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SDTV refers to a digital TV (DTV) standard with a resolution approximately matching either the PAL or NTSC analog TV standard. SDTV resolution is loosely based on the ITU-R BT.601 standard for digitizing analog video, providing resolutions of 720x480 for NTSC and 720x576 for NTSC. Before HDTV prices came down many consumers purchased either SDTVs or EDTVs, believing them to be high definition because they were digital. While these displays do provide significant advantages over standard analog TVs when it comes to digital content such as DVDs, they must downscale all HD signals, resulting in much lower quality than even a 720p HDTV.
SDTV vs NTSC and PAL
SDTVs are digital, meaning analog NTSC and PAL standards don't apply to them. However, they can still be used for watching analog television with the appropriate tuner or other device to receive the signal and convert to digital. SDTV formats include 480i and 480p as a replacement for NTSC or 576i and 576p for PAL. The 'i' or 'p' denotes either interlaced or progressive video. Although video may be stored either way, the actual TVs are almost always progressive, meaning interlaced video will be deinterlaced at some point. If the video signal is sent across an analog connection, such as S-Video or Composite Video it will always be interlaced when it gets to the TV. Progressive scan DVD players can deliver progressive video to these displays.
SDTVs have the same aspect ratio (AR) as their analog equivalents of 4:3, or 1.33:1. Also like those TVs, widescreen video displayed on a SDTV must be letterboxed, adding borders to the top and bottom and vertically squeezing the image, to maintain the correct aspect ratio.
A related standard, EDTV also uses the same number of horizontal line as either NTSC or PAL, but has an AR of 16:9 (1.78:1) to match anamorphic DVDs. Although this means widescreen video is shown with its original vertical resolution, it also means that 4:3 video must either be pillarboxed, adding borders to the sides, stretched or zoomed to fill the screen. Like SDTVs, EDTVs are not considered HDTVs, and must downscale all HD video for display.
With their limited resolution and 4:3 AR, SDTVs are only suitable for standard definition DVDs and SDTV broadcasts