Editing Digital television (section)

=== Formats and bandwidth ===
[[File:Digital & Analog TV screen quality comparison-1.jpg|thumbnail|right|Comparison of image quality between [[ISDB-T]] (1080i broadcast, top) and [[NTSC]] (480i transmission, bottom)]]
Digital television supports many different picture formats defined by the [[broadcast television systems]] which are a combination of size and [[aspect ratio (image)|aspect ratio]] (width to height ratio).

With [[digital terrestrial television]] (DTT) broadcasting, the range of formats can be broadly divided into two categories: [[high-definition television]] (HDTV) for the transmission of [[high-definition video]] and [[standard-definition television]] (SDTV). These terms by themselves are not very precise and many subtle intermediate cases exist.

One of several different HDTV formats that can be transmitted over DTV is: {{resx|1280|720}} [[pixel]]s in [[progressive scan]] mode (abbreviated ''[[720p]]'') or {{resx|1920|1080}} pixels in interlaced video mode (''[[1080i]]''). Each of these uses a [[16:9]] aspect ratio. HDTV cannot be transmitted over analog [[television channel]]s because of [[channel capacity]] issues.

SDTV, by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast. [[NTSC]] can deliver a {{resx|640|480}} resolution in 4:3 and {{resx|854|480}} in [[16:9]], while [[PAL]] can give {{resx|768|576}} in [[4:3]] and {{resx|1024|576}} in [[16:9]]. However, broadcasters may choose to reduce these resolutions to reduce [[bit rate]] (e.g., many DVB-T channels in the UK use a horizontal resolution of 544 or 704 pixels per line).<ref>[http://dtt.me.uk Latest snapshots - Freeview/DTT bitrates] {{Webarchive|url=https://web.archive.org/web/20071122175239/http://dtt.me.uk/ |date=2007-11-22 }} (Mendip transmitter, UK)</ref>

Each [[commercial broadcasting]] [[terrestrial television]] DTV channel in North America is allocated enough bandwidth to broadcast up to 19 [[megabit]]s per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead, the broadcast can use [[Program and System Information Protocol]] and subdivide across several video [[subchannel]]s (a.k.a. feeds) of varying quality and compression rates, including non-video [[datacasting]] services.

A broadcaster may opt to use a standard-definition (SDTV) digital signal instead of an [[HDTV]] signal, because current convention allows the bandwidth of a DTV channel (or "[[multiplex (TV)|multiplex]]") to be subdivided into multiple [[digital subchannel]]s, (similar to what most FM [[radio station]]s offer with [[HD Radio]]), providing multiple feeds of entirely different [[television program]]ming on the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's bit budget or multicasting. This can sometimes be arranged automatically, using a [[statistical multiplexer]]. With some implementations, image resolution may be less directly limited by bandwidth; for example in [[DVB-T]], broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission [[bit rate]] and make reception easier for more distant or mobile viewers.