The move from analog to digital TV is well under way. Most homes have some form of digital TV, but the conversion from analog to digital is still an evolving process. High-definition TV is digital all the way, and is becoming much more widely available.
The key concept behind any kind of digital TV is that the audio and video programming is converted from an analog signal into a series of digital bits of binary code that consist of combinations of 1 and 0. The primary technology behind any kind of digital TV (at least in the United States and Canada) is something called MPEG:
Several video and audio compression and digitization standards are based on MPEG. Most are named by adding a number to the end of MPEG. The MPEG-2 standard is by far the most common in the video world, with MPEG-4 coming on strong. (Another, older standard, MPEG-1, is also supported by DVDs and is used for VideoCD, which is more common outside the United States.)
The digital television (cable or satellite) signal and DVDs that most people receive today use MPEG-2 as their encoding to digitally transport or store standard analog NTSC signals. Often, when you use a digital TV system (such as a DSS satellite service like DIRECTV) or a prerecorded digital source (such as a DVD), you get an analog signal that has been transmitted or stored digitally. The signal itself — the program that goes into your TV — is often still analog. Digital over digital is the ideal, and it will be the law of the land as analog TV gets turned off in 2009.
Even though the video signal coming out of a DVD player is NTSC (an interlaced format), some DVD players can convert this into a progressive-scan version of NTSC, which you can use if you have a progressive-scan TV.
When this digitized signal gets to your house (over a digital cable system, a Direct Broadcast Satellite system, or on a DVD), a set-top box, satellite receiver, or DVD player converts the signal back to analog NTSC TV, which your TV understands and can display. This digital transmission signal coming into your house usually looks and sounds better than an analog one because the digital transmission path is cleaner and isn’t susceptible to the interference that usually messes up analog signals.
Many things that are called digital — like most (but not all) digital cable, digital satellite TV services, and regular DVDs — provide you with a nice digital picture that is not high-definition TV. These digital video signals are usually just the same resolution as the older analog TV signal — they’re simply stored and/or transmitted by digital systems. There’s nothing wrong with these digital systems, but many people think that digital cable or digital satellite or DVD is the same thing as HDTV, and that’s not true.
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Source:http://www.dummies.com/how-to/content/digital-tv-signals.html
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