Analogue modulation may be divided into amplitude modulation and angle modulation. Amplitude modulation (AM) may be categorized as AM with side bands and carrier (AM/DSB), vestigial side band (VSB), double side - band suppressed carrier (DSB/SC), single side - band suppressed carrier (SSB/SC), and independent side - band suppressed carrier (ISB/SC).
AM/DSB is very popular for radio telephony. For TV transmission with a large bandwidth. VSB is preferred because of the reduced bandwidth of this modulation system. DSB/SC or SSB/SC provides a further reduction in power and bandwidth requirement. SSB/SC finds an extensive use in multiplexed coaxial system and can carry several messages simultaneously.
All AM systems are, however, prone to noise which directly affect the signal amplitude.
In angle modulation, the instantaneous angle of a sinusoidal carrier is varied as per the instantaneous amplitude of the modulating signal. The system leads to phase modulation (PM) and frequency modulation (FM). FM and PM waves require a much larger bandwidth than AM, but are capable of giving a sufficiently improved signal to noise rather than the latter. It also leads to considerable saving in power. |
