What is Digital?
Why not just use analog?
By: David K. Every
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2002-07-15 05:54:27
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 lot of people hear and use the term "digital", but do not really know what it means. I also occasionally get the question, "why is digital better than analog?"
In humans, digital means digits (fingers), in clocks it means that the clock shows the actual digits (numbers) and not a hand that points to the numbers. Which reminds me of an old engineering joke; "Did you hear about Frankenstein's digital watch? He used real digits". See digits could mean real fingers, and, well, never mind - engineers have their own sense of humor and it probably loses something in the translation. Anyways, I was getting to computers version of digital.
In computers, digital is another way of saying "binary". That means that there are really only two states; either on or off, either power signal high or low, or the value is either zero or one -- those all mean the same thing.
Every other value is built by grouping these on/off states. So if you need to represent a value between zero and seven, then you'd just pair up three digital lines (bits). Together, that can represent 8 different values (zero to seven). A table to represent this would look as follows:
off, off, off = 0
off, off, on = 1
off, on, off = 2
off, on, on = 3
on, off, off = 4
on, off, on = 5
on, on, off = 6
on, on, on = 7
And you can add more and more "bits" (lines), until you get the resolution you need.
In contrast, analog would be a way of having more than just on or off, and having states (voltages) that lie between minimum and maximum.
In analog, if you want to represent a value between zero and seven, then you would have five steps (levels) between the minimum and maximum, to represent each possibility. If the voltage was off the value would be a zero, 1/8ths of the way to maximum then it would represent a 1, 2/8ths would be a 2, and so on.
Now to a lot of people it seems that digital and binary is more complex, and that you can get more information in analog because each line holds more possible values. And this is true. In fact, the first computers were analog for just that reason. But when you learn more about binary and electronics things change.
The problem is that in electronics it takes time for a line to change from one voltage (level) to another, and then more time to stabilize/settle down (and not be too high or too low of voltage). The more steps (levels) you have, then the more careful you have to be, and the longer it takes to get a good value set, and to be able to read that value; or in other words, the slower a computer would have to be.
In digital electronics, you can set on and off very close together, which means it is quicker to change from one state to the other. And you also don't really care if the voltage is higher than on (one), or lower than off (zero); it just has to pass some threshold, which you call zero or one. So digital electronics can change state more times in the same amount of time; or in other words, it can do its calculations faster (you can look at the value more times each second).
Also in electronics, you can get "noise" or interference from other electronic devices, from static electricity, power lines, radio waves, from the sun, and so on. This interference can be more easily detected and ignored in digital electronics than in analog ones.
Now some person might ask if there are alternatives to digital or analog; and of course there is. One of them is something called a quantum computer that exists in laboratories. Quantum mechanics gives me a headache, but basically it uses a concept called a quantum bit (or QuBit), which allows for a "bit" to be in both states (on and off) at the same time. The potential is fast computers or new ways of thinking; but we are decades away from that. It takes a demented mind to really understand quantum mechanics, and now I need an aspirin for talking about it, so if you want to read more, go search the Internet.
For now, the choices are pretty much digital and analog. And for the last 50 years or so, it is easier to design digital computers (and many other devices) that run quicker and are more reliable than analog ones, and hopefully this article helped you understand why.
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