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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 source may require more bandwidth than is available in order to be transmitted without loss of fidelity. The lossy compression that is used in all digital HDTV storage/transmission systems will then cause the received picture to appear distorted when compared to the uncompressed source.
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 by speeding it up by 4.1%. In countries using the NTSC standard, (60 fps) 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)
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 (720i), 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 poor 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 $250. An LCD capable of native 1080p resolution costs close to a thousand dollars.
In North America, Fox, ABC, and ESPN (ABC and ESPN are both owned by Disney) currently broadcast 720p content. NBC, Universal HD (both owned by General Electric), CBS, HBO-HD, INHD, HDNet and TNT currently broadcast 1080i content.
MPEG-2 is most commonly used as the compression codec for digital HDTV broadcasts. Although MPEG-2 supports up to 4:2:2 YCbCr chroma subsampling and 10-bit quantization, HD broadcasts use 4:2:0 and 8-bit quantization to save bandwidth. Some broadcasters also plan to use MPEG-4 AVC, such as the BBC which is trialling such a system via satellite broadcast, which save considerable bandwidth compared to MPEG-2 systems. Some German broadcasters already use MPEG-4 together with DVB-S2 (ProSieben, Sat1 and Three Premiere Channels). Although MPEG-2 is more widely used at present, it seems likely that in the future all European HDTV may be MPEG-4, and Ireland and Norway, which have not yet begun any digital television broadcasts, are considering MPEG4 for SD Digital as well as HDTV on terrestrial broadcasts.
HDTV is capable of "theater-quality" audio because it uses the Dolby Digital (AC-3) format to support "5.1" surround sound.
The pixel aspect ratio of native HD signals is a "square" 1.0, or 1 pixel length = 1 pixel width. New HD compression and recording formats such as HDV use rectangular pixels for more efficient compression and to open HDTV acquisition for the consumer market.
For more technical details see the articles on HDV, ATSC, DVB, and ISDB, respectively.
Within television studios and other production and distribution facilities, HD-SDI SMPTE 292M interconnect standard (a nominally 1.485 Gbit/s, 75-ohm serial digital interface) is used to route uncompressed HDTV signals. The native bitrate of HDTV formats cannot be supported by 6-8MHz standard-definition television channels for over-the-air broadcast and consumer distribution media, hence the widespread use of compression in consumer applications. SMPTE 292M interconnects are generally unavailable in consumer equipment, partially due to the expense involved in supporting this format, and partially because consumer electronics manufacturers are required (typically by licensing agreements) to provide encrypted digital outputs on consumer video equipment, for fear that this would aggravate the issue of video piracy.
Newer dual-link HD-SDI signals are needed for the latest 4:4:4 camera systems (Sony HDC-F950 & Thomson Viper), where one link/coax cable contains the 4:2:2 YCbCr info and the other link/coax cable contains the additional 0:2:2 CbCr information.
- All commercial HD is digital, so the signal will either deliver an excellent picture, a picture with noticeable pixelation, a series of still pictures, or no picture at all. The system cannot produce a snowy or washed out image from a weak signal, effects from signal interference, such as herringbone patterns, or vertical rolling.
- HD programming and films will be presented in 16:9 widescreen format (although films created in even wider ratios will still display "letterbox" bars on the top and bottom of even 16:9 sets.) Older films and programming that retain their 4:3 ratio display will be presented in a version of letterbox commonly called "pillar box", displaying bars on the right and left of 16:9 sets (rendering the term "fullscreen" a misnomer). Or, one can usually choose to zoom the image to fill the screen.
- The colors will generally look more realistic, due to their greater bandwidth.
- The visual information is about 2-5 times more detailed overall. The gaps between scanning lines are smaller or invisible. Legacy TV content that was shot and preserved on 35 mm film can now be viewed at nearly the same resolution as that at which it was originally photographed.
- Two new pre-recorded disc formats support HDTV resolutions, namely HD DVD (supporting 720p and 1080i; future players will support 1080p) and Blu-ray (supporting up to 1080p). Players for both systems are expected to be backward-compatible with DVDs. However, the two formats are not compatible with each other.
- The increased clarity and detail make larger screen sizes more comfortable and pleasing to watch.
