What’s the difference between AM and FM radio?

Although most of us had heard over the course of our lives the terms “AM” and “FM”, have you ever wondered what exactly do these terms mean? What is the difference between them? Well, today we will dive into the interesting world of radio and find out. There’s one main difference between these two, and it’s in the type of signals they receive, let’s see the rest.

The difference between AM and FM radio in the little details

What do AM and FM stands for? AM is short for amplitude modulation. It was first developed in the early 20th century, chiefly, by the Canadian inventor Reginald Aubrey Fessenden, the same time experimental AM radio stations began to be set up, but would only gain widespread use in the 1920s. The following three decades would be known as The Golden Age of Radio, which would meet its end in the late 1950s when television started to take over as the main entertainment medium.

This is also the time when AM would finally gain a noticeable challenger in radio transmission as well. FM, or frequency modulation, was invented in the early 1930s, specifically 1933 by the American inventor Edwin Howard Armstrong. The development and expansion of FM followed a similar trend as AM before. While the first experimental radio stations using FM were set up through the 1930s and 1940s, FM would have its breakthrough among the mainstream public only two decades later in the 1950s, a time when the Golden Age of radio was ending.

what's the difference between am and fm radio stations
Photo: sciencedirect

Related: When was AM and FM radio invented?

AM and FM radio receivers

Both AM radio and FM radio receivers are superheterodyne, which means it uses frequency mixing to convert the signal it received into an intermediate frequency (IF) which is more convenient for processing. The IF of AM is 470 kHz, while the IF of FM is 10.8 MHz. While an RF amplifier is optional when it comes to AM receivers it is mandatory in the FM variant. Limiter circuits are not used at all in AM receivers and are used only after IF amplifiers in FM receivers. The de-emphasis circuit is not used in AM receivers but it is used in FM receivers. Automatic gain control (AGC) in AM is used after the detector and in FM after the limiter. The input frequency range of AM radio receivers is divided into two parts. One is from 530 kHz to 1650 kHz and from 3 MHz to 30 MHz. FM has one frequency range from 65 MHz to 108 MHz.

This can vary between countries most today have a frequency range between 88 and 108 MHz. As was already mentioned FM receivers have a higher sound quality than AM receivers. There is no capture effect in AM, but it is present in FM receivers. FM receivers are also cheaper than their AM counterparts. AM receivers tend to be bigger than FM receivers.

AM and FM radio stations

As was already mentioned, during the 1920s and all the way until the late 1950s AM radio stations were the king of entertainment content. Many radio dramas that were considered classic come from this era, which when combined with a healthy dose of classical music set the dominance of AM radio in what is now known as the Golden Age of Radio. But with the rapid emergence of FM radio and television in the late 1950s and early 1960s, this dominance began to fall. Although at first AM radio stations tried to stay competitive with their FM counterparts, the high-fidelity of FM made its dominance inevitable.

Today FM radio stations mainly broadcast music, as their high-fidelity makes them perfect for this role. The Top 40 format, which usually plays the 40 most popular songs, is usually used on all of them. It is also used for the general broadcast of high-quality sound. AM radio stations have, for the most part, abandoned trying to compete with FM radio for the music crowd and have moved on to sports broadcasts, religious programming, talk shows, and the like. AM radio stations are on a downward spiral with more and more stations going out of business every year.

It is also of note that FM radio stations have not caught on in Oceania as they have in the rest of the world and, as a consequence of that, AM radio still thrives there.

AM and FM radio broadcasting

Reading the previous text you might get the impression that FM is way better than AM, and in most ways it is. The higher sound fidelity, smaller receivers, and their cheaper prices are only a part of why FM is where it is today, but there still is one technical area where AM is noticeably better. When it comes to reception distance FM has a maximum range of 50 to 60 km (or 30 to 40 miles), while AM reception distance can go as far as 160 km (or 100 miles). Also during the night, AM signals travel for thousands of kilometers by reflecting off the ionosphere, while FM radio signals just go through it and into space. This happens because AM has a longer wavelength.

AM and FM radio broadcasting
Photo: agilebroadcast

AM and FM radio waves

In essence AM and FM radio work the same way, their difference stems from the way the carrier wave is altered. A carrier wave is a waveform that is modified with a piece of signal-bearing information, its purpose is to convey useful information. In the case of AM radio, the carrier wave is modified based on its amplitude, or maximum strength, to record the sound information. FM does not rely on amplitude and instead uses its frequency to record sound information. While both signals are susceptible to changes in amplitude in the case of FM signals it does not matter since it conveys sound information through its frequency, and since AM signals rely on their amplitude to convey sound a change in amplitude will cause static to appear. read more How to record FM radio broadcasts?

AM and FM radio transmitters

AM and FM transmitters work in different ways:

  • In an AM transmitter, the oscillator generates the carrier signal, which then gets amplified by a buffer and a driver amplifier. The signal’s power level must be sufficient to power the power amplifier. At the same time, an audio signal is created and then amplified. It is then fed to a speech-processing circuit which makes it so that only the valid frequencies are passed on. Then the power amplifier outputs an AM modulated signal which is radiated by the antenna and ready to be received by a receiver.
  • In an FM transmitter, a crystal oscillator generates a carrier signal, same as in AM transmitter, which is amplified by a buffer amplifier and then reaches the phase modulator. An audio signal is generated and then amplified by an audio amplifier and then also reaches the phase modulator. The carrier signal then goes through a frequency multiplier, then a mixer, and then another frequency multiplier. The signal is then amplified by a driver amplifier and then by a power amplifier before being transmitted through the antenna to be received by a receiver.

The epilogue of the What’s the difference between AM and FM radio?

A hundred years have passed since AM radio became mainstream and seventy since FM started to take over. Will these old technologies be replaced any time soon? Unless you live in Norway you have nothing to fear, yet at least. For example, AM and FM stations will continue operating in the United Kingdom at least until 2032. There have been plans to start slowly switching off these old analog formats in 2015. But due to the popularity of AM and FM, even if 60% of people mainly listen to digital, the change has been scraped for at least another decade with Ofcom renewing all commercial analog licenses.

The trend is similar in other countries as well with many seeing no reason to shut of the reliable AM and FM services. Sweden scrapped its plans to switch completely to digital in 2015. In Canada, they have even stopped renewing digital licenses because of low consumer interest. There is a similar trend in Finland. The United States is unique in this regard since they use the standard HD radio which can broadcast both analog and digital at the same time. Even in Norway the majority of the population (66% to be exact), in the year 2017, opposed the analog switch off. Perhaps we should let the new and old technologies exist and work together.

FAQs

  1. So which is better to use, FM or AM?

It depends on how you want to use your radio, but generally, FM is better and more cost-effective. The only reason you would want to use AM is to cover a larger area.

  1. Can I legally operate my own broadcast without any registration?

It depends on the laws in your country, but if you can get broadcasting equipment in a legitimate way chances are that it is legal. Still better safe than sorry check your local laws before trying to start your own broadcast, also be mindful that the equipment you can get usually has a small range (200 to 300 meters) and a limited frequency band.

  1. Is there any way I can get the frequency for my local radio stations (both AM and FM)?

There are many sites dedicated to finding your local radio stations. One of those being tunein.com.

  1. Is there any place on Earth where there is no AM or FM?

As of the writing of this article, Norway is the only country without AM or FM radio. In 2017 they had decided to change them in favor of digital audio broadcasting.

  1. Where can I learn more about AM and FM radio?

There are many articles about radios on our site with many more to come!

 

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