Episode: 603
Title: HPR0603: QSK Episode 4: AM vs. FM
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr0603/hpr0603.mp3
Transcribed: 2025-10-07 23:45:55

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Welcome to Hacker Public Radio, the following presentation is a syndication of the QSK podcast
used with kind permission on Russ Woodman. We are using this episode today because we do not
have enough shows on our own. Please consider recording a show today.
You may land me at Hacker Public Radio not on for more information.
This podcast is a proud member of the Fusion Podcast Network. Find us at fusionpodcastnetwork.com.
Welcome to the Black Sparrow Media, the internet broadcast network.
Hello everybody, this is Russ and welcome once again to the QSK netcast.
This is episode number four and I do want to welcome everybody back to the show.
My voice is a little bit under the weather right now as I'm a little bit under the weather right
now. I hope everybody can deal with that. I'm sure it's better for you than it is for me right now.
The first thing I'd like to do is to thank Chad Wallenberg and Claudio Miranda over at
the Linux Basement Podcast for talking about my show on their show in episode number 60.
Thanks a lot guys for mentioning me over there and hopefully that will bring some more people
my way. I really do appreciate it and I know you said you haven't had a chance to listen to
the show very much so I hope you will come back and take in a few episodes. Let me know how
you like the show. I'd also like to mention that I am a proud member of the Black Sparrow Media
internet broadcast network. You can find that over at blacksparrowmedia.net.
I'm also a member of the Fusion Podcast Network. You can find that at FusionPodcastnetwork.com
slash WordPress and that's Fusion with a Z. And I'm also a member of the augcast planet which you
can find at augcastplanet.org. These are all fine podcasts networks and I encourage you to go
check them out and listen to all of the shows that appear there. You're already listening to mine
so go check out some other great content. As far as the show tonight is concerned this is a topic
that I've thought about a few times over the years. It came up the other day in an IRC chat
probably two weeks ago. Me and a few other folks in the IRC got into a discussion of radios
as often happens because for some reason everybody thinks I know everything there is to know about
radio communications even though nothing could really be further from the truth.
But one notion that I've wanted to dispel for the general public over time is the difference
between AM radio and FM radio and I'm talking about AM and FM radio in the in the aspect of
broadcast radio as most people would recognize it. I'm not talking about amateur radio in this
particular instance although some of that is going to definitely come up as I go through my little
talk. The first thing to note about wireless communication particularly radio is that most of
the time the radio signal the electromagnetic wave that makes up a radio signal is centered around
the carrier wave and a carrier wave is the central part of the radio signal. Interestingly though the
carrier wave doesn't really do anything. It contains enough information to let a receiver know that
there is something on a particular frequency but it doesn't contain any input or modulation data
in and of itself. There is nothing there in the way of an input signal to be reproduced.
Now you'd recognize this if you had an FM radio turned on when you tune to a station,
an FM radio station that is broadcasting but let's say the DJ has gone to the bathroom
or his disk has run out and he doesn't realize it so they're broadcasting nothing in the way of
a signal. Well you still know there's a signal there though because your FM radio is dead quiet
and if you have a tuning indicator it will show that there's a signal there even though you're not
really hearing anything and what you're actually hearing is an unmodulated carrier wave.
It's a carrier wave that is on the frequency but it doesn't contain any modulation. There's no
input signal so all your hearing is essentially dead air. That's what we tend to call it in the
common parlance. If you tune off of a station to an area where there is nothing on a frequency
everyone's pretty familiar with what you hear. You hear noise. It's basically left over
radiation that's coming through the atmosphere and hitting your radios antenna and it comes through
his noise. It's basically just a mishmash of electromagnetic signals. Your tuner doesn't know
what to do with it so it just creates noise. So let's take a look first at the carrier wave itself.
I looked on Wikipedia and this is basically what it has to say about a carrier wave.
