Standard-definition television or SDTV refers to television systems that have a lower resolution than HDTV systems. The term is usually used in reference to digital television, in particular when broadcasting at the same (or similar) resolution as analog systems.
In ATSC, SDTV can be broadcast in 704 pixels × 480 lines (16:9 and 4:3 aspect ratios) or 640 pixels × 480 lines (4:3 ratio). The refresh rate can be any of 24, 30 or 60 pictures per second.
SDTV in 4:3 aspect ratio has the same appearance as the regular analogue TV (NTSC, PAL, PAL2 , SECAM) minus the ghosting, snowy images and static noises.
Standards that can broadcast digital SDTV include DVB, ATSC and ISDB. The latter two were originally developed for HDTV, but they have proved to be more often used for their ability to deliver multiple SD video and audio streams via multiplexing, than to use the entire bitstream for one HD channel.