Analogue and digital signals

An ANALOGUE SIGNAL is one where the waveform in the information follows the original waveform exactly at all times

A DIGITAL SIGNAL is one where the original waveform is sampled at regular intervals and a number given to the value of the disturbance at each of these points.

Binary numbers are used for these sampled values.

Binary is a way of expressing numbers in ones (high voltage value) or zeros (low voltage value) – there is nothing in between. You can have either a 1 or a 0.
In mathematical language you are expressing numbers to the base 2 instead of our normal decimal system where we use the base 10.

The number of digits in the group gives is the BIT NUMBER. For example all the above numbers are FOUR BIT NUMBERS. Many of your computers are 32 BIT machines – they deal with numbers like:

00110011010011100011000110101011

So we can express the following in numbers in binary :

27 THIRTY TWO 0 SIXTEENS 1 EIGHTS 1 FOURS 0 TWOS 1 ONES 1 = 011011
53 THIRTY TWO 1 SIXTEENS 1 EIGHTS 0 FOURS 1 TWOS 0 ONES 1 = 110101

Anlogue to digital conversion

An example of the conversion from an analogue to a digital signal is shown below.

Different sampling rates

The following three graphs show the problems with sampling. If you only sample at a few times the resulting curve does not really match the original very well. The red sampled points and the yellow sampled points give different curves from sampling both the red and yellow points and even this does not quite fit the original wave. Therefore the more often you sample the original wave the better the ditital output will be.

In a digital CD the original analogue waveform is sampled a staggering 44 000 times a second and for a DVD it can be double this.

The reason for using binary and not ordinary decimal numbers is to do with interference of the original signal or noise.