Low-definition television

From Wikipedia, the free encyclopedia
  (Redirected from 240p)
Jump to: navigation, search

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).

For the first years of its existence, YouTube offered only one, low-definition resolution of 320x240p at 30fps or less, only extending - first to widescreen 426×240, then to genuinely higher resolutions - once the video service had become well established, it could afford radically improved storage space and transmission bandwidth, and could rely on a good proportion of its users having high-speed internet connections. More recently, YouTube has also extended further into the LDTV realm by adding an even lower 256×144p resolution with a halved framerate - giving an overall effect reminiscent of early online video streaming attempts using RealVideo or similar, where 160×120p at single-figure fps was deemed acceptable - to cater to those whose network connections could not even widthstand the rigors of "240p".

A VHS videotape can be considered SDTV due to its resolution (approximately 360 × 480i/576i), but using VHS for professional production will yield results subjectively comparable to LDTV because of VHS's low bandwidth, particularly in the field of color reproduction, which can be as little as a few hundred kHz, translating to a few dozen color clocks per line. VHS does, however, still provide high motion and a relatively high vertical resolution via interlacing, which is a feature uncommon in true LDTV material, and reasonable luma resolution.

In comparison, professional-level Betacam SP tape produces approx 440×486i/576i, and some college TV studios use Super VHS at ~560×486i/576i, along with an increased colour carrier bandwidth. Both of these systems, whilst showing a marked improvement over VHS, ultimately offer lower resolution images than DVD, but are still comparable to (and thus remain useful for) lower-bandwidth broadcast television, which is compromised in the analogue domain by a narrower available frequency range for each individual channel, and in the digital by a literally lower horizontal pixel count (often 480 or 544, versus the 720 of DVD and full-rate SDTV broadcasts). Again, both systems offer high motion and a high vertical resolution by way of interlacing, and are more justifiably counted as SD rather than LD.

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 originally used a progressive LDTV signal[citation needed] (352×240 or 352×288), which is half the vertical and horizontal resolution of full-bandwidth SDTV. However, most DVD and SVCD players, as well as VCD 2.0 players (which can display still images at 704×480/576i and offer limited DVD menu-esque functions), will internally upscale VCD material to 704x480/576i (or 480/576p for progressive-scan players) 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, unlike the single-field output of classic computers and consoles, VCD video is not high-motion and only plays back at 25 or 30 frames per second. A similar recording standard (quarter-resolution and half framerate) is also used for super-long-play home DVD recording, although it does not typically adhere to the same bitrate or encoding specifications, and could therefore be considered LDTV. (Regular "LP" DVD recording is closest in spec to a high grade VHS or Betamax recording, having half the normal horizontal resolution and a lower bitrate, but otherwise being identical to a full-rate "SP" recording, including high-motion interlace, and thus still qualifies as SDTV)

With the introduction of 16-bit computers and game consoles, 480/576i output was supported for the first time, but rarely used due to its heavy demands on processing power and memory, as well as its tendency to produce annoying flicker at horizontal edges unless employed quite carefully (typically demanding a degree of anti-aliasing which was either not available or computationally exorbitant). Thus, 240/288p 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, 480/576i use became more common, and 240/288p 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 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 Notes
MMS-Small 96p 128×96 4:3 Lowest size recommended for use with 3GPP video transmitted by MMS to/from cellular phones, matching resolution of smallest generally-used color cellphone screen.
QQVGA 120p 160×120 4:3 Used with some webcams and early colour-screen cellular phones, commonly used in early desktop computer and online video applications. Lowest commonly-used video resolution.
QCIF Webcam 144p 176×144 4:3 Approximately one-sixth analogue PAL resolution (one-half horizontal, one-third vertical). Also the size recommended for "medium" quality MMS videos.
YouTube 144p 144p 256×144 16:9 Also halves replay framerate respective to higher resolution options.
NTSC square pixel 240p 320×240 4:3 Comparable to "low resolution" output of many popular home computers and games consoles, including VGA "Mode X". Used in some webcams and for video recordings in early/budget digital cameras and cameraphones, and low-end smartphone screens. Original YouTube resolution. Maximum recommended size for "large" MMS videos.
SIF (525) 240p 352×240 4:3 NTSC-standard VCD / super-long-play DVD. Narrow/tall pixels.
NTSC widescreen 240p 426×240 16:9 Same as current YouTube "240p" mode; screen resolution of some budget portable DVD players. Roughly one-third full NTSC resolution (half vertical, two thirds horizontal).
CIF, SIF (625) 288p 352×288 4:3 PAL-standard VCD / super-long-play DVD. Wide/short pixels. Also a common webcam / video conferencing resolution and
PSP 288p 480×272 30:17 Notionally 16:9 with slight left/right edge cropping. Used in many portable DVD player screens and other small-format devices besides.
360p 360p 480×360 4:3 Uncommon, used in some lower-mid-market smartphone screens and as an intermediate screen resolution for some 1990s PC-format videogames.
Wide 360p 360p 640×360 16:9 Current YouTube "360p" resolution, and typically used as base "SD" standard by VoD services due to subjective similarity (and similar pixel counts) to a mid-grade free-to-air broadcast picture. Historically used as an ad-hoc standard for intermediate-quality / CDR-sized MPG4 conversions on P2P file sharing networks. Effectively the resolution offered by any higher-definition 16:9 video being displayed on a standard 640×480 ("VGA") computer screen, the lowest (and least computationally demanding) resolution generally supported by mainstream desktop operating systems. Offers 75% of the pixel count of a true anamorphic NTSC DVD image, or 89% of a letterboxed 16:9 image.

See also[edit]

References[edit]