Low-definition television

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Low-definition television or LDTV refers to television systems that have a lower screen resolution than standard-definition television systems. The term is usually used in reference to digital television, in particular when broadcasting at the same (or similar) resolution as low-definition analog TV systems. Mobile DTV systems usually transmit in low definition, as do all slow-scan TV systems.

Sources[edit]

The most common source of LDTV programming is the Internet, where mass distribution of higher-resolution video files could overwhelm computer servers and take too long to download. Many mobile phones and portable devices such as Apple’s iPod Nano, or Sony’s PlayStation Portable use LDTV video, as higher-resolution files would be excessive to the needs of their small screens (320 × 240 and 480 × 272 pixels respectively). The current generation of iPod Nanos have LDTV screens, as do the first three generations of iPod Touch and iPhone (480 × 320).

A VHS videotape could be considered SDTV due to its resolution (approximately 480i x 320), but using VHS for professional production will yield results comparable to LDTV because of VHS's low bandwidth. VHS supports interlace and high motion, which are not typical of LDTV signals. Professional-level Betacam SP tape produces approx 440 × 486; some college TV studios use Super VHS at ~560 × 486.

Older video game consoles and home computers generated a nonstandard NTSC or PAL signal which sent a single field type which prevented fields from interlacing.[1][2] This is equivalent to 240p and 288p respectively, and was used due to requiring less resources and producing a progressive and stable signal. Conversely, the FCC forbade TV stations from broadcasting in this format. The Video CD format was introduced on such a console (CD-i), and it likewise uses a progressive LDTV signal (352 × 240 or 352 × 288), which is half the vertical resolution of SDTV.

With the introduction of 16-bit game consoles, 480i was supported for the first time, but rarely used due to limited memory and processing power. Thus, 240p remained the primary format on all fifth generation consoles (Sega Saturn, PlayStation and Nintendo 64). With the advent of sixth generation consoles and the launch of the Dreamcast, 480i use became more common, and 240p usage declined.

More recent game systems tend to use only properly interlaced NTSC or PAL in addition to higher resolution modes, except when running games designed for older, compatible systems in their native modes. The PlayStation 2 generates 240p/288p if a PlayStation game calls for this mode, as do many Virtual Console emulated games on Wii. Nintendo's official software development kit documentation refers to 240p as 'non-interlaced mode' or 'double-strike'.[3][4]

Shortly after the launch of the Wii Virtual Console service many users with component video cables experienced problems displaying some Virtual Console games due to certain TV models/manufacturers not supporting 240p over a component video connection. Nintendo's solution was to implement 'Wii Component Cable Interlace Mode' which forces the emulator to output 480i instead of 240p,[5] however many games released prior have still not been updated.[6]

Teleconferencing LDTV[edit]

Upcoming sources of LDTV using standard broadcasting techniques include mobile TV services powered by DVB-H, DMB, or ATSC-M/H. However, this kind of LDTV transmission technology is based on existent LDTV teleconferencing standards that have been in place for a decade or more.

Resolutions[edit]

Standard Class Resolution Aspect Ratio
144p 176×144 4:3
NTSC square pixel 240p 320×240 4:3
SIF(525) 240p 352×240 5:4 (narrow pixels)
NTSC widescreen 240p 427×240 16:9
CIF, SIF(625) 288p 352×288 4:3 (wide pixels)
PSP 288p 480×272 30:17
360p 360p 480×360 4:3
Wide 360p 360p 640×360 16:9

See also[edit]

References[edit]