Standard-definition television
Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition. SDTV and high-definition television (HDTV) are the two categories of display formats for digital television (DTV) transmissions. "Standard" refers to the fact that it was the prevailing specification for broadcast (and later, cable) television in the mid- to late-20th century.
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 NTSC system. Common SDTV refresh rates are 25, 29.97 and 30 frames per second.[lower-alpha 1] Both systems use a 4:3 aspect ratio.
Standards that support digital SDTV broadcast include DVB, ATSC, and ISDB. 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. In North America, digital SDTV is broadcast in the same 4:3 aspect ratio as NTSC signals, with widescreen content often being center cut.[1] However, the aspect ratio of widescreen content may be preserved in a 4:3 frame through letterboxing. 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 mid-2000s depending on region. Older programs with a 4:3 aspect ratio are broadcast with a flag that switches the display to 4:3.[lower-alpha 2]
Digital SDTV eliminates the ghosting and 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-frames or blockiness from missing macroblocks.[lower-alpha 3]
Pixel aspect ratio
The table below summarizes pixel aspect ratios for the scaling of various kinds of SDTV video lines.
| Video format | Display aspect ratio (DAR) | Resolution | Pixel aspect ratio (PAR) | After horizontal scaling |
|---|---|---|---|---|
| 480i | 4:3 | 704×480 (horizontal blanking cropped) |
10:11 | 640×480 |
| 720×480 (full frame) | 654×480 | |||
| 480i | 16:9 | 704×480 (horizontal blanking cropped) |
40:33 | 854×480 |
| 720×480 (full frame) | 872×480 | |||
| 576i | 4:3 | 704×576 (horizontal blanking cropped) |
12:11 | 768×576 |
| 720×576 (full frame) | 786×576 | |||
| 576i | 16:9 | 704×576 (horizontal blanking cropped) |
16:11 | 1024×576 |
| 720×576 (full frame) | 1048×576 |
The pixel aspect ratio is always the same for corresponding 720 and 704 pixel resolutions because the center part of a 720-pixel-wide image is equal to the corresponding 704-pixel-wide image. 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 line 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 for horizontal 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.
For SMPTE 259M-C compliance, a SDTV broadcast image is scaled to 720 pixels wide[lower-alpha 4] 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. The display ratio for broadcast widescreen is commonly 16:9,[lower-alpha 5] the display ratio for a traditional or letterboxed broadcast is 4:3.[lower-alpha 6]
An SDTV image outside the constraints of the SMPTE standards requires no non-proportional scaling with 640 pixels[lower-alpha 7] 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
Notes
- 50 and 60 rates are sometimes used as frame-doubled versions of 25 and 30 rates to mitigate interlace artifacts.
- Some broadcasters prefer to reduce the horizontal resolution by anamorphically scaling the video into a pillarbox.
- The audio encoding is the last to suffer loss due to the lower bandwidth requirements.
- Only 704 center pixels contain the image and 16 pixels are reserved for horizontal blanking though a number of broadcasters fill the whole 720 frame.
- Pixel aspect ratio of 40:33 for anamorphic
- Pixel aspect ratio of 10:11
- Defined by the adopted IBM VGA standard