Standard-definition television (SDTV) is a television system that uses a resolution that is not considered to be either high-definition television (HDTV 720p and 1080p) or enhanced-definition television (EDTV 480p). The two common SDTV signal types are 576i, with 576 interlaced lines of resolution, derived from the European-developed PAL and SECAM systems; and 480i based on the American National Television System Committee NTSC system.
In the US, digital SDTV is broadcast in the same 4:3 aspect ratio as NTSC signals. However, in other parts of the world that used the PAL or SECAM analog standards, standard-definition television is now usually shown with a 16:9 aspect ratio, with the transition occurring between the mid-1990s and mid-2000s. Older programs with a 4:3 aspect ratio are shown in 4:3.
Standards that support digital SDTV broadcast include DVB, ATSC and ISDB. The last two were originally developed for HDTV, but are more often used for their ability to deliver multiple SD video and audio streams via multiplexing, than for using the entire bitstream for one HD channel.
In ATSC Standards, SDTV can be broadcast in 720 pixels × 480 lines with 16:9 aspect ratio (40:33 rectangular (unsquare) pixel), 720 pixels × 480 lines with 4:3 aspect ratio (10:11 rectangular pixel) or 640 pixels × 480 lines with 4:3 ratio. The refresh rate can be 24, 30 or 60 frames per second.
Digital SDTV in 4:3 aspect ratio has the same appearance as regular analog TV (NTSC, PAL, SECAM) without the ghosting, snowy images and white noise. However, if the reception is poor, one may encounter various other artifacts such as blockiness and stuttering.
Read more about Standard-definition Television: Pixel Aspect Ratio
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