Saturday, March 15, 2008

Broadcast station format considerations

At the least, HDTV has twice the linear resolution of standard-definition television (SDTV), thus showing greater detail than either analog television or regular DVD. The technical standards for broadcasting HDTV also handle the 16:9 aspect ratio images without using letterboxing or anamorphic stretching, thus increasing the effective image resolution.

The optimum format for a broadcast depends upon the type of videographic recording medium used and the image's characteristics. The field and frame rate should match the source and the resolution. A very high resolution source may require more bandwidth than available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage and transmission systems will distort the received picture, when compared to the uncompressed source.

Types of media

Standard 35mm photographic film used for cinema projection has comparable resolution to HDTV systems, but is exposed and projected at a rate of 24 frames per second. To be shown on television in PAL-system countries, cinema film is scanned at the TV rate of 25 frames per second, causing an acceleration of 4.1 per cent, which is generally considered acceptable. In NTSC-system countries, the TV scan rate of 30 frames per second would cause a perceptible acceleration if the same were attempted, and the necessary correction is performed by a technique called 3:2 pull-down: over each successive pair of film frames, one is held for three video fields (1/20 of a second) and the next is held for two video fields (1/30 of a second), giving a total time for the two frames of 1/12 of a second and thus achieving the correct average film frame rate.

See also: Telecine

Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in the form 480i60 or 576i50. These may be relatively easily up-converted to a higher resolution interlaced format such as 720i, but removing the interlace to match the common progressive 720p format may distort the picture or involve filtering that even reduces the resolution of the result.

See also: Deinterlacing

Non-cinematic HDTV video recordings intended for broadcast are typically recorded either in 720p or 1080i format as determined by the broadcaster. 720p is commonly used for Internet distribution of high-definition video, because most computer monitors operate in progressive-scan mode. 720p also imposes much less strenuous storage and decoding requirements compared to both 1080i and 1080p. 1080p is used for Blu-Ray and HD DVD recordings.

List of stations

  • In Australia, the 576p50 format is also considered a HDTV format, as it has higher vertical resolution through the use of progressive scanning. When Australia started DVB-T in 2001 several networks broadcast high-definition in a 576p format as this could give better quality on 50 Hz scanning CRT TVs and was not as demanding on MPEG-2 bit-rate. Now that flat-screens are predominating and these have an interlace to progressive scan conversion there is little difference in picture quality. Also MPEG-2 encoders have improved so the more conventional 720p and 1080i formats are now used. Technically, the 576p format is internationally defined as Enhanced-definition television and many DVD players can provide a 576p signal usually on HDMI outputs.
  • In Germany, HDTV is broadcast via Cable and Satellite on 2 channels. PremiereHD and AnixeHD. ProSiebenHD and Sat1HD have stopped broadcasting in HD until 2010.

All channels are broadcast using the h.264 codec. The public channels ARD and ZDF are considering starting their HDTV channel(s) in 2010, though it's still unknown how many channels will be broadcast and if terrestrial is going to be a part of their strategy to get HDTV into German living rooms.

No comments: