ample rate selection produce samples at nominally the same positions in the picture. Making this feature common simplifies conver- ITU BT.1358 defines the progressive SDTV video sion from one standard to another. signals, also known as 480p or 576p, or Enhanced Digital For 480i and 576i video systems, several Y sampling Television (EDTV). The sample rate is doubled to frequencies were initially examined, including four times 27 MHz (4:3 aspect ratio) or 36 MHz (16:9 aspect ratio) Fsc. However, the four times Fsc sampling rates did not in order to keep the same static orthogonal sampling grid support the requirement of simplifying international ex- as that used by BT.601. change of programs, SO they were dropped in favor of a single common sampling rate. Because the lowest sample rate possible (while still supporting quality video) was HDTV sample rate selection a goal, a 12 MHz sample rate was preferred for a long time, but eventually was considered to be too close to the ITU BT.709 defines the 720p, 1080i, and 1080p video Nyquist limit, complicating the filtering requirements. signals, respectively. With HDTV, a different technique When the frequencies between 12 MHz and 14.3 MHz was used-the number of active samples per line and the were examined, it became evident that a 13.5 MHz number of active lines per frame is constant regardless of sample rate for Y provided some commonality between the frame rate. Thus, in order to keep a static orthogonal 480i and 576i systems. Cb and Cr, being color difference sampling grid, each frame rate uses a different sample signals, do not require the same bandwidth as the Y, SO may clock rate. be sampled at one-half the Y sample rate, or 6.75 MHz. The "4:2:2" notation now commonly used originally applied to NTSC and PAL video, implying that Y, U and V 480i and 480p systems were sampled at 4x, 2x, and 2x the color subcarrier frequency, respectively. The "4:2:2" notation was then Interlaced analog composite video adapted to BT.601 digital component video, implying that the sampl