What is the difference between AM and FM radio and why is FM more popular?
In: Technology
There are technical differences in how FM and AM modulation work. The result is the sound quality is better for a longer distance but then quickly go to no reception at all. AM gradually decreases in sound quality.
Because of the amount of spectrum given to AM and FM stations even in good reception, there is a huge sound quality difference. AM have 10kHz spacing between stations in the US, 9kHz in some other places. The signal contains a mirror image of itself so the sound is limited to 5kHz and 4.5kHz. Humans can hear sound up to around 15kHz for adults and 20kHz for younger people. This mean if you broadcast music on AM radio it will never sound very good. This not a fundamental problem with AM modulation but one of what part of the spectrum is available for AM radio.
FM has 200kHz available, it make it possible to fit a mono signal of 15 kHz and tow channels for stereo and still have space over.
AM modulation is easier and cheaper for transmitters and receivers. It was the first type of broadcast radio. It transmits at 0.5 to 2 MHz range, it results in a long-range for the station, the drawback is there is only enough spectrum for 10kHz per channel. The longer range is not always an advantage if means the distance between two station that sends of the same frequency needs to be very long.
FM is a newer technology with more complicated transmitters and receivers, The frequency range is usually 87.5 to 108.0 MHz the larger frequency range makes it possible to give each station a large part of the spectrum. It alos reduces range but that means the same frequency can be reused at closer distances.
The cost part is more relevant historically than today. FM broadcast spectrum was dedicated for that use in 1945 in the US. Even so, it was not until 1960 it became common in the US. With modern electronics, the cost of receiving both is equally low. I am not sure if there is a transmission cost difference.
You could transmit AM radio at the FM frequency or in other bands. You could then give the station a larger frequency range so the sound is better with good reception. It has not been done because FM is even better for sound quality. If you put FM radio at the AM band you would need to reduce its sound quality to or fit fewer stations is the available spectrum.
So today the main difference of AM and FM radio is sound quality and how large area the signal can cover from a single broadcaster.
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