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.

Decimal numbers	Binary equivalent		Decimal numbers	Binary equivalent
0	0000		8	1000
1	0001		9	1001
2	0010		10	1010
3	0011		11	1011
4	0100		12	1100
5	0101		13	1101
6	0110		14	1110
7	0111		15	1111

The number of digits in the group gives us 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

We can express the following two decimal numbers in eight bit binary :

27 ONE HUNDRED AND TWENTY EIGHT 0 SIXTY FOUR 0 THIRTY TWO 0 SIXTEENS 1 EIGHTS 1 FOURS 0 TWOS 1 ONES 1 = 00011011
53 ONE HUNDRED AND TWENTY EIGHT 0 SIXTY FOUR 0 THIRTY TWO 1 SIXTEENS 1 EIGHTS 0 FOURS 1 TWOS 0 ONES 1 = 00110101

Conversion of analogue to digital - sampling an analogue signal

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


The graph shows an analogue signal - the blue line. The value of this signal is 'sampled' at a number of points, in this case 5 ms apart and the value at each of these points is shown in the table.

Time (ms)	Signal intensity (decimal)	Signal intensity (binary)		Time (ms)	Signal intensity (decimal)	Signal intensity (binary)
0	78	01001110		75	56	00111000
5	104	01101000		80	80	01010000
10	65	01000001		85	62	00111100
15	71	01000111		90	130	10000010
20	80	01010000		95	95	01011111
25	35	00100011		100	30	00011110
30	116	01110100		105	62	00111110
35	110	01101110		110	75	00111000
40	46	00101110		115	20	00010100
45	60	00111100		120	120	01111000
50	98	01100010		125	115	01110011
55	72	01001000		130	36	00100100
60	60	00111100		135	83	01010011
65	82	01010010		140	92	01011100
70	82	01010010

The following graph shows only these sampled values with the analogue line removed.

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 wave the better. In a digital CD the original analogue waveform is sampled a staggering 44 100 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.