'In Amplitude Modulation the transmitted signal expressed mathematically as u(t)=Ac[1+m(t)]cos(2πfct+φc)Double-Sideband part: Acm(t)cos(2πfct+φc),Carrier Component part: Accos(2πfct+φc).

It is sometimes convenient to express m(t) as
m(t)=amn(t)
where mn(t) is normalized such that its minimum value is -1. This can be done, for example, by defining mn(t)=m(t)/max|m(t)|
The scale factor a is called the modulation index.'90