This standard defines the digital encoding parameters of SD television for studios. It is the international standard for digitizing component television video in both 525 and 625 line systems and is derived from SMPTE RP125. ITU-R BT.601 deals with both color difference (Y, R-Y, B-Y) and RGB component video and defines sampling systems, RGB/Y, R-Y, B-Y matrix values and filter characteristics. It does not actually define the electro-mechanical interface; see ITU-R BT. 656.
ITU-R BT.601 is normally taken to refer to color difference component digital video (rather than RGB), for which it defines 4:2:2 sampling at 13.5 MHz with 720 (4) luminance samples per active line. The color difference signals R-Y and B-Y are sampled at 6.75 MHz with 360 (2) samples per active line. Its depth may be 8 or 10 bits.
Some headroom is allowed so, with 10-bit sampling, black level is at 64 (not 0) and white at level 940 (not 1023) – to minimize clipping of noise and overshoots. With 210 levels each for Y (luminance), Cr and Cb (the digitized color difference signals) = 230 – over a billion unique colors can be defined.
The sampling frequency of 13.5 MHz was chosen to provide a politically acceptable common sampling standard between 525/59.94 and 625/50 systems, being a multiple of 2.25 MHz, the lowest common frequency to provide a static sampling pattern for both.