How does AM/FM Radio work?

can someone tell me the difference, with a technical explanation?

and why FM is stereo and AM is usually not?

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Chief’s Domain

Kewl. Let’s see how badly I can mangle this. :wink:

AM stands for Amplitude Modulation; FM stands for Frequency Modulation. AM was the first broadcast radio standard intended for the masses. In 1920, the technology for dealing with radio frequencies was pretty new, and so the AM broadcast band was created with many stations sharing a limited amount of radio spectrum, and everybody was pretty limited in the bandwidth they could use. As Hi-Fi and stereo hadn’t been invented yet, nobody much cared. Then, after radio technology had had some time to mature, they started thinking about using higher frequency bands for broadcasting to the masses. The advantage of using higher frequencies is that you can give everybody a lot more bandwidth. Enough to support a decent frequency response (AM is limited to 5 KHz max, IIRC, while FM can go up to 24 KHz or so.) With the improved quality of FM audio, and the popularity of that new-fangled stereo stuff, a way was devised to broadcast FM signals in stereo. Thus started the status quo. For a while, FM was the place to go for serious classical music with good sound quality, and AM picked up everything else; FM receivers commanded a premium price compared to the simple and cheap AM radios, and the album-oriented rock format had yet to be invented. :wink:
Then, in the Seventies, with the advent of cheap integrated circuits, it became possible to pack enough information in the limited AM broadcast signal bandwidth to offer stereo while maintaining compatibility with non-stereo AM recievers. Several schemes were proposed, none of them compatible with any other scheme, and the main response was a deafening yawn. By the time AM stereo was technically feasible, the same technology made FM stereo just as cheap as AM. As FM still had the edge in frequency response, and was far less prone to interference than AM, there was no compelling reason to run out and buy a AM stereo receiver, especially as the various AM stereo schemes weren’t compatible with each other.
So, that’s why today AM radio is where you find all-news radio, talk radio, and other stations where the limited bandwidth is not a big issue, and FM is where you listen to music.

A cubicle is just a padded cell without a door.

Let me see if I can remember correctly . . .

AM = Amplitude Modulation
FM = Frequency Modulation

Now . . . when you change the tuner on you radio, it picks up a new station. Then it strips the signal of the proper frequency or amplitude, and the remaining wave is interpeted into sound.

As to why FM is in stereo, I haven’t got a clue.


For once I get the chance to answer first, and I take too long and give a subpar explanation!

A radio signal is a wave called “carrier” that is “modulated” to carry information. There are several ways to modulate the carrier, Am and FM being two of them.

AM was invented and used first as it is technically simpler but it is more subject to static. FM is much more immune to static but only became widespread once transistors and modern technology made it feasable.

For this reason, AM (with lower bandwidth) remains a talk format and FM with lower noise and higher bandwidth is used for hifi.

There are many other forms of modulation for specific uses.

expansion on previous posts:

Amplitude Modulation (AM)
Amplitude means the signal strength. So, a higher sound volume gets translated to a higher strength signal. (On a scope, you’d see a taller wave.

Frequency Modulation (FM)
Rather than modulate the signal on the amplitude (strength) of the signal, think of it turned 90 degrees, so the modulation of a stronger signal causes a bigger spread in the frequencies transmitted from the center frequency.

An optimization of FM: If you graph out how an FM signal looks, you’ll find the high end frequencies (of the transmitted signal, not the original audio) are duplicates of the low-end frequencies. Also, the frequencies in the middle aren’t really of much value. So, you can eliminate the middle and one of the outside sets of frequencies without losing much information (signal quality). This process is known as Single Side Band (SSB).

Does the above make sense? I can expound if it’s too messy.

First off, congratulations on even knowing that AM stereo exists. There are threads every so often on by people who are frustrated, because they went to the local electronics store, and when they asked if the store sells any AM stereo receivers, the employees looked at them like they had two heads.

can someone tell me the difference, with a technical explanation As others noted, AM stands for Amplitude Modulation, and FM for Frequency Modulation. AM is a much older and much simpler technology, which actually dates back to the telephone. FM (technically wideband FM) was specifically invented in the late 1930s by Howard Armstrong to be immune from static. FM circuitry in much more complicated than AM.

and why FM is stereo and AM is usually not? Commercial FM broadcast stations were first established in the United States in early 1940s, but it took 30 years of struggle before FM listenership began to match the original AM band. From the beginning, FM stations were generally engineered to be high fidelity, partly to match the high fidelity records that started coming out in the 1950s.

