Editing Standard-definition television

{{Short description|Digital TV with similar definition to analog broadcasts}}
{{Use American English|date=September 2020}}
{{Redirect|SDTV|a television broadcasting network in Shandong Province|Shandong Television}}
{{Refimprove|date=June 2022}}

[[File:TV-line-count-world.svg|thumb|500px|right|SDTV resolution by nation: for historical reasons, different countries use either [[480i]] or [[576i]] as their standard-definition picture format]]

'''Standard-definition television''' ('''SDTV'''; also '''standard definition''' or '''SD''') is a television system that uses a resolution that is not considered to be either [[high-definition television|high]] or [[enhanced definition]].<ref name=":0">{{cite web |title=Standard definition television (SDTV) |url=https://www.atsc.org/atsc-glossary/standard-definition-television-sdtv/ |access-date=2023-06-18 |website=ATSC: NextGen TV |language=en-US}}</ref> ''Standard'' refers to offering a similar resolution to the [[broadcast television systems#ITU standards|analog broadcast systems]] used when it was introduced.<ref name=":0" /><ref>{{cite web |title=HDTV |url=http://www.siliconimaging.com/hdtv.htm |access-date=2023-06-18 |website=siliconimaging.com}}</ref>

== History and characteristics ==
SDTV originated from the need for a standard to digitize analog TV (defined in [[BT.601]]) and is now used for digital TV broadcasts and home appliances such as game consoles and DVD disc players.<ref name=":02" /><ref>{{cite web |title=BT.601: Studio encoding parameters of digital television for standard 4:3 and wide screen 16:9 aspect ratios |url=https://www.itu.int/rec/R-REC-BT.601/ |publisher=ITU}}</ref>

Digital SDTV broadcast eliminates the [[Ghosting (television)|ghosting]] and [[Noise (video)|noisy images]] associated with analog systems. However, if the reception has interference or is poor, where the error correction cannot compensate one will encounter various other artifacts such as image freezing, stuttering, or dropouts from missing [[Intra-frame coding|intra-frames]] or blockiness from missing [[macroblock]]s. The audio encoding is the last to suffer a loss due to the lower bandwidth requirements.{{Citation needed|date=April 2024}}

Standards that support digital SDTV broadcast include [[DVB]], [[ATSC]], and [[ISDB]].<ref name=":1" /> The last two were originally developed for [[HDTV]], but are also used for their ability to deliver multiple SD video and audio streams via [[multiplexing]].

== PAL and NTSC ==
The two SDTV signal types are [[576i]] (with 576 [[interlaced]] lines of resolution,<ref name="auto">{{cite web |title=What means 576i? |url=http://www.afterdawn.com/glossary/terms/576i.cfm |website=Afterdawn.com}}</ref> derived from the European-developed [[PAL]] and [[SECAM]] systems), and [[480i]] (with 480 interlaced lines of resolution,<ref name=":02">{{cite web |title=What means 480i? |url=https://www.afterdawn.com/glossary/term.cfm/480i |website=afterdawn.com}}</ref> based on the American [[NTSC]] system). SDTV [[refresh rate]]s are 25, 29.97 and 30 [[frames per second]], again based on the analog systems mentioned.

In North America, digital SDTV is broadcast in the same [[Fullscreen (aspect ratio)|4:3 fullscreen aspect ratio]] as NTSC signals, with [[16:9 aspect ratio|widescreen]] content often being [[center cut]].<ref name=":1">{{cite web |title=All-Digital Television Is Coming (And Sooner Than You Think!) |url=http://www.fcc.gov/cgb/consumerfacts/digitaltv.html |url-status=dead |archive-url=https://web.archive.org/web/20080929041527/http://www.fcc.gov/cgb/consumerfacts/digitaltv.html |archive-date=Sep 29, 2008 |website=Federal Communications Commission}}</ref>

