HDTV (This article is still in draft form)

HDTV stands for High Definition Television. In the United States, HDTV is defined by the ATSC or Advanced Television System Committee. For introduction of digital television to the United States, the ATSC introduced a complementary set of standards covering different resolutions, refresh rates and aspect rations. These standards encompass SDTV (Standard Definition Television) and HDTV.

   HDTV standards are all in digital format .....
                   ....... but not all digital formats are HDTV.

In addition to encouraging HDTV, Congress through the FCC wished to have US broadcasters move from the old analog NTSC (National Television System Committee), system to a digital equivalent. The digital equivalent forms a large part of the ATSC set of standards but is not HDTV.

For the last few years, television broadcasters have been investing in new transmission and studio equipment to enable them to move their broadcasts to the new digital channels. These changes have occurred in parallel with the deployment of HDTV. The combination has led to consumer confusion over both HDTV, DTV (Digital Television) and SDTV.

The ATSC standards encompass 18 different formats and only x of these are HDTV. The different formats are usually referred to by the number of vertical lines and the type of screen refresh. Therefore 720p describes a HDTV signal with 720 vertical lines using a progressive refresh rate. Similarly 1080i describes an HDTV signal with 1080 lies vertically refreshed as an interlaced signal.

SDTV formats remain close to the NTSC analog format. NTSC uses interlaced refresh with a rate of 60 per second (60 Hz) giving 30 picture frames per second. In an interlaced system, only half of the signal is refreshed on each refresh cycle. On the following refresh cycle, the other half of the picture is refreshed. It takes 60, half-picture refreshes to get 30 full frames.
This technique was introduced in early analog television to fool the eye into seeing a continuous picture while keeping the broadcast bandwith low. The NTSC system tranmits 525 lines but only 480 lines actually carry picture information (the others are used for technical and system enhancement purposes).

In ATSC digital equivalents, SDTV formats are known a 480p or 480i.

A progressive scan system, refreshes the whole picture every refresh cycle so carries twice as much picture information as an interlaced system with the same refresh rate. The extra information makes for a better television picture but requires better and more expensive electronics. When an interlaced system displays a half-picture frame, it displays every other line. If the first half-frame displays the even-numbered lines, then the next frame will display the odd numbered lines. Hence the name interlaced as each successive half-frame is interlaced between the previous frame lines, heklping to fool the eye into seeing continuous movement.

NTSC pictures are wider than they are high. If
an NTSC picture is 4 units wide, then it will be 3 units high. This is known as the aspect ratio. An NTSC signal will be described as having a 4:3 aspect ratio.

HDTV formats are capable of displaying a much wider screen and have a 16:9 aspect ratio. Therefore, if an HDTV picture is 32 inches wide, the picture height will be 18 inches.

In sales literature this size of screen will usually be described by the length of the diagonal from corner to corner or (approximately) 37 inches.

Television refresh rates were originally chosen in a diufficult trade off between resolution, human psychology of vision and equipment cost. Early electronics were difficult and expensive to synchronize. The compromise used 60 Hz so that the television set could derive some basic timing from the a.c. electrical system. For similar reasons, the European PAL, SECAM and British 405-line systems used 50 Hz where the a.c. power system uses that frequency.

By contrast, a movie film camera runs at 24 full frames per second. The film looks like a 24p system in ATSC terms. Since film-based movies are a large part of television entertainment, the ATSC specifications accomodate this speed. Therefore there are three different refresh rates in ATSC: 60, 30 and 24 per second. This flexibility improves the viewing experience for a movie when compared to NTSC. The NTSC standard required a complicated system of duplicating and dropping movie frames just to be able to get the 24 frame per second film to display on a 60 frame per second interlaced system. Much of the time the effect is subtle but it does lead to a different viewing experience when compared to the movie theater.

aframe rates 29.97 interlaced only
total lines 525, active lines 4xx

Digital

DTV - Digital Television

HDTV - High Definition Digital Television

ATSC - Advanced Television System Committee.
frame rates 24, 30, 60, interlaced (I) and progressive (P)
total lines , active lines

405-line - Original British B&W analog television system. Replaced in 1960s by PAL system

PAL - Phase Alternating Line 625-line analog television system used world-wide

References

ATSC, "ATSC Digital Television Standard," Advanced Television Systems Committee, Washington, D.C., Doc. A/53, Sep.16, 1995.

Aspect Ratio
4:3
16:9