ITU-R BT.601 - technical definition

An ITU-R standard for component digital video (YCbCr). It was designed to provide a common digital standard for interoperability between the three analog video/TV systems (NTSC, PAL and SECAM). ITU-R BT.601 enables their signals to be converted to digital and then easily converted back again to any of the three for distribution.

The choice of a luma sampling frequency of 13.5 MHz and a chroma frequency of 6.75MHz (4:2:2) yields 720 samples of active data per line for both NTSC and PAL/SECAM. See YUV, YCbCr and chroma subsampling.

BT.601 and BT.709
BT.601 was established in 1987 for 525-line (NTSC) and 625-line PAL/SECAM. In 1990, BT.709 was introduced for high definition (HDTV) with specifications for 1125 and 1250 lines. In 2000, BT.709-4 added 1080 lines to conform to the DTV standard. Following are the sampling frequencies used for both standards.

                  Luma        Chroma
   Standard       Sampling    Sampling

   BT.601 SDTV    13.5 MHz     6.75 MHz  (4:2:2)
   BT.709 HDTV    74.25 MHz   37.125 MHz (4:2:2)