Intellicommunications a carrier wave or carrier is a wave form that is modulated with an input
signal for the purpose of conveying information. This carrier wave is usually of much higher frequency
than the input signal and the purpose of the carrier is to transmit the information through space
as an electromagnetic wave. Now when you're talking about an AM or FM broadcast that you might hear
on an AM or FM radio, both of them contain a carrier wave. Now what they do to the carrier wave
when you attach it to an input signal is what makes one amplitude modulation or AM and one frequency
modulation or FM. Now when you hear the words it should be almost self evident what the difference
is. If you have an input medium like a microphone and you have a machine that will translate that
input signal combined it with a carrier wave and then send it out as an electromagnetic wave,
amplitude modulation, what it does is it modifies the carrier wave by changing the amplitude
of the carrier wave based on the volume and pitch of the input signal. So you basically have a
sine wave if you're familiar with what a sine wave looks like and as you change or modulate that
based on the input signal the lobes of the sine wave will actually get taller or shorter
because you're modifying the amplitude. By contrast and frequency modulation
instead of the lobes getting taller or shorter because you're modifying the amplitude
you're actually measuring the frequency. In other words how many of those lobes there are
over a given period of time. As the frequency increases you're going to have more tightly packed
sine wave lobes in a particular amount of time and at lower frequency you'll have fewer and
they'll be further apart. Interestingly AM broadcasting is pretty power inefficient.
You can lose two thirds or more of the power of a signal in an AM broadcast.
The reason for this is because most of the power is going into the carrier wave just to push
the signal out as an electromagnetic wave. Very little of the power is actually used in
modulating that wave to create the signal. The other problem is that in a standard AM broadcast
the signal is duplicated. It has what are called side bands. In other words there's a certain amount
of bandwidth above the carrier wave and a certain amount of bandwidth below the carrier wave
which are used to convey the signal and as it happens in a standard AM broadcast
those signals are redundant. The one below the carrier wave is a mirror image of the one above
the carrier wave and technically you only need one to make an AM broadcast work. In amateur radio
we have what's called single sideband which means you have a signal that's an AM broadcast but you
suppress one of the sidebands whether it's the upper or the lower and therefore you only have to
use half of the power because you're only transmitting half of the bandwidth. Now the other thing
you can do to a signal like that is you can suppress the carrier wave. This makes things extremely
efficient because instead of using two thirds of your power for the carrier and the rest of your
power to put out a redundant sideband signal, two sideband signals in fact, you only have to put
one sideband and no carrier. The problem with not having a carrier however is that the receiver
doesn't know how to decode the signal necessarily because there is no carrier
informing it at what frequency the carrier wave is coming in because there is no carrier wave.
So what you have to do in that case is you have to use what's called a beat frequency oscillator
and this can be found on a lot of receiver devices and it's built into amateur radios for example
and what it does is it creates its own carrier wave to be mixed with the incoming sideband signal
which will then recreate the original signal. The interesting thing about this is if you happen
to have a beat frequency oscillator on your device and you toy with it some you'll find that the
voice or the transmission you hear may be altered. It may sound really bizarre if you don't have the
oscillator tuned properly. It will sound really low and grumbly if the beat frequency oscillator
is turned too low and things can sound very chipmunkish if it's turned too high but with enough
playing you'll get it tuned to the original carrier wave frequency even though it doesn't exist.
You'll figure out how it matches up and then the received signal that you hear will sound
perfectly normal. So that's a quick look at the AM side of things and then there's the FM side
of things. We already talked a little bit about this but basically instead of modifying the
amplitude of the carrier wave based on the input signal you modify the frequency. Now what this does
is it makes the bandwidth constant and for FM broadcasts the bandwidth tends to be considerably wider
than an AM broadcast which means you can pack more information into the signal and do it more
efficiently. What this happens to do is make it more resistance to outside noise and outside
interference. So when you're talking about an AM broadcast or an FM broadcast most people realize
that an FM broadcast is quieter and sounds better overall and that's why. You know when you talk
about an AM radio everyone knows that the sound quality is pretty much crap. In an FM broadcast the
sound quality is usually very good. So something people may have noticed though is that when you have a
radio in front of you the dials are different you're using the same radio but when you talk about
an FM broadcast the numbers on the dial go from 88 to 108 and in this particular instance those
numbers represent megahertz and those are the frequencies over which you can tune your radio
in the FM broadcast band. In the AM broadcast band your numbers will go from 540 give or take
to about 1760. There may be some slight variance in that based on whatever radio you have
and those represent killerhertz. So the FM broadcast band is from 88 megahertz to 108 megahertz
and the AM broadcast band is actually from 0.54 megahertz to about 1.77 megahertz. You can see that the
AM broadcast band is much much lower in frequency than the FM broadcast band. What's interesting
about that is low frequency broadcasts both in amateur radio and in the broadcast arena can be
propagated by skywave propagation. There's a phenomena particularly at night called F-layer
skip and low frequency transmissions like those of an AM broadcast station can be reflected off the
F-layer of the ionosphere and be heard over long distances. This is why somebody in Arizona or
Florida might be able to hit here an AM broadcast station from Chicago late at night. Now very high
frequencies like those in the FM broadcast band don't have this quality. So you will not hear a VHF
station or an FM broadcast station reflected off the atmosphere. You're going to hear them
broadcast as far as their direct broadcast based on their power output will carry them
for a typical 50,000 or 100,000 watt FM station. This broadcast diameter can be anywhere from
about 50 miles to 100 or 120 miles or so based on terrain and other factors. The same holds true
for AM during the daytime. You're going to be limited to the strength of your power output
but at night because those signals bounce off the atmosphere they can be heard over long
distances. However due to fluctuations in the atmosphere's F-layer those signals may seem to
waver or come and go and anyone who's experienced this phenomenon knows exactly what I'm talking about.
