Phase modulation is most commonly used to convey digital signals. All high performance modems today use phase modulation. Similar to FM (frequency modulation), is Phase modulation. (We will show how they are the same in the next chapter.) If we alter the value of the phase according to a particular function, we will get the following generalized PM function:

sPM = Acos(2πfct + αs(t))

It is important to note that the fact that for all values of t. If this relationship is not satisfied, then the phase angle is said to be wrapped.