There comes a time when everything old… is old. TV signals that are designed to work on old TV sets are referred to as “standard definition” or “SD” for short. SD describes the way old VHS tapes, the way that late-night infomercials look, and the way that everyone used to watch TV.
If you’re really going to get nitpicky, old-school TV signals aren’t SD, they’re NTSC. Older TVs used analog signals because, well, because digital wasn’t a thing in the 1930s when TV was invented. The United States television standard was an analog signal within a 6MHz channel, carrying 525 lines of vertical resolution in an interlaced format at 60Hz. Only about 480 lines were used for visible picture; the rest were used for information that made the whole system work, carrying weird names like “vertical blank.”
SD didn’t really become a “thing” until there was something to compare it to. The idea of high-definition television actually started in the 1960s and by the 1980s Japanese broadcasters were doing experimental HD recordings and broadcasts. By the time the USA got involved, the key to making the whole thing work was going digital.
Digital technology allowed for a high-definition signal to actually fit into that same 6MHz channel through the use of some pretty fancy (by 2005 standards) compression techniques. The US government adopted standards proposed by the ATSC commitee for broadcasting a digital signal with a resolution of up to 1920×1080 on a regular TV channel. At the same time they came up with a standard for broadcasting regular old television digitally. Technically, only digital signals that mimic older TV signals are actually “SD.”
Standard-definition describes a signal with resolution of either 640×480 or 720×480 (for widescreen applications like DVD) showing 30 full frames per second. That’s about one-quarter of the information in a high-definition picture, one-sixteenth the information in a 4K picture, and about one-fourty-eighth (1/48th) the information in your average smartphone picture. It’s pretty paltry when you look at it that way. But then again, you knew that just from looking at it.