So now back to the original point of this talk which was to dispel a certain misconception about
AM and FM broadcasting. Most people when you talk about AM and FM will only understand that
in the context of AM radio and FM radio broadcasts and they assume that when you're broadcasting
an AM you're broadcasting at the frequencies allocated to AM broadcasting and when you talk
about FM you're talking about broadcasting in the frequencies allocated to FM broadcasting.
Well the truth is nothing could be further from the truth. The frequency that a broadcast exists on
where that carrier wave is initiated has nothing to do with the mode of that broadcast. In other
words if you have an AM broadcast station that transmits on say 1000 kilohertz there's nothing
that says except for FCC regulations that you can't have another station that broadcasts at 1000
kilohertz using FM or using single sideband or some other mode of communication like
phase shift key or continuous wave which is otherwise known as Morse code. Interestingly
continuous wave or Morse code is a transmission method that is derived from simply altering
the carrier wave itself. There is no mixing of input signal you're simply modulating the carrier
wave and as I said before the FCC and other government bodies regulate what modes can be used on
what frequencies and by whom. Now the AM broadcast band in the United States for example
goes from 540 to about 1770 kilohertz. Now there's an American and in other parts of the world
amateur radio band which goes from 1800 kilohertz to 2000 kilohertz. Now that's just above
the AM broadcast band and frequency but if you're an AM broadcaster you only get to broadcast in AM.
Amateur radio operators on the other hand get to broadcast in just about any mode they want to.
They can broadcast in slow scan television in single sideband in AM in FM in continuous wave
or pretty much anything else you can think of and that's also to say that
up in the FM say in 100 megahertz if the FCC allowed it you could easily broadcast in single
sideband or AM as well but the law says that you have to broadcast in FM that's just the way
things work but there's also an amateur radio band just above FM in around 140 megahertz where
amateur radio operators can as before broadcast in any mode they want AM FM single sideband
continuous wave phase shift keying whatever you can think of so the point here is to make it clear
that just because that something exists on a certain frequency doesn't mean that the mode
of transmission on that frequency has to be a certain thing and when somebody talks about an AM
radio or an FM radio just because in the United States FCC law says they have to operate between
a certain range of frequencies doesn't mean that those things can't be operated in other frequencies
using other modes in other parts of the radio spectrum now in terms of amateur radio for example
they have proven communications in on various modes with frequencies as low as nine kilohertz
this is in the VLF band the very low frequency band now to give you some perspective remember that
the AM broadcast band starts at 540 kilohertz and we're talking about below nine kilohertz this is
very very low frequency communication has also been successfully made on frequencies as high as
10 and if I'm not mistaken 30 gigahertz and even higher and some of these transmissions actually
propagate a very long way on very low power so hopefully this has been an interesting little talk
about frequency spectrums and broadcast methods including AM and FM and my hope is that the average
person will understand a little better the difference between broadcasting at a certain frequency
and using various modes to do the broadcasting so that they're not locked into the concept that an AM
radio operates at a certain set of frequencies and an FM radio operates at a certain set of frequencies
and that's all there is to it hopefully I didn't sound particularly pet peeveish about that or
anything it's just something that is fairly simple to understand and I just want everybody to have
that information since it's available to me and hopefully I've just imparted it to you so thanks
very much for listening to QSK netcast number four just as I have enjoyed bringing this information
to you I hope you've enjoyed listening to it and I should be back in a week or so with QSK
episode number five until then everybody have a fantastic week and I will catch you soon
you
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