FM vs AM radio- why the sound difference?

Back in my school days, we learned that the AM signals were more susceptible to interference and atmospheric noise. That alone probably influences the quality of the sound most of all.

Furthermore, AM stations needed to keep a 10kHz frequency spacing. If your station broadcasts at 1000kHz, the adjacent stations (on the dial) needed to broadcast at 990kHz or 1010kHz. That 10kHz also limits the audio frequencies that you can broadcast to between 20Hz and 10kHz. Since "normal" audio frequencies reach up to 20kHz, you've lost some of the information above 10kHz.

FM stations, on the other hand were allowed 100kHz spacing, so the full 20Hz to 20kHz sound spectrum could be broadcasted.

For this reason, most AM stations are "talk" show stations. Voice frequencies range from 200Hz to around 2kHz so ordinary talking was easily handled by AