Modulation methods

Modulation methods
• amplitude modulation: two different amplitudes of a carrier, for example 1500 Hz are used to
represent a 1 and a 0.
• frequency modulation: this is also call frequency shift keying (FSK). A 0 and a 1 are presented by
two different carrier frequencies. This is the most common method of modulation for telephone
line modems.
• phase modulation: there are two different types of phase modulation:
a) a phase shift of 180( in the carrier occurs each time a binary zero is transmitted. No phase
change takes place for a binary 1.
b) phase shift keying (PSK) in which a zero and a 1 are represented by two carrier signals
180° out of phase
The above methods all have two signal levels so each signal element represents one bit of information.
It is possible to have variations and combinations of these techniques which result in more than one bit
per baud.