BT601

ITU-R BT.601, commonly referred to as Rec. 601, is a standard for digital representation of standard-definition television signals. It defines the encoding of color for SD video using YCbCr color difference signals and specifies chroma subsampling, sampling rates, and quantization. The standard is a foundational part of SDTV workflows and informs digital formats used in broadcasts, DVDs, and early digital interfaces.

Technical details include a 4:2:2 chroma subsampling scheme, where the luma component (Y) is sampled at 13.5 MHz and the chroma components (Cb, Cr) at 6.75 MHz. Most implementations use 8-bit data with a limited color range: Y typically 16–235 and Cb/Cr 16–240. The standard also defines the matrix for converting between RGB and YCbCr, with commonly used equations such as Y' = 0.299R + 0.587G + 0.114B, Cb = -0.168736R - 0.331264G + 0.5B + 128, and Cr = 0.5R - 0.418688G - 0.081312B + 128. These parameters ensure consistent color reproduction across SD equipment and content.

Usage and scope: BT.601 underpins Standard-Definition formats and is closely associated with digital SD video interfaces (including SMPTE 259M). It is distinct from ITU-R BT.709, which governs high-definition video. While BT.601 remains foundational for SD content, modern HD and UHD workflows often rely on Rec. 709 and Rec. 2020, with upscaling and color-space conversion bridging between standards when necessary.