Suppose you had a page from a book. An analog signal is like sending a photograph of the page while the digital signal is like sending the letters which form the sentences in the book. Which do you think will be more accurate if there is noise ? Does that tell you why most of the computers we use are digital. (There are computers which use analog signals and now there are TV's which use digital signals!)

I am not sure which digital signals you are referring to-- so I will make a guess.

A computer outputs a signal to drive its monitor which is different from the video signal used to transmit television pictures. In a sense, these two signals are not too different, since both the television and monitor scan their screens and display the information (recorded in the signal as changes in signal strength) as changes in the brightness of the scanning dot on the screen. (You can see the scanning lines on a tv by looking at it through a repetetive shutter such as a spinning disk with holes cut in it). However, a computer monitor must make a display that is much sharper than a TV set, so that lettering can be easily read. So it separates the Red, Green, and Blue signals into separate wires, and thus controls the screen directly. (All of the colors you see are mixed from these three components on the screen). Green is used instead of Yellow to increase the brightness of the blue component of the screen. The high resolution (large number of lines possible on the screen) can cause these signals to contain very high frequencies (>100MHz).

In a conventional video TV set, all of the colors are mixed into a Black and White signal (so that the video is compatible with B&W TV's) and a color difference signal which is offset by 3.579545MHz. Since the resolution of TV is low, there is not much energy in this band anyway. (However, if several narrow lines are placed together vertically, the scanning dot going by can make frequencies this high... leading to the false color bands seen in some television test patterns). Essentially, the BW signal tells how bright to make the display, and the color difference tells what color it should be. This trick allows only 1 wire to be used, making it possible to broadcast the signal as a conventional T.V. signal. The tricky color technique and the use of interlacing (two successive frames are shifted and contain different data) allow the total video signal to be packaed into 4.5Mhz, including the color parts. This number was required to be compatible with earlier TV sets which used simpler circuits. (Remember that TV was invented log before transistors became common -- let alone "chips".)

If you are interested in more information on this, please check for NTSC (the video standard) and/or RS-170 (one of the computer standards) in your library or web access. Another good source are books such as the TV/Video experimenter-- However: If you experimentwith a T.V. or monitor, please be advised, a magnet produces interesting effects -- but will permanently ruin your set! Also, never open a set -- even when off, as the anode voltages in a T.V. commonly run in the 18,000 to 28,000 volt range and can remain even when the set is off!

I think that there are two ways to answer your question, "Not much" and "Just about everything". It all depends on which question you are really asking. The explanation that goes with your first answer goes like this:

Both digital and analog signals server the same purpose, they take some information (like a picture on the tv or computer screen), encode it (I'll explain what that means in a second), send it somewhere else (like from the tv station to your house or from your computer to your friend in France), and decode it (the opposite of encoding).

Now what do I mean by encode? Let me give you and example. Let's pretend that I am lost in my car and call you on my cell phone. All I can tell you is that I want to get to Burger King and that I am right now parked at the corner of Main St. and First. You have a map that will show me how to get there, but I can't see your map because I'm in my car all the way across town! What we need to do is to transmit the information on the map from you to me. The most complete way to do that is for you to come find me and give me the map, but that is very time consuming and you might have something else you want to do with your time (like go watch "Titanic" or "Amistad").

Another solution is for you to "encode" the information on the map into words, send those words to me through my cell phone, and let me decode your words into the information I need; ie. Take Main north for three blocks, turn East onto Fourth and go six more blocks, Burger King is on the left. It doesn't even matter what language you "encode" the information into, as long as we both speak it.

Now back to your question. Lets say the information is a trailer for the re-release of Star Wars. If I wanted to watch it on my computer it would go like this: some helpful guy who works for George Lucas would encode the trailer digitally and put it on the Star Wars web site, I would find that web site and trasfer the encoded (digital) information to my home computer, then my computer would decoded the signal and play the trailer for me. For tv it would work like this: some helpful programming executive would decide to run the trailer tonight at eight, when eight o'clock comes around he would encode the trailer in an analog signal that he would broadcast to my tv (in my case through a cable but it works by antenna too) then my tv would decode the signal and show me the trailer.

So you can see, that is a really long way of saying "Not much". Both digital and analog are simply differnt ways of encoding information to be transmitted. And that brings me to the explanation for your second answer:

Taken another way, the answer to your question is quite different, because digital and analog are very different in the details. Using my example from above about encoding, if we were both bilingual you could have "encoded" the information on the map into either English or Hindi (a language spoken in India and the durrounding region). Now, in theory the two languages serve an identical purpose, transferring the information about the map from you to me; but when we get right down to it spoken English and spoken Hindi don't sound a whole lot alike! In the same sense digital and analog can be thought of as two different languages, with a different vocabulary and grammar.

In digital encoding all of the information is broken up into seperate chunks, that is where it gets its name, digital is another way of saying chunky (not chunky like Roseanne Barr, chunky like beef stew); another word commonly used to describe chunkiness is "discrete" (again, that's discrete like chunky, not discrete like someone who can keep a secret). A good example of something that is discrete is a flight of stairs: you are either on one step or the next, but you can't stand anywhere in between (if you only had one leg anyway). These discrete chunks can be represented by 1's and 0's, and every piece of information has it's own special "translation" in 1's and 0's. For instance the number 13 would be 1101 (if you are interested in learning more about how I "translated" that number ask your teacher for a good book about changing bases between number systems). Now when two computers talk to each other digitally they can transmit huge amounts of information, but they say it all in 1's and 0's, never 2/3, or 143.976 (that's where the discreteness comes in).

Now, analog encoding is what's called continuous. That means that a slice of analog information can have any value it wants, 5/7, 345.34, the square root of 2. A slide is a good example of something that is continuous: you can be half way down the slide, 2/3 down the slide, 0.324 down the slide, whatever. Analog encoding was invented long before digital, that's the way radio waves work and we've had radios since the early 1900's. The problem is, it's

A digital signal is a series of 0s and 1s. It looks like this:

... 01010001011111100010101 ...

You can encode information by using a particular series of 0's and 1's.

For example, I could assign each letter in the alphabet a code, so that

00000 = 0
00001 = a
00010 = b
00011 = c
etc.

There are 26 letters in the alphabet. How many digits do you need to use per letter to be sure each letter gets a unique digital code?

Now, how does this code get transmitted from one computer to another?

We can connect the two with a wire, and apply 1 Volt to the wire to represent 1, and 0 Volts to the wire to represent 0. Then at any particular time the voltage on the wire would be either 1 Volt or 0 Volts, but nothing in between.

An analog signal can take on any value, not just 0s and 1s. So if take my cable TV signal and measure the voltage on the cable, then I could measure any voltage between say, 0 Volts and 5 Volts. (I don't know the exact voltages, but you get the idea.) I mean, any voltage, like 3.29 Volts.

The way a TV works is that there is an electron beam behind the screen. Now the screen has a coating of phosphor that glows when the electron beam hits it. The beam sweeps across the screen back and forth, like someone mowing a lawn, "drawing" one line at a time.

The analog TV signal (the voltage on the cable) affects how intense the electron beam is at any particular time. This determines how bright that particular spot is on the TV screen.

Could you think of how we could use a digital signal to tell the TV how bright to make the electron beam?