Analogue broadcasting is a world without choices. Every analogue television uses 480 lines, the same interlaced scanning at 30 frames (complete pictures) per second and the same 4:3 aspect ratio screen. This system is abbreviated as 480i or 480/30i.
In comparison, the ATSC recommendations give broadcasters and viewers a world of choice. The signal for transmission may be scanned either Interlaced (i) or Progressive (p). Interlaced scanning means that each frame is sent as two “fields” – one with the odd-numbered scanning lines (A) and the next with even-numbered lines (B), similar to current TVs. In progressive scanning, all lines of the frame are scanned sequentially and sent as a single frame. There’s also a choice of 24, 30 or 60 frames per second and a choice of standard 4:3 or wide-screen 16:9 aspect ratio. In total there are 18 formats, each suited to a specific purpose.