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Digital TV (HDTV)
(Also see Analog TV further down)

Digital Television - (DTV) - (Often known as High Definition Television {HDTV}; may also be known as Advanced Television {ATV}, Integrated Digital Television.) (Though people often use DTV as a synonym for HDTV, DTV isn't always HDTV) - "Digital Television" is a comprehensive term for digital and digitised television. It includes HDTV, Datacasting and Multicasting.
   
In the 80s, when cable programming became serious competition to the film industry, the Japanese company NHK formally introduced High Definition Television technology to Hollywood. At the time it was called "NHK Hi-vision". It transmitted better picture and sound, and had a wider screen. All of that was something the film industry very much wanted. Unfortunately, the HDTV signal required much more bandwidth (20 MHz) compared to a standard NTSC analog signal (6 MHz). In addition there were other problems dealing with the signal.
 
Then in 1996, things really started heating up.  Over 23 well-funded international proposals from corporations and educational institutions were submitted to the U.S. Federal Communications Commission. These proposals tried to answer:

(1) Would a HDTV transmission be analog, a mixture of analog and digital, or purely digital?
(2) How would the signal transmit: cable, broadcast, satellite, etc?
(3) What part of the broadcast spectrum would HDTV occupy?
(4) How would a 20 MHz signal be fitted into a 6 MHz one?

In 1990, the FCC decided a HDTV digital signal could be simultaneously broadcasted until analog signals were phased out. In order to receive this signal, people would need to buy either a digital TV set or a converter such as a set-top box. A digital TV tuner card could work for their computer. Eventually, four proposals seemed serious, but no one was the winner. A suggestion was made to form a "Grand Alliance" between these contenders: MIT, Philips, AT&T, General Instrument, Zenith Sarnoff and Thomson.  After much discussion in 1996, the FCC adopted the Advanced Television Systems Committee (ATSC) Digital Television Standard based on an MPEG-2 compression scheme proposed by the "Grand Alliance." Also in 1996 the Telecommunications Act was passed. Then in 1997 the FCC allocated pure digital spectrum, (not analog or a blend,) to broadcasters.  In addition they decided to require broadcasters to transmit digital programming on a graduated schedule by 2006, (which was extended.)

During the 1990s, there were a number of important related developments:

(1) The cable industry became a powerhouse across the country.
(2) The PC revolution gave the television and film producing industry software tools to digitally edit and manage their work, especially those from Macromedia, Avid and Adobe.
(3) The CDROM industry became a leader in the development of interactive multimedia applications.
(4) Satellite companies, eager to get a bigger piece of the market, introduced smaller 18 inch to a yard in diameter residential satellite dishes for homes (Direct Broadcast Satellite {DBS}). These dishes receive transmissions of hundreds of channels of digitally encoded NTSC broadcast signals to digital-to-analog set-top boxes nationally and internationally. Typically they offer more interactive television than their cable counterparts.
(5) A mix of analog and digital consumer electronics devices also appeared such as CD-ROMs, VCRs, camcorders, laser disks, and digital video disks.
(6) The success of the Internet seriously affected the television industry.  Among other things, a lot of TV viewers, and thus advertising revenues related to them, was lost to folks who were spending time concentrating on the Internet.  Something had to be done.

Fully digital television requires much more technically advanced equipment than does analog TV. Digital television receivers rely on advanced electronic circuitry to decode the digital signals in real time. Unlike the backward compatibility between color and black and white analog sets of yesteryear, fully digital broadcasts cannot be displayed on analog equipment without additional components.

The digital broadcast video (and audio) signals are encoded as a series of pulses where the pulse height and distribution define the signal. Rather than being directly related to the voltage applied to the electron guns of analog TVs, digital to analog converters (DACs) are needed to convert the signal pulses into the proper voltage levels. A digital TV signal gets decoded into 3 simultaneous voltage levels, one each for the 3 electron guns of the digital TV. The voltages are applied in the same manner as for analog TV, only the method of developing the voltages is different. In terms of scanning, even digital TV is an analog process, since the scanning signals are analog (at least on the standard CRT picture tube; this is not the case with LCD displays.)    

