Why do some radio stations still use amplitude instead of frequency modulation? Why is frequency modulation (FM) not used on the current amplitude modulation (AM) radio stations? The Police have FM 2 way radios that use only 5 kilocycles of modulation. Instead of the current AM radio stations that use the same bandwidth of 5 kilocycles. The limiter &amp; discriminator or ratio detector filter out the static &amp; loud pops that are on AM; Thus the FM would have much less static &amp; noise than AM. FM channels would occupy the same bandwidth as AM channels. For example if WABC in New York used frequency modulation over the current frequency of 770 they would need to have the oscillator changed but the power amp would stay the same. But the receiving radios would need to be replaced with a radio to demodulate frequency modulation at 770.
Because (as you note) all the receivers would have to be replaced.
Because international agreements allot that part of the spectrum to AM.
Because an FM transmitter with 5kHz modulation needs 25kHz channel spacing to avoid problems with adjacent channels.
Because an FM transmitter with 5kHz modulation only has an audio bandwidth of 3kHz - which is far less than AM (depending on the standard used 5khz, 10kHz or 15kHz.)