Standard-definition television (SDTV) is a television system that uses a resolution that is not considered to be either enhanced-definition television (EDTV) or high-definition television (HDTV). The term is usually used in reference to digital television, in particular when broadcasting at the same (or similar) resolution as analog systems. The two common SDTV signal types are 576i, derived from the European-developed PAL and SECAM systems with 576 interlaced lines of resolution; and 480i, based on the American National Television System Committee NTSC system.
In the USA, digital SDTV is broadcast in the same 4:3 aspect ratio as NTSC signals.[1] However, in areas 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.
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When standard-definition television signals are transmitted in digital form, its pixels have rectangular shape, as opposed to square pixels that are used in modern computer monitors and modern implementations of HDTV. The table below summarizes pixel aspect ratios for various kinds of SDTV video signal. Note that the actual image (be it 4:3 or 16:9) is always contained in the center 704 horizontal pixels of the digital frame, regardless of how many horizontal pixels (704 or 720) are used. In case of digital video signal having 720 horizontal pixels, only the center 704 pixels contain actual 4:3 or 16:9 image, and the 8 pixel wide stripes from either side are called nominal analogue blanking and should be discarded before displaying the image. Nominal analogue blanking should not be confused with overscan, as overscan areas are part of the actual 4:3 or 16:9 image.
| Video Format | Resolution | Pixel Aspect Ratio | Equivalent square-pixel resolution |
| PAL 4:3 | 704×576 | 12:11 | 768×576 |
| PAL 4:3 | 720×576 | 12:11 | 786×576 |
| PAL 16:9 | 704×576 | 16:11 | 1024×576 |
| PAL 16:9 | 720×576 | 16:11 | 1048×576 |
| NTSC 4:3 | 704×480 | 10:11 | 640×480 |
| NTSC 4:3 | 720×480 | 10:11 | 654×480 |
| NTSC 16:9 | 704×480 | 40:33 | 854×480 |
| NTSC 16:9 | 720×480 | 40:33 | 872×480 |
The pixel aspect ratio is always the same for corresponding 720 and 704 pixel resolutions because the center part of a 720 pixels wide image is equal to the corresponding 704 pixels wide image
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