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what actually is the difference between AM and FM radio?
Both are methods of encoding and broadcasting radio signals. The difference is how they do
that. Radio signals travel as electromagnetic waves — invisible to us, but as fast as the
speed of light, and on the spectrum of electromagnetic radiation.
Now, this is where it starts to get a little confusing. And it’s got to do with the words we use.
Radio waves are around us all the time, but we can’t detect them on our own. They are a long-
wave form of electromagnetic radiation. So, more confusingly, radio waves are part of the
same spectrum as light, not sound! That’s why they travel as fast as light.
Mechanical and Electromagnetic Waves
Radio waves can’t be “heard” and have nothing to do with sound waves. Sound waves are
mechanical vibrations of air particles (which is why sound travels so much slower than
light, at a speed of 767 mph), but radio waves are electromagnetic energy and a part of
the same spectrum as light. So they travel at about 670,080,887 mph. Like, real fast.
Mechanical waves require a medium to pass through, and there are basically two
types: Longitudinal waves are sound waves and they move through air, water, and even
solid surfaces, but they need to be able to move the particles of that medium; and transverse
waves move through water and other media in perpendicular oscillations.
On the other hand, electromagnetic waves — the types of which include light, microwaves,
infrared, x-rays, ultraviolet, and radio — do not require a medium through which to travel.
That’s why they can move through deep space and through physical barriers.
Transmission and Reception
Radio waves surround us all the time, but the only way we can pick them up is with a radio
receiver. The term radio also refers to the technology that allows information to be
transmitted and received over radio waves. You can have individual pairs of transmitters
and receivers that come together, like two-way radios or walkie-talkies, or one-way
broadcasts from a single powerful transmitter to multiple receivers, which is like the giant
radio towers in your city and the tiny radio in your living room.
From the Studio to Your Home
So if you’re listening to your favorite radio show (like I am right now while writing this), that
radio program began as a bunch of sound waves, and was then captured by microphones
and turned into electrical signal. It’s then either broadcast immediately from the studio or
stored as a recording to use later, but here’s where the AM/FM thing happens.
In order to broadcast these electrical signals, the studio needs to join it to a radio wave
called a carrier wave, and that attachment process is called modulation. There are two
ways of modulating (or changing) the carrier wave:
1. By affecting the amplitude or height of the carrier wave (AM: A for amplitude, M for
modulation):
2. By affecting the frequency or how fast the carrier wave travels (FM: F for frequency,
M for modulation).
From the antenna on top of the radio station, the highest point in the air it can reach, the
signal is then broadcast as electromagnetic waves. The receiver on your personal radio
then picks up the waves, amplifies them, and converts them back into sound through the
speaker. If there’s no signal attached, you’ll hear nothing, because the carrier waves haven’t
been modulated.
Although radio waves coming from many stations surround us constantly, your radio doesn’t
receive them all at the same time, because stations broadcast at different specific
frequencies. You have to tune in to a specific frequency to find the right signal. The numbers
on your radio dial represent frequencies used by your local radio stations. If the FM dial is
set at 89, the radio signal you hear is broadcasted at 89 MegaHertz (MHz), or 89,000,000
cycles per second.
Differences in Sound Quality
The difference in the way AM and FM radio signals are encoded means differences in sound
quality, performance, and broadcast range between the two types of station. This explains
why FM stations sound better than AM stations, but AM stations can be heard from further
away.
AM radio varies the amplitude of the broadcast signal so the power at which that signal is
broadcast is also changed, since amplitude represents the strength of the signal. Some
receivers can’t pick up low amplitude signals at all. FM radio, always remains at constant
amplitude, so signal strength does not change.
FM uses a higher frequency range and a bigger bandwidth than AM. AM radio operates from
535 kHz (kiloHertz) to 1605 kHz. When you tune the dial on your radio, the number
changes by 10 kHz each time. This means that each station has 10 kHz of bandwidth on which
to broadcast. FM radio on the other hand operates between 88 MHz (MegaHz) and 108
MHz, and your radio increments every 200 kHz.
Each FM station is allocated 150 kHz of bandwidth, which is 15 times that of an AM station.
This means that an FM station can transmit 15 times as much information as an AM station
and explains why music sounds so much better on FM. Since music has more electrical
information contained within it than a monophonic voice audio signal, FM typically
broadcasts music and AM generally sticks to talking programs.