- Dolby Digital 5.1 sound is broadcast along with standard HDTV video signals, allowing full surround sound capabilities. (Standard broadcast television signals usually only include basic stereo audio.)
- Both designs make more efficient use of electricity than SDTV designs of equivalent size, which can mean lower operating costs.
Main article: Analog high-definition television systems
The British 405-line black-and-white system, introduced in 1936, was the first to advertise itself as "high definition," although it was high definition only in comparison with previous mechanical and electronic television systems, and not in the sense of the term as it is used today. On the other hand, the 819-line French black-and-white television system introduced after World War II arguably was high definition in the modern sense, as it had a line count and theoretical maximum resolution considerably higher than those of the 625-line systems introduced across most of postwar Europe, and the later European 625-line color systems (PAL and SECAM).
Japan was the only country where commercial analog HDTV was launched and had some success. In other places, such as Europe, analog (HD-MAC) HDTV failed. Finally, although the United States experimented with analog HDTV (there were about 10 proposed formats), it soon moved towards a digital approach.
Main article: High-definition pre-recorded media and compression
HDTV can be recorded to D-VHS (Data-VHS), W-VHS (analog MUSE only), to an HDTV-capable digital video recorder, such as DirecTV's high-definition TiVo, a HDTV-ready Windows Media Center, or Dish Network's DVR 921, 942 or VIP622.
However, the massive amount of data storage required to archive uncompressed streams make it unlikely that an uncompressed storage option will appear in the consumer market soon. Realtime MPEG-2 compression of an uncompressed digital HDTV signal is also prohibitively expensive for the consumer market at this time, but should become inexpensive within several years (although this is more relevant for consumer HD camcorders than recording HDTV). Analog tape recorders with bandwidth capable of recording analog HD signals such as W-VHS recorders are no longer produced for the consumer market and are both expensive and scarce in the secondary market.
As part of the FCC's "plug and play" agreement, cable companies are required to provide customers that rent HD set-top boxes with a set-top box with "functional" Firewire (IEEE 1394) upon request. None of the direct broadcast satellite providers have offered this feature on any of their supported boxes, but some cable TV companies have. As of July 2004, boxes are not included in the FCC mandate. This content is protected by encryption known as 5C. [2]. This encryption can prevent someone from recording content at all or simply limit the number of copies.
Main characteristics of three DTTV systems |
Systems |
ATSC |
DVB-T |
ISDB-T |
Source coding |
Video |
Main Profile syntax of ISO/IEC 13818-2 (MPEG-2 – Video) |
Audio |
ATSC Standard A/52 (Dolby AC-3) |
ISO/IEC 13818-2 (MPEG-2 – Layer II Audio) and Dolby AC-3 |
ISO/IEC 13818-7 (MPEG-2 – AAC Audio) |
Transmission system |
Channel coding |
|
Outer coding |
R-S (207, 187, t = 10) |
R-S (204, 188, t = 8) |
Outer interleaver |
52 R-S block |
12 R-S block |
Inner coding |
rate 2/3 Trellis code |
PCC: rate 1/2, 2/3, 3/4, 5/6, 7/8; constraint length = 7, Polynomials (octal) = 171, 133 |
Inner interleaver |
12 to 1 Trellis code |
bit-wise, frequency |
bit-wise, frequency, selectable time |
Data randomization |
16-bit PRBS |
Modulation |
8-VSB and 16-VSB |
COFDM
QPSK, 16QAM and 64QAM
Hierarchical modulation: multi-resolution constellation (16QAM and 64QAM)
Guard interval: 1/32, 1/16, 1/8 & 1/4 of OFDM symbol
Two modes: 2k and 8k FFT |
BST-COFDM with 13 frequency segments
DQPSK, QPSK, 16QAM and 64QAM
Hierarchical modulation: choice of three different modulations on each segment
Guard interval: 1/32, 1/16, 1/8 & 1/4 of OFDM symbol
Three modes: 2k, 4k and 8k FFT |
Cited references
- ^ Carlo Basile et al. (1995). "The U.S. HDTV standard: the Grand Alliance". IEEE Spectrum (4): 36–45.
- ^ 5C Digital Transmission Content Protection White Paper (pdf) (1998-07-14).
General references
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