Digital and Analog

Think of an electrical signal, carrying information,  traveling along a wire. There are many other types of signal but this example is all we need for now. This signal is just a voltage which varies with time. This signal may be analog or digital in nature.

Analog		The voltage can very in amplitude and frequency. A microphone produces an analog electrical signal when you talk into it. The signal going to a loudspeaker is an analog signal.
Digital		The voltage is either one thing or another, e.g. +5V and 0V

We can represent a digital signal by lots of 1's and 0's. i.e. 1001101001001111010010110100110101101

The signal is made up of lots of "bits" of information. Each bit is either a 1 or a 0. A group of 8 bits is called a byte.

So how can lots of 1's and 0's carry lots of information?

Numbers - Numbers in binary (base 2) are made up up 1's and 0's so all we have to do is convert our base 10 numbers into binary.

Text - We use an international coding system called ASCII code. With this we can represent a letter (or number) with an 8 bit code. Here is part of the code. Notice how numbers are coded.

When you press a key on your computer keyboard it is converted into an 8 bit binary code which travels through a wire to your computer.

When you send an e-mail message to a friend you are actually sending lots of 1's and 0's. Your friends computer decodes this message back into text.

Why are digital signals better?

All signals pick up noise in transmission. If an analog signal picks up noise it is very difficult for an electronic system to tell whether the noise is meant to be part of the signal or not, because of this it is very hard to filter out the noise. If it assumes the noise is a high frequency component, which it often is, then after filtering you lose the high frequency part of the signal that was meant to be there.

If a digital signal picks up noise it is much easier to filter it out and retrieve the original signal.

Using a digital system we can also send much more information at the same time as we shall see on another page.