Low-definition television
Low-definition television (LDTV) refers to TV systems that have a lower screen resolution than standard-definition TV systems. The term is usually used in reference to digital TV, 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
The Video CD format uses a progressive scan LDTV signal  (352×240 or 352×288), which is half the vertical and horizontal resolution of full-bandwidth SDTV. However, most players will internally upscale VCD material to 480/576 lines for playback, as this is both more widely compatible and gives a better overall appearance. No motion information is lost due to this process, as VCD video is not high-motion and only plays back at 25 or 30 frames per second, and the resultant display is comparable to consumer-grade VHS video playback.

For the first few years of its existence, YouTube offered only one, low-definition resolution of 320x240 at 30fps or less, later extending first to widescreen 426×240, then to gradually higher resolutions; once the video service had become well established and had been acquired by Google, it had access to Google's radically improved storage space and transmission bandwidth, and could rely on a good proportion of its users having high-speed internet connections. In 2013, YouTube extended further into the LDTV realm by adding an even lower 256×144 resolution with a halved framerate giving an overall effect reminiscent of early online video streaming attempts using RealVideo or similar services, where 160×120 at single-figure framerates was deemed acceptable to cater to those whose network connections could not even sufficiently deliver 240p content.

Video games
Older video game consoles and home computers often generated a technically compliant NTSC or PAL signal (480i and 576i respectively) but only sent one field type rather than alternating between the two. This created a 240 or 288 line progressive signal, which in theory can be decoded on any receiver that can decode normal, interlaced signals. Since the shadow mask and beam width of standard CRT televisions were designed for interlaced signals, these systems produced a distinctive fixed pattern of alternating bright and dark scan lines; many emulators for older systems offer video filters to recreate this effect.

With the late 1980s introduction of 16-bit and 32-bit computers/game consoles, such as the Atari ST, the Commodore Amiga, the Super NES, and the Sega Genesis, outputs up to 480i/576i were supported for the first time, but rarely used due to heavy demands on processing power and memory. Standard resolutions also had a tendency to produce noticeable flicker at horizontal edges unless employed quite carefully, such as using anti-aliasing, which was either not available or computationally exorbitant. Thus, 240p/288p remained the primary format on most games of the fourth and fifth generation consoles (including the Sega Saturn, the Sony PlayStation and the Nintendo 64). With the advent of sixth generation consoles and the launch of the Dreamcast, 480i/576i use became more common, and 240p/288p 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 the Nintendo Wii. Nintendo's official software development kit documentation for the Wii refers to 240p as 'non-interlaced mode' or 'double-strike'.

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 a video mode which forces the emulator to output 480i instead of 240p, however many games released prior were never updated.

Teleconferencing LDTV
Upcoming sources of LDTV using standard broadcasting techniques include mobile TV services powered by DVB-H, 1seg, 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

See also

 List of common resolutions
 8640p, 4320p, 2160p, 1080p, 1080i, 720p, 576p, 576i, 480p, 480i, 360p, 240p
 Digital television
 Digital audio broadcasting
 DVB, ATSC, ISDB
 SDTV, EDTV, HDTV
 Narrowband television
 MPEG
 Handheld television

References

Digital television
Broadcast engineering
Broadband