View source for Rec. 601

{{Short description|Standard from the International Telecommunication Union}}
{{Multiple issues|{{one source|date=June 2023}}
{{original research|date=June 2023}}}}
{{Infobox technology standard
| title             = Rec. 601
| long_name         = 
| native_name       = <!-- Name in local language. If more than one, separate using {{plain list}} -->
| native_name_lang  = <!-- ISO 639-2 code e.g. "fr" for French. If more than one, use {{lang}} inside native_name items instead -->
| image             = CIExy1931 Rec 601.svg
| size              = 300
| caption           = Rec. 601 [[chromaticity]] diagram, with 625-line ([[PAL]] and [[SECAM]]) and 525-line ([[NTSC#SMPTE C|NTSC SMPTE C]] primaries) areas shown in black and white
| status            = Approved
| year_started      = <!-- {{Start date|YYYY|MM|DD|df=y}} -->
| first_published   = {{Start date and age|1982}}
| version           = BT.601-7
| version_date      = {{Start date and age|2011|03|08}}
| preview           =
| preview_date      = 
| organization      = 
| committee         = 
| editors           = 
| authors           = [[ITU-R]] ([[Comité consultatif international pour la radio|CCIR]])
| base_standards    = Rec.601, BT.601, CCIR 601
| related_standards = ISO/IEC [[MPEG]], ITU-T [[H.26x]]
| abbreviation      = 
| domain            = [[Digital image processing]]
| license           = 
| website           = {{URL|https://www.itu.int/rec/R-REC-BT.601/}}
}}
'''[[ITU-R]] Recommendation BT.601''', more commonly known by the abbreviations '''Rec.&nbsp;601''' or '''BT.601''' (or its former name '''CCIR&nbsp;601'''), is a standard originally issued in 1982 by the [[Comité consultatif international pour la radio|CCIR]] (an organization, which has since been renamed as the [[ITU-R|International Telecommunication Union{{snd}} Radiocommunication sector]]) for encoding [[interlaced]] [[analog video]] signals in [[digital video]] form.<ref>{{cite web |url=https://www.itu.int/rec/R-REC-BT.601/ |title=BT.601: Studio encoding parameters of digital television for standard 4:3 and wide screen 16:9 aspect ratios |publisher=ITU}}</ref> It includes methods of encoding 525-line 60&nbsp;[[Hertz|Hz]] and 625-line 50&nbsp;Hz signals, both with an active region covering 720 [[Luminance (video)|luminance]] samples and 360 [[chrominance]] samples per line. The [[color]] encoding system is known as [[chroma subsampling#4:2:2|YCbCr&nbsp;4:2:2]].

The Rec.&nbsp;601 video raster format has been re-used in a number of later standards, including the ISO/IEC [[MPEG]] and ITU-T [[H.26x]] compressed formats, although compressed formats for consumer applications usually use [[chroma subsampling]] reduced from the 4:2:2 sampling specified in Rec.&nbsp;601 to [[chroma subsampling#4:2:0|4:2:0]].

The standard has been revised several times in its history. Its seventh edition, referred to as BT.601-7, was approved in March 2011 and was formally published in October 2011.

==Background and history==
In the early 1980s, digital television equipment was beginning to emerge, but each manufacturer was developing their own proprietary digital versions of existing analog standards like [[PAL]], [[SECAM]], and [[NTSC]].<ref name="wood">{{cite web |last1=Wood |first1=David |title=Rec. ITU-R BT.601 25th Anniversary and still ´in force´ - the bridge that joined the analogue and digital worlds (1982-2007) |url=https://www.itu.int/net/ITU-R/index.asp?category=information&rlink=rec-601&lang=en |website=www.itu.int |access-date=10 March 2024}}</ref>

At an ITU meeting in autumn 1981, CCIR Study Group 11 approved document 11/1027 describing the parameter values for a unified digital video format. This was adopted as Draft Rec. AA/11 "Encoding Parameters for Digital Television for Studios" by the CCIR Plenary Assembly in February 1982, later becoming ITU-R Rec. 601.<ref name="wood"/>

The key feature allowing a globally accepted digital standard were the use of component coding and choosing a luminance sampling frequency that was a common multiple of the line frequencies used in analog standards. This "orthogonal" sampling approach originated from Stanley Baron of NBC.<ref name="wood"/>