All the original FM stations were in mono. When stereo phonograph records started coming out in the 1950s, the FM stations as a group were in very bad financial shape, because they were having trouble convincing the general public to buy FM receivers. So, as an additional way to differentiate themselves from AM stations, the FM stations applied to the Federal Communications Commission to set up standards to allow them to broadcast in stereo. There were a bunch of competing technical standards, and the FCC picked a single standard for all FM stations to follow. Not all FM stations immediately went to stereo broadcasting. I remember seeing a 1970 FM station listing for the Washington, DC area, which still listed which stations were in stereo and which were mono.

According to the FCC’s AM Stereo page, around this same time the AM stations also asked that the FCC come up with an AM stereo standard, but the FCC said it wasn’t needed. By the early 1980s, with AM listening now declining rapidly, the FCC relented. But unlike FM, where a single standard was adopted, the FCC approved four different incompatible standards, and said “let the market place decide”. This “four standards” non-decision caused extensive confusion, and kept many stations on the sidelines, waiting for one standard to emerge. Finally, in 1993 the FCC adopted the Motorola standard as the sole standard.

But by now, AM listenership has declined so much, it’s hard to believe AM stereo will ever become popular. The stations don’t want to invest in the transmitters until there are receivers, and most receiver makers won’t make receivers because there are only a handful of AM stations transmitting in stereo. The AM band is like a depressed neighborhood, where everybody thinks “it is natural that” that all the upscale shops have been replaced by liquor stores. Personally, I think the FCC should adopt the equivalent of zoning laws, and require all AM stations to broadcast in high fidelity stereo, much as they require them to stay on frequency.

It is a myth that AM stations currently have, or ever had, technical limitations that restricted them 5 kilohertz response. (Programming that comes over phone lines often had lower fidelity, but that is a phone line limitation.) “AM radio was never limited to 5 kilohertz!” threads regularly appear on, where people in the know say that the average AM receiver in the late 1920s was better constructed and had better sounding audio than most AM receivers we buy today. Even in today’s “high-end” receivers, the AM sections are usually junk. Few people today know how close high fidelity AM stereo sound quality can come to FM, because they’ve never had the chance to listen a well engineered station over a well designed receiver.
The AM STEREO Page is a good source of additional information on AM stereo.

hightechburrito: As to why FM is in stereo, I haven’t got a clue.

Because FM stations send out two signals, one for left and one for right.


Wrong thinking is punished, right thinking is just as swiftly rewarded. You’ll find it an effective combination.

Two corrections-

  1. SSB is a variation of AM, not FM.
  2. Stereo FM involves transmitting the L+R (mono) and L-R (difference between left and right) signals, not just sending separate left and right signals. This makes it easy to make a mono FM receiver.

Have you ever wondered why you can get AM radio stations from hundreds of miles away at night, but not on FM? That’s due to the difference in frequencies they use- AM is down at 0.55 to 1.6 MHz, while FM is much higher at 88-106 MHz, which doesn’t usually travel as far. Also, some AM stations boost their signal at night to reach certain areas.



Am and SSB signals are much more efficient than FM, they take up much less bandwidth than FM so you can jam more stations into a given band. AM is much easier to pick up than FM, due to the technologies used. An AM signal is pretty much intelligible down to the signal threshold(if your radio can receive the signal you can listen to it), while FM needs all of its signal to be received for it to be understandable. That is why your FM signals tend to cut out rather shrpley. The third advantage of AM is that you need a lower power output at a given frequency to receive the station.

You want brilliance BEFORE I’ve had my coffee!!!

Arjuna34 wrote

Yep. Sorry.

Stereo FM involves transmitting the L+R (mono) and L-R (difference between left and right) signals, not just sending separate left and right signals. This makes it easy to make a mono FM receiver.

Yes ,but in addition, a referance signal is also broadcast some 19khz below the carrier freq.

You need this to decode l+r ,l-r signal .
This referance signal is of a lower power than the rest of the signal so that if reception is weak it is the first to be lost and the radio will revert to mono.
A signal may be too poor for stereo quality but be perfectly o.k for mono which effectively has double the strength.

When you tune in to a stereo station the little indicator that lights up is actually showing that the referance signal has been picked up.
AM is more vulnerable to noise.When AM is transmitted the width of the signal(amplitude)is made to vary but in FM the amplitude stays the same.

Imagine you have gate which must let something through it,and imagine that our AM signal is a varying width pipe,we have to open the gate wide to allow it through but that means there is room for unwanted stuff to pass when the pipe was not at its widest.That unwanted stuff is noise
With a pipe of uniform width(FM)signal we open the gate just wide enough for the pipe, and so nothing else can get past.

This is in effect is what is done using electronics.

