The noise elimination system using the amplitude-locked loop with cochannel FM interference

The frequency modulation (FM) signal was conceived to minimise the effect of additive noise - since additive noise is the principal source of signal corruption in amplitude modulation (AM), the pre-cursor of frequency modulation. When the FM signal is received the additive noise is removed by limiter action. This process is a necessary part of FM demodulation. Without hard limiting, the signal is corrupted by both phase noise and by amplitude noise. It had not been possible to achieve FM demodulation without first using a hard limiter - not until the realisation of the amplitude locked loop (ALL).