Technology Trade-offs

With analog systems, problems may arise with repeated processing, causing an extension of the blanking intervals and softening of the blanking edges. Using 720 digital samples for the active line period accommodates the range of analog blanking tolerances of both the 480i and 576i systems. Therefore, repeated processing may be done without affecting the digital blanking interval. Blanking to define the analog picture width need only be done once, preferably at the display or upon conversion to analog video.

Initially, BT.601 supported only 480i and 576i systems with a 4:3 aspect ratio (720×480i and 720×576i active resolutions). Support for a 16:9 aspect ratio was then added (960×480i and 960×576i active resolutions) using an 18 MHz sample rate.

EDTV Sample Rate Selection

ITU BT.1358 defines the progressive SDTV video signals, also known as 480p or 576p, or Enhanced Digital Television (EDTV). The sample rate is doubled to 27 MHz (4:3 aspect ratio) or 36 MHz (16:9 aspect ratio) in order to keep the same static orthogonal sampling grid as that used by BT.601.

HDTV Sample Rate Selection

ITU BT.709 defines the 720p, 1080i, and 1080p video signals, respectively. With HDTV, a different technique was used—the number of active samples per line and the number of active lines per frame is constant, regardless of the frame rate. Thus, in order to keep a static orthogonal sampling grid, each frame rate uses a different sample clock rate.