A standard for sampling analog waveforms to convert them into digital data. The official sampling standard for 625/50 and 525/60 television is ITU-R BT.601. ITU-R BT.709 and SMPTE 274M specify sampling for HD formats. They are generally written as the number of pixels per line x number of lines per frame/vertical refresh rate (in Hz) progressive or interlaced (P or I), e.g. 1920 x 1080/50I. Sometimes the pixel count of the lines is omitted (but understood), making the example 1080/50I.