In other parts of the world that used the PAL or SECAM color systems, digital standard-definition television is now usually shown with a [[16:9 aspect ratio]], with the transition occurring between the mid-1990s and late-2000s depending on the region. Older programs with a 4:3 aspect ratio are broadcast with [[Active Format Description|a flag]] that switches the display to 4:3. Some broadcasters prefer to reduce the horizontal resolution by anamorphically scaling the video into a [[pillarbox]].{{Citation needed|date=April 2024}}

== Pixel aspect ratio ==
{| class="wikitable"
|+ [[Pixel aspect ratio]]s for the scaling of various kinds of SDTV video lines
! Video format
! Display aspect ratio (DAR)
! Resolution
! Pixel aspect ratio (PAR)
! After horizontal scaling
|-
| rowspan="2" | [[480i]]
| rowspan="2" | 4:3
| {{resx|704×480}} ([[Raster interrupt|horizontal blanking]] cropped)
| rowspan="2" | 10:11
| {{resx|640×480}}
|-
| {{resx|720×480}} (full frame)
| {{resx|655×480}}
|-
| rowspan="2" | [[480i]]
| rowspan="2" | 16:9
| {{resx|704×480}} ([[Raster interrupt|horizontal blanking]] cropped)
| rowspan="2" | 40:33
| {{resx|854×480}}
|-
| {{resx|720×480}} (full frame)
| {{resx|873×480}}
|-
|-
| rowspan="2" | [[576i]]
| rowspan="2" | 4:3
| {{resx|704×576}} ([[Raster interrupt|horizontal blanking]] cropped)
| rowspan="2" | 12:11
| {{resx|768×576}}
|-
| {{resx|720×576}} (full frame)
| {{resx|788×576}}
|-
| rowspan="2" | [[576i]]
| rowspan="2" | 16:9
| {{resx|704×576}} ([[Raster interrupt|horizontal blanking]] cropped)
| rowspan="2" | 16:11
| {{resx|1024×576}}
|-
| {{resx|720×576}} (full frame)
| {{resx|1050×576}}
|}

The pixel aspect ratio is the same for 720- and 704-pixel resolutions because the visible image (be it 4:3 or 16:9) is contained in the center 704 horizontal pixels of the digital frame. In the case of a digital video line having 720 horizontal pixels (including horizontal blanking), only the center 704 pixels contain the actual 4:3 or 16:9 image, and the 8-pixel-wide stripes on either side are called [[nominal analog blanking]] or [[horizontal blanking]] and should be discarded when displaying the image. Nominal analog blanking should not be confused with [[overscan]], as overscan areas are part of the actual 4:3 or 16:9 image.

For [[SMPTE 259M-C]] compliance, an SDTV broadcast image is scaled to 720 pixels wide for every 480 NTSC (or 576 PAL) lines of the image with the amount of non-proportional line scaling dependent on either the display or [[pixel aspect ratio]]. Only 704 center pixels contain the actual image and 16 pixels are reserved for horizontal blanking, though a number of broadcasters fill the whole 720 frames.{{cn|date=April 2024}} The display ratio for broadcast widescreen is commonly 16:9 (pixel aspect ratio of 40:33 for [[anamorphic widescreen|anamorphic]]); the display ratio for a traditional or [[letterboxed]] broadcast is 4:3 (pixel aspect ratio of 10:11).

An SDTV image outside the constraints of the SMPTE standards requires no non-proportional scaling with 640 pixels (defined by the adopted IBM [[VGA]] standard) for every line of the image. The display and pixel aspect ratio is generally not required with the line height defining the aspect. For widescreen 16:9, 360 lines define a widescreen image and for traditional 4:3, 480 lines define an image.

== See also ==
{{portal|Television}}

* {{anl|Digital Audio Broadcasting}}
* {{anl|Moving Picture Experts Group}}
* {{anl|Rec. 601}}

== References ==
{{reflist}}

== External links ==
* [http://lurkertech.com/lg/video-systems Programmer's Guide to Video Systems]

{{TV resolution}}
{{Video formats}}

[[Category:ATSC]]
[[Category:Digital television]]
[[Category:Broadcast engineering]]
[[Category:Broadband]]
[[Category:Television technology]]