Modulation Schemes - Introduction
The purpose of modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for severable reasons:
- For easy (low loss, low dispersion) propagation as electromagnetic waves
- So that they may be simultaneously transmitted without interference from other signals
- So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength)
- So as to be able to multiplex that is to combine multiple signals for transmission at the same time.
For example AM radio is 550-1600 KHz, FM radio is 88 MHz-108 MHz, TV is 52-88 MHz (channels 1-6), 174-216 MHz (channels 7-12) and 470-900 MHz (UHF),
Microwave and satellite signals are of the order of several GHz and infra red fiber optic signals are of the order of 200-300 THz.