Preparation preceded the CCIR approval, including laboratory testing around the world to validate the proposed parameter values. International negotiations and efforts to build consensus were led by figures like [[Mark Krivosheev]], [[Richard Green (telecommunication)|Richard Green]], and representatives from Japan and Europe.<ref name="wood"/>

==Signal format==
The Rec.&nbsp;601 signal can be regarded as if it is a digitally encoded analog [[component video]] signal, and thus the sampling includes data for the horizontal and vertical sync and blanking intervals. Regardless of the frame rate, the luminance sampling frequency is 13.5&nbsp;MHz. The samples are uniformly [[Quantization (signal processing)|quantized]] using 8- or 10-bit [[PCM]] codes in the [[YCbCr]] domain.

For each 8-bit luminance sample, the nominal value to represent black is 16, and the value for white is 235. Eight-bit code values from 1 through 15 provide footroom and can be used to accommodate transient signal content such as filter undershoots. Similarly, code values 236 through 254 provide headroom and can be used to accommodate transient signal content such as [[Ringing artifacts|filter overshoots]]. The values 0 and 255 are used to encode the sync pulses and are forbidden within the visible picture area. The Cb and Cr samples are [[Offset binary|unsigned]] and use the value 128 to encode the neutral color difference value, as used when encoding a white, grey or black area.

==Primary chromaticities==
[[File:SMPTE-C RGB color cube.png|thumb|BT.601 525-line color cube <small>(image encoded with an ICC profile)</small>]]
[[File:PAL SECAM RGB Color cube.png|thumb|BT.601 625-line color cube <small>(image encoded with an ICC profile)</small>]]
Slightly different primaries are specified for the 625-line ([[PAL]] and [[SECAM]]) and 525-line ([[NTSC#SMPTE C|NTSC SMPTE C]] primaries) systems. Earlier versions of the standard (prior to BT.601-6, approved in January 2007) did not contain an explicit definition of the color primaries.

{| class="wikitable" 
|+ RGB color space parameters for Rec.&nbsp;601
|-
! rowspan="2" |[[Color space]]
! colspan="2" |[[White point]] ([[Illuminant D65|D<sub>65</sub>]])
! colspan="6" |[[Primary color]]
|-
! ''x''<sub>W</sub> !! ''y''<sub>W</sub>
! ''x''<sub>R</sub> !! ''y''<sub>R</sub>
! ''x''<sub>G</sub> !! ''y''<sub>G</sub>
! ''x''<sub>B</sub> !! ''y''<sub>B</sub>
|-
|align="center"| 625 lines
|rowspan=2| 0.3127 ||rowspan=2| 0.3290
| 0.640 || 0.330
| 0.290 || 0.600
| 0.150 || 0.060
|-
|align="center"| 525 lines
| 0.630 || 0.340
| 0.310 || 0.595
| 0.155 || 0.070
|}

==Transfer characteristics==
Rec. 601 defines a [[Nonlinear system|nonlinear]] [[Transfer functions in imaging|transfer function]] which is [[Linear system|linear]] near 0 and then transfers to a [[gamma correction|gamma curve]] for the rest of the [[luminance]] range:

: <math>E = \begin{cases}
 4.500 L                & L < 0.018,\\
 1.099 L^{0.45} - 0.099 & L \ge 0.018.
\end{cases}
</math>

== Awards ==
The CCIR received a 1982–83 [[Technology and Engineering Emmy Award]] for its development of the Rec.&nbsp;601 standard.<ref>[http://www.itu.int/ITU-R/index.asp?category=information&rlink=rec-601&lang=en Rec. ITU-R BT.601 25th Anniversary and still "in force"{{snd}} the bridge that joined the analogue and digital worlds (1982–2007)], ITU-R, 2007.</ref>

== See also ==
* [[Digital component video]]
* [[YCbCr]]
* [[Rec. 709]], the corresponding standard for [[high-definition television]] (HDTV)
* [[Rec. 2020]], ITU-R Recommendation for [[ultra-high-definition television]] (UHDTV)
* [[ITU-R BT.656]], ITU-R Recommendation for parallel and serial transmission formats for BT.601 video
* [[Pixel aspect ratio]]

== References ==
{{reflist}}
{{color space}}

[[Category:Digital television]]
[[Category:Film and video technology]]
[[Category:ITU-R recommendations]]
[[Category:Color space]]