HD explained

High-definition television (HDTV) is a digital television broadcasting system with a significantly higher resolution than traditional formats (NTSC, SECAM, PAL). While some early analogue HDTV formats were broadcast in Europe and Japan, HDTV is usually broadcast digitally, because digital television (DTV) broadcasting requires much less bandwidth if it uses enough video compression.

Three HDTV standards are currently defined by the International Telecommunication Union (ITU-R BT.709). They include 1080i (1,080 actively interlaced lines), 1080p (1,080 progressively scanned lines), and 720p (720 progressively scanned lines). All standards use a 16:9  aspect ratio, leading many consumers to the incorrect conclusion of equating widescreen television with HDTV. All current HDTV broadcasting standards are encompassed within the ATSC and DVB specifications.
HDTV is also capable of "theatre-quality" audio because it uses the Dolby Digital (AC-3) format to support "5.1" surround sound. It should be noted that while HDTV is more like a theater in quality than conventional television, 35 mm and 70 mm film projectors used in theatres still have the highest resolution and best viewing quality on very large screens. Many HDTV programs are produced from movies on film as well as content shot in HD video.

HDTV sources

The rise in popularity of large screens and projectors has made the limitations of conventional Standard Definition TV (SDTV) increasingly evident. An HDTV compatible television set will not improve the quality of SDTV channels. To get a better picture HDTV televisions require a High Definition (HD) signal. Typical sources of HD signals are as follows:

• Over the air with an antenna. Most cities in the US with major network affiliates broadcast over the air in HD, however currently there is no HD terrestrial broadcast in the UK. To receive this signal an HD tuner is required. Most newer HDTV televisions have an HD tuner built in. For HDTV televisions without a built in HD tuner, a separate set-top HD tuner box would be required.
• Cable television companies often offer HDTV broadcasts as part of their digital broadcast service. This is usually done with a set-top box issued by the cable company. Some cable carriers also offer HDTV On-Demand playback of movies and commonly viewed shows.
• Satellite-based TV companies, such as Sky Digital in the UK and Ireland offer HDTV to customers as an upgrade. New satellite receiver boxes and a new satellite dish are often required to receive HD content.
• Video game systems, such as the Xbox (NTSC only), Xbox 360, and Playstation 3, can output an HD signal.
• Two optical disc standards, Blu-ray and HD DVD, can provide enough digital storage to store hours of HD video content.

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.
• Whether progressive scan (p) or interlaced scan (i) are used. Progressive scan redraws all the lines (a frame) of a picture in each refresh. Interlaced scan redraws every second line (a field) in one refresh and the remaining lines in a second refresh. Interlaced scan increases picture resolution while saving bandwidth but at the expense of some flicker or other artifacts.
• The number of frames or fields per second.
• The format 720p60 is 1280 × 720 pixels, progressive encoding with 60 frames per second (60Hz). The format 1080i50 is 1920 × 1080 pixels, interlaced encoding with 50 fields (25 frames) per second. Often the frame or field rate is left out, indicating only the resolution and type of the frames or fields, and leading to confusion. Sometimes the rate is to be inferred from the context, in which case it can usually be assumed to be either 50 or 60, except for 1080p which is often used to denote 1080p24, 1080p25 or 1080p30 at present but will also denote 1080p50 and 1080p60 in the future.

A frame or field rate can also be specified without a resolution. For example 24p means 24 progressive scan frames per second and 50i means 25 interlaced frames per second, consisting of 50 interlaced fields per second. Most HDTV systems support some standard resolutions and frame or field rates. The most common are noted below.

Changes in notation
The terminology described above was invented for digital systems in the 1990s. A digital signal encodes the color of each pixel, or dot on the screen as a series of numbers. Before that, analogue TV signals encoded values for one monochrome, or three-color signals as they scanned a screen continuously from line to line. By comparison, radio encodes an analogue signal of the sound to be sent to an amplified speaker, typically up to 20 kHz, but video signals are in the MHz range, which is why they are much higher in the broadcast spectrum than audio radio. Analogue video signals have no true "pixels" to measure horizontal resolution. The vertical scan-line count included off-screen scan lines with no picture information while the CRT beam returned to the top of the screen to begin another field. Thus NTSC was considered to have "525 lines" even though only 486 of them had a picture (625/576 for PAL). Similarly the Japanese MUSE system was called "1125 line", but is only 1035i by today's measuring standards. This change was made because digital systems have no need of blank retrace lines unless the signal was converted to analog to drive a CRT. 

Standard resolutions 

When resolution is considered, both the resolution of the transmitted signal and the (native) displayed resolution of a TV set are taken into account. Digital NTSC- and PAL/SECAM-like signals (480i60 and 576i50 respectively) are transmitted at a horizontal resolution of 720 or 704 "pixels". However these transmitted DTV "pixels" are not square, and have to be stretched for correct viewing. PAL TV sets with an aspect ratio of 4:3 use a fixed pixel grid of 768 × 576 or 720 × 540; with an aspect ratio of 16:9 they use 1440 x 768, 1024 × 576 or 960 × 540; NTSC ones use 640 × 480 and 852 × 480 or, seldom, 720 × 540. High Definition usually refers to one million pixels or more.

In Australia, the 576p50 format is also considered a HDTV format, as it has doubled temporal resolution though the use of progressive scanning. Thus, a number of Australian networks broadcast a 576p signal as their High-definition DVB-T signal, while others use the more conventional 720p and 1080i formats. Technically, however, the 576p format is defined as Enhanced-definition television.

Standard frame or field rates

23.977p (allow easy conversion to NTSC)
24p (cinematic film)
25p (PAL, SECAM DTV progressive material)
30p (NTSC DTV progressive material)
50p (PAL, SECAM DTV progressive material)
60p (NTSC DTV progressive material)
50i (PAL & SECAM)
60i (NTSC, PAL-M) 

Comparison with SDTV
HDTV has at least twice the linear resolution of standard-definition television (SDTV), thus allowing much more detail to be shown compared with analog television or regular DVD. In addition, the technical standards for broadcasting HDTV are also able to handle 16:9 aspect ratio pictures without using letterboxing or anamorphic stretching, thus further increasing the effective resolution for such content.

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 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 theatre typically has a high resolution and is photographed at 24 frames per second. 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 percent. In countries using the NTSC standard, 30 frames per second, a technique called 3:2 pull-down 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.

Non-cinematic HDTV video recordings are recorded in either 720p or 1080i format. The format used 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 scan frames, 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.

Technical details
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 sub-sampling 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 trialing such a system via satellite broadcast, which will 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 AVC, and Ireland and Norway, which have not yet begun any digital television broadcasts, are considering MPEG-4 AVC for SD Digital as well as HDTV on terrestrial broadcasts.