Saturday, December 15, 2007

High-Definition Video - Format considerations

The optimum format for a broadcast depends on the type of media used for the recording and the characteristics of the content. The field and frame rate should match the source, as should the resolution. On the other hand, a very high resolution may require more bandwidth than is available. The lossy compression that is used in all digital HDTV systems will then cause the picture to be distorted.

Photographic film destined for the theater typically has a high resolution and is photographed at 24 frame/s. Depending on the available bandwidth and the amount of detail and movement in the picture, the optimum format for video transfer is thus either 720p24 or 1080p24. When shown on television in countries using PAL, film must be converted to 25 frames per second either by speeding it up by 4% or using pulldown techniques. In countries using the NTSC standard, (60 fields per second) a technique called 3:2 pulldown is used. One film frame is held for three video fields, (1/20 of a second) and then the next is held for two video fields (1/30 of a second) and then the process repeats, thus achieving the correct film rate with two film frames shown in 1/12 of a second. (See also: Telecine) (Note: This is slightly more complicated because film is photographed at exactly 24.00 frames per second while NTSC digital video at 24p is recorded at 23.976 frames per second since color NTSC video is actually recorded at 59.97 fields per second (not 60.00) which is a difference of 1000/1001 from black and white NTSC video. Therefore, telecine from film to NTSC video also requires a slow down of the projection rate for both the picture and the audio by 0.1%.)

Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in the form 480i60 or 576i50. These may be upconverted to a higher resolution format (1080i), but removing the interlace to match the common 720p format may distort the picture or require filtering which actually reduces the resolution of the final output. (See also: Deinterlacing)

Non-cinematic HDTV video recordings are recorded in either 720p or 1080i format. The format depends on the broadcast company if destined for television broadcast, however in other scenarios the format choice will vary depending on a variety of factors. In general, 720p is more appropriate for fast action as it uses progressive fields, as opposed to 1080i which uses interlaced fields and thus can have a degradation of image quality with fast motion. In addition, 720p is used more often with internet distribution of HD video, as all computer monitors are progressive, and most graphics cards do a sub-optimal job of de-interlacing video in real time. 720p Video also has lower storage and decoding requirements than 1080i or 1080p, and few people possess displays capable of displaying the 1920x1080 resolution without scaling. 720p appears at full resolution on a common 1280x1024 LCD, which can be found for under US$160 as of June 2007. An LCD capable of native 1080p resolution still costs over five hundred US dollars.

In North America, Fox, ABC, and ESPN (ABC and ESPN are both owned by Disney) currently broadcast 720p content. PBS, NBC, Universal HD (both owned by General Electric), CBS, UPN, Showtime, INHD, HDNet and Time-Warner-owned HBO-HD, the WB and TNT currently broadcast 1080i content.

In the United Kingdom, BBC HD and Sky HD broadcast in 1080i. It is worth noting, however, that the two main HD providers, Virgin Media and Sky, provide set-top boxes which are capable of upscaling the video to 1080p.

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