Modulation Index is defined as the ratio between the amplitude of modulating signal and the carrier wave.
If the modulation index is greater than , that means the amplitude of the modulating wave is higher than that of the carrier wave. This causes over- modulation of the carrier wave which results in distortion. Thus, a low modulation index is always maintained.
Bandpass Filter is a circuit that allows a range of frequencies to pass through and blocks all other frequencies.