2 basic modes of communication:-
(i). Frequency modulation
(ii). Amplitude modulation
Amplitude modulation: In this modulation mode, amplitude carrier wave varies according to instataneous value of modulating signal. Let modulating signal [information signal]
`S(t)=a_(m)sin(omega_(m)t)`
`C(t)=a_(c)(sinomega_(c)t)`
Now modulating signal is superimposed on carrier signal carrier signal is a high frequency signal, which contains the information in its envelope (its variation w.r.t., modulating signal)
`X(t)=[ac+s(t)]sinwct`
`=ac[1+(a_(m))/(a_(c))sinw_(m)t)sinwct`
`x(t)=c(t)[1+(a_(m))/(a_(c))sinw_(m)t]`
Here `(a_(m))/(a_(c))=Ma` (modulation index)
`x(t)` is amplitude modulated signal.
Communication: Using te trigometric relation `x(t)=ac(t)sinw_(c)+(M_(a)a_(c))/(2)cos(wc-wm)t-(M_(a)a_(c))/(2)cos(wc+wm)t`
Here `w_(c)-w_(m)` and `w_(c)+w_(m)` are respectively called the lower side and upper side frequencies the modulated signal now consists of the carrier wave of frequency wc plus two sinusoidal waves each with a frequency slightly difference from, know as side bands. As long as the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.