The other reason you get better quality from FM is that the range of frequencies that make up an FM broadcast(bandwidth) is far greater than an AM broadcast.
BTW all you techies FM works on Quadrature,that is the rate of change of frequency which is a significantly differant thing to the amount of frequency shift.


Do not wait for the last judgement-It takes place every day
CAMUS-The Fall


If you are unsure about what modulation and carrier waves are and why they have to be used just add to this thread.
Looks like there are a few bods well capable of giving you the gen round here besides me. :slight_smile:

thanks a lot everyone… you answered my questions pretty well :slight_smile:
as for the rest, i have a book for my class, so that should help
otherwise i will post again

Check out my site:
Chief’s Domain

>> It is a myth that AM stations currently have, or ever had, technical limitations that restricted them 5 kilohertz response

Whitetho, I am not sure what you would consider a “myth” buth let’s look at the math. The total bandwidth of the AM band is about 1Mhz and the total bandwidth of the FM band is 18MHz.

A carrier modulated in amplitude with a 5 Khz signal, would use 10 Khz of bandwidth and you could put (in theory) 100 stations in the AM band.

Now, a carrier modulated in frequency with a 20 Khz signal plus a subcarrier for the stereo uses something like 125Khz of bandwidth. In the FM band you can get (again in theory) about 120 stations. But try using this modulation in the AM band and in theory you could get about 8 stations and in practice, with guard bands you could get 5 or 6. Do you rally think that is viable? If it was a viable alternative they would not have allocated the FM band in the fisrt place, just used the new FM modulation in the AM band.

So there are solid reasons why stereo FM modulation is not allowed in the AM band as just one station would take up the bandwidth of 15 AM stations. That’s not a myth, it’s the math.
bandwidth would use 10 Khz

In the Americas, it is true that the standard AM broadcast band frequencies are spaced every 10 kilohertz, but that doesn’t mean their frequency response is limited to just 5 kilohertz. (In most of the rest of the world, in the preceding sentance substitute: “mediumwave band”, “every 9 kilohertz”, and “4.5 kilohertz”).

The simple reason is that the sidebands of adjacent frequencies overlap, within prescribed limits. A good summary for the current U.S. standards is an extract from a Motorola document that was included in a thread:
High Fidelity AM Broadcasts, which begins: “Contrary to popular belief, AM stations in the United States are not required to roll off audio above 5KHz.”

Also a short review of why “The 5 kHz audio limit on AM is a myth” is included in a post in the
Why is AM reception bad on my $300 stereo receiver? thread on

whitetho, I think you have misread that document. What it says precisely is that AM stations cause interference in adjacent channels because the do not strictly limit their bandwidth.

What I said stands. An FM-quality broadcast requires way more bandwidth than a 5 Khz AM broadcast and you could only fit very few in the AM band. As I said, it is not the myth, it’s the math. Do it and you will see for yourself.

I should have expanded my earlier post to note that, while it is true that “the 5 kilohertz limit is a myth”, because of interference concerns, the current U.S. standards do limit audio frequencies above 10 kilohertz for AM stations. As noted in the first link of my last post: “These specs define the AM channel bandwidth as 20 KHz (or 10 KHz above and below it).”

For the technically inclined, the current U.S. standard is “(b) Emissions 10.2 kHz to 20 kHz removed from the carrier must be attenuated at least 25 dB below the unmodulated carrier level, emissions 20 kHz to 30 kHz removed from the carrier must be attenuated at least 35 dB below the unmodulated carrier level, emissions 30 kHz to 60 kHz removed from the carrier must be attenuated at least [5 + 1 dB/kHz] below the unmodulated carrier level, and emissions between 60 kHz and 75 kHz of the carrier frequency must be attenuated at least 65 dB below the unmodulated carrier level. Emissions removed by more than 75 kHz must be attenuated at least 43 + 10 Log (Power in watts) or 80 dB below the unmodulated carrier level, whichever is the lesser attenuation, except for transmitters having power less than 158 watts, where the attenuation must be at least 65 dB below carrier level.” according to a 73.314  Field strength measurements page.

Since the AM frequencies are 10 kilohertz apart, this means the sidebands of the adjacent frequencies overlap. But you never put two AM stations 10 kilohertz apart in the same city, so this isn’t a major problem.

Although 10 kilohertz audio doesn’t match FM, it is far better than most people have ever heard from an AM station. Most AM receivers are designed not to pick up hifi AM signals. So, unfortunately, even if your local AM station broadcasts hifi stereo, on most receivers it’s still going to sound like it’s coming from a tin can connected to a string.

Another good sites for information about hifi AM stereo is The AM Stereo Listbot Archives.