As you know, there are a number of differences between digital and analog television, several are noted below:

1) The DTV hardware reads or turns the broadcast signal (depending on the hardware involved) into bits and bytes, which is the language of computers.  This makes computers and HDTVs compatible.  As the primary language (code) for the World Wide Web is HTML, which is a computer code turned into bits and bytes, this makes for a much more efficient way to experience and interact with the Internet.

2) Digital TV signals are much less susceptible to interference. With MPEG compression (and other) technologies, an error-free picture is possible, even if minor signal errors are present. With an analog broadcast signal, minor signal errors can cause minor picture degradation (ghosting etc.)  As interference becomes worse, the picture becomes worse.  With a digital signal, because of the way the error correction works, the picture will still look perfect until the threshold FEC signal to noise ratio is reached. Ratios below the threshold lead to an unacceptable picture (which is called catastrophic degradation.)

3) The use of compression (which DTV does) means that a standard 6 MHz TV channel bandwidth can carry around 4 or 5 separate digital TV channels (versus one with the analog signal) and have the same good resolution.  

4) DTV can offer at least twice the picture resolution of straight analog TVs, this making possible a cinema-quality image as well as sound quality like that of a compact disc.

5) Signals for adjacent digital TV channels do not interfere with each other like those in analog systems. Therefore more channels can be occupied. 

6) Unlike analog systems, the resolution of the digital TV broadcast can be varied. 

7) Because digital signals can be compressed when they're sent to the DTV, the viewer can receive a great many more channels. This allows for the development of channels with content that only a select group of viewers would be interested in. With some sort of back channel the viewer can interact with others more freely.

8) With some sort of return path (back channel) the viewer can interact with others more freely.

More Definitions of Digital TV (and Demonstrations)
 Dozens more Interactive TV and Digital TV Links are here.

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Analog TV – (a.k.a. SDTV, Standard Definition Television, Standard Definition) - These are by far the most widely used type of television sets. These televisions receive signals sent as a continuously varying analog waveform. The waveform's value determines the amount of voltage that will be applied to the electron guns in the back of the picture tube.  This directly affects the luminance (the technical term for the picture brightness (a.k.a. intensity,) and chrominance (the technical term for the color) of the picture.  Analog TVs are far and away the dominant type of TVs used by the public and have been for decades. In analog TV systems, the voltage applied to the electron guns is proportional to the constantly changing video signal voltage.
   
Analog TVs however have limitations:

Analog TV channels are susceptible to interference caused by physical features (hills, buildings etc.)  This interference often takes the form of  "ghosting" on the picture screen.

The normal TV channel occupies a bandwidth of 6 MHz. With analog TVs, the viewer can see only a limited total number of 6 MHz channels, obviously limiting the total number of possible channels the consumer can receive and view, (without outside electronic help).  In addition, many of the frequency bands that could be used by TV channels, have to be kept empty because analog TV channels spaced too close together can interfere with one another.

Although luminance and chrominance signals are separated, they can interfere with one another, especially in regions with low signal strength. This can lead to problems with the color display.

To watch digital TV on an analog TV you need a DTV tuner (in the form of a set-top/receiver) and some form of TV connection. Since analog televisions are not capable of displaying the higher resolution HDTV broadcasts, the set-top/receiver can be set to convert the signal to what is known as 480I, which is roughly equivalent to a DVD player or Direct TV satellite image.

Analog TV Formats - The 3 main formats used for analog TV differ in the number of lines in each complete picture, the frame refresh rate, and in the detailed encoding of the color (chrominance) and brightness (luminance) information. With each the chrominance & luminance signals are separated. This ensures that an analog color TV signal will display satisfactorily on a monochrome receiver (which has no chrominance.) 

The 3 main formats are:

NTSC: National Television Systems Committee

Countries include: USA, Canada, Japan, and Korea
Total Lines: 525
Active Lines: 484
Frame refresh rate: 6o Hz

PAL: Phase Alternating Line

Countries include: EU (except for France, Greece), China, India
Total Lines: 625 Lines
Active Lines: 575
Frame refresh rate: 50 Hz

SECAM: Sequential Couleur Avec Memoire

Countries include: France, Greece, and Russia
Total Lines: 625 Lines
Active Lines: 575
Frame refresh rate: 5o Hz

To see what system is used in other countries, see the below URL. It lists what system individual countries use: www.dvdaust.com/world_video_standards.htm






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