Digital TV


Formats and bandwidth for DTV

There are many different  formats for DTV. Each broadcaster has one digital TV channel, but one channel can carry multiple sub-channels if the broadcaster chooses that option. Here's how it works.

Digital television supports many different picture formats defined by the combination of size, aspect ratio (width to height ratio) and interlacing. With digital terrestrial television broadcasting, the range of formats can be broadly divided into two categories: HDTV and SDTV. These terms by themselves are not very precise, and many subtle intermediate cases exist. High-definition television (HDTV), one of several different formats that can be transmitted over DTV, uses different formats, amongst which: 1280 × 720 pixels in progressive scan mode or 1920 × 1080 pixels in interlace mode. Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 × 1080 at a 60 Hz progressive scan frame rate — known as 1080p) HDTV cannot be transmitted over current analog channels.

Standard definition TV (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.For 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries. For 16:9 broadcasts, the 704 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line).

Each commercial terrestrial DTV channel in North America is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computer.

Digital television supports many different picture formats
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