Modulation index is a descriptor used to show the ratio between the maximum and minimum voltages in any given modulated signal for music, data or voice waves. When a modulation index value is equal to one, the maximum value of the signal ranges from one to zero.
When the value of the index is zero, it can be assumed there is no modulation occurring from the signal or wave carrier. When the value of the index is greater than one, it is an indication that the signal carrier was cut off. This creates an unwanted harmonics effect for the signal that is outputted by the transmitter.
At the base level of the modulation index is the value of a wave at a static condition. This calculation can be expressed in a formula as the modulation value of the wave divided by the amplitude. This ratio value can also be written as a percentage, at which point it is showing the depth of the modulation from the base value.
As of 2014, this modulation index is used by broadcast companies to limit the amount of variation in their transmitted signal. Typically, most broadcast transmitters are designed to include modulation index limiters that keeps its value below 100 percent.