high-definition television (HDTV) - Notation, Comparison to SDTV, Format considerations, Technical details

Any television system using substantially more scanning lines than the 500–600 of established broadcast standards, with improved picture quality in a wide-screen format. In the 1980s, the Japanese NHK proposed a completely new system, Hi-Vision, as a worldwide standard, using 1125 lines at 60 fields per second (60 Hz). However, the need to provide transmissions having some compatibility with existing receivers led to alternative approaches, such as the Eureka project in Europe (related to PAL, using 1250 lines/50 Hz) and several proposals in the USA (with NTSC compatibility, such as Advanced Compatible TV, using 1050 lines/60 Hz).

High-definition television (HDTV) is a television broadcasting system with a significantly higher resolution than traditional formats (NTSC, SECAM, PAL) allow. Except for early analog formats in Europe and Japan, HDTV is broadcast digitally, and therefore its introduction sometimes coincides with the introduction of digital television (DTV): this technology was first introduced in the USA during the 1990s, by the Digital HDTV Grand Alliance (grouping together AT&T, General Instrument, MIT, Philips, Sarnoff, Thomson, and Zenith).

While a number of high-definition television standards have been proposed or implemented on a limited basis, the current HDTV standards are defined in ITU-R BT.709 as 1080 active interlaced or progressive lines, or 720 progressive lines, using a 16:9 aspect ratio.

Notation

In the context of HDTV, the formats of the broadcasts are referred to using a notation describing:

The number of lines in the vertical display resolution.

For example, the format 720p60 is 1280 × 720 pixels, progressive encoding with 60 frames per second (60 hertz known as Hz).

A frame or field rate can also be specified without a resolution.

Most HDTV systems support some standard resolutions and frame or field rates.

Changes in notation

It should be noted that the terminology described above was invented for digital systems in the 1990s.

Standard resolutions

We distinguish (a) the resolution of the transmitted signal and (b) the (native) displayed resolution of a TV set.

Standard frame or field rates

23.976p (allow easy conversion to NTSC) 24p (cinematic film, including 18fps/20fps silent film speed) 25p (PAL, SECAM DTV progressive material) 30p (NTSC DTV progressive material) 50p (PAL, SECAM DTV progressive material) 60p (NTSC DTV progressive material) 50i (PAL, PAL-M & SECAM) 60i (NTSC)

Comparison to SDTV

HDTV has at least twice the lineal resolution of standard-definition television (SDTV), thus allowing much more detail to be shown compared to analog television or regular DVD.

Close-up view

HDTV at four times the spatial resolution of SDTV.

SDTV resolution.

Format considerations

The optimum formats for a broadcast depends on the type of media used for the recording and the characteristics of the content. 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. 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. (See also: Deinterlacing)

Non-cinematic HDTV video recordings are recorded in either 720p or 1080i format.

In North America, Fox, ABC, and ESPN (ABC and ESPN are both owned by Disney) currently broadcast 720p content.

Technical details

MPEG-2 is most commonly used as the compression codec for digital HDTV broadcasts. 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. 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.

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.

Newer dual-link HD-SDI signals are needed for the latest 4:4:4 camera systems (Sony HDC-F950 &

Advantages of HDTV expressed in non-engineering terms

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. 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).

Early systems

The term "high definition" was used to describe the electronic television systems of the late 1930s and 1940s beginning with the former British 405-line black-and-white system, introduced in 1936; however, this and the subsequent 525-line U.S. NTSC system, established in 1941, were high definition only in comparison with previous mechanical and electronic television systems, and NTSC, along with the later European 625-line PAL and SECAMs, is described as standard definition 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.

Japan was the only country where commercial analog HDTV was launched and had some success.

Contemporary systems

Recording and compression

HDTV can be recorded to D-VHS (Data-VHS), W-VHS (analog only), to a HDTV-capable digital video recorder (for example DirecTV's high-definition TiVo or SkyHD's set-top box), a HDTV-ready Windows Media Center, DirecTV's Plus HD DVR H20 or Dish Network's DVR 921, 942 or VIP622.

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).

In the USA, 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.

Table of terrestrial HDTV transmission systems

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	convolutional (I=12, M=17, J=1)	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, 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 &