Standard-definition television (or SDTV) is a television system that has a resolution that meets standards but 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. In the USA, SDTV refers to digital television broadcast in 4:3 aspect ratio, the same aspect ratio as NTSC signals. When a television set is labeled "SDTV", this means that the set includes an ATSC tuner, but scans its picture in the same 480i pattern used in NTSC.
In most of Europe standard-definition television is usually shown with a aspect ratio as most people have a television with a aspect ratio. Older programs with a aspect ratio are shown in . The aspect ratio is also known as 576i.
All content recorded for television broadcast in the UK and the Republic of Ireland has a aspect ratio.
Standards that support digital SDTV broadcast include DVB, ATSC Standards 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 aspect ratio (40:33 rectangular (unsquare) pixel), 720 pixels × 480 lines with aspect ratio (10:11 rectangular pixel) or 640 pixels × 480 lines with 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.
Pixel aspect ratio
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.
{| class="wikitable"
|- align=left
|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.
See also
ATSC Standards
Digital television
Digital audio broadcasting
DVB
Enhanced-definition television
High-definition television
ISDB
MPEG
SBTVD
References
External links
Programmer's Guide to Video Systems
Category:ATSC
Category:Digital television
Category:Broadcast engineering
Category:Broadband
