[[ITU-R]] Recommendation BT.656''', sometimes also called '''ITU656''', is a simple [[digital video]] protocol for streaming uncompressed [[PAL]] or [[NTSC]] [[standard-definition television]] ([[625 lines|625]] or [[525 lines]]) signals. The protocol builds upon the [[4:2:2]] digital video encoding parameters defined in ITU-R Recommendation [[BT.601]], which provides [[interlaced]] video data, streaming each field separately, and uses the [[YCbCr]] color space and a 13.5 MHz sampling frequency for pixels.<ref>{{Cite book |url=https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.656-5-200712-I!!PDF-E.pdf |title=Recommendation ITU-R BT.656-5 - Interface for digital component video signals in 525-line and 625-line television systems operating at the 4:2:2 level of Recommendation ITU-R BT.601 |publisher=ITU-R |year=2007}}</ref>

The standard can be implemented to transmit either 8-[[bit]] values (the standard in consumer electronics) or 10-bit values (sometimes used in studio environments). Both a parallel and a serial transmission format are defined. For the parallel format, a 25-pin [[D-subminiature|D-Sub]] connector pinout and [[Emitter-coupled_logic|ECL]] logic levels are defined. The serial format can be [[Serial_digital_interface|transmitted over 75-ohm coaxial cable]] with [[BNC_connector|BNC connectors]], but there is also a fibre-optical version defined.

The parallel version of the ITU-R BT.656 protocol is also used in many TV sets between chips using [[CMOS]] logic levels. Typical applications include the interface between a PAL/NTSC decoder chip and a [[digital-to-analog converter|DAC]] [[integrated circuit]] for driving a [[Cathode ray tube|CRT]] in a TV set.

== Data format ==
A BT.656 data stream is a sequence of 8-bit or 10-bit [[Word (computer architecture)|words]], transmitted at a rate of 27 Mword/s. Horizontal scan lines of video [[pixel]] data are delimited in the stream by 4-byte long SAV (Start of Active Video) and EAV (End of Active Video) code sequences. SAV codes also contain status bits indicating line position in a video field or frame. Line position in a full frame can be determined by tracking SAV status bits, allowing receivers to 'synchronize' with an incoming stream.

Individual pixels in a line are coded in [[YCbCr]] format. After an SAV code (4 bytes) is sent, the first 8 bits of Cb (chroma U) data are sent then 8 bits of Y (luma), followed by 8 bits of Cr (chroma V) for the next pixel and then 8 bits of Y. To reconstruct full resolution Y, Cb, Cr pixel values, [[chroma subsampling|chroma upsampling]] must be used.

== References ==
{{Reflist}}

== See also ==
* [[Rec. 601]]
* [[Serial digital interface]]

[[Category:Digital television]]
[[Category:Film and video technology]]
[[Category:Serial digital interface]]
[[Category:ITU-R recommendations]]
[[Category:International Telecommunication Union]]

{{video-tech-stub}}

ITU-R BT.656 - Revision history

98.115.164.53: copyedit