3.1 Digital and Analogue Signalling
Before examining analogue modulation techniques it would be useful to review the difference between analogue and digital signalling.
When it was originally conceived the public telephone network used analogue signalling. This type of signalling was ideal for human speech. However, with the increased business use of computer technology that relied on digital signalling, a way had to be found to connect this equipment between businesses. The natural idea was to make use of the communication channel that already linked businesses, ie: the telephone system. The draw back was the disparity between the analogue system (telephone network) and the digital equipment (computer systems).
A device would have to be used to sit in between the digital equipment and the telephone system and act as a translator. This communication device was called a modulator/demodulator or modem for short. The purpose of this device was to take a digital signal and transform it into an analogue signal at the senders end. The signal could then be sent and at the receiver’s end the signal would be converted from analogue back into a digital signal.
In analogue transmission the sending device produces a high frequency signal that acts as a basis for the information signal; this signal is referred to as the carrier signal or carrier wave.
The function of the carrier signal is to ‘carry’ information from one place to another.
Data is sent or encoded onto the carrier by modifying one or more of the signals characteristics, eg: amplitude. This is known as modulation or shift keying.
Next: 3.2 Analogue Signals