**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 number | Binary equivalent | Decimal number | 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 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

The value of the analoque signal is first 'sampled' once every 10 ms.

Time (ms) | Signal intensity (decimal) | Signal intensity (binary) |

0 | 78 | 01001110 |

5 | 104 | 01101000 |

10 | 65 | 01000001 |

15 | 71 | 01000111 |

20 | 80 | 01010000 |

25 | 35 | 00100011 |

30 | 116 | 01110100 |

35 | 110 | 01101110 |

40 | 46 | 00101110 |

45 | 60 | 00111100 |

50 | 98 | 01100010 |

55 | 72 | 01001000 |

60 | 60 | 00111100 |

65 | 82 | 01010010 |

70 | 76 | 01001100 |

75 | 56 | 00111000 |

80 | 80 | 01010000 |

85 | 62 | 00111110 |

90 | 130 | 10000010 |

95 | 95 | 01011111 |

100 | 30 | 00011110 |

105 | 62 | 00000000 |

110 | 75 | 01001011 |

115 | 20 | 00010100 |

120 | 120 | 01110000 |

125 | 115 | 01110011 |

130 | 36 | 00100100 |

135 | 83 | 01010011 |

140 | 92 | 01011100 |

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, often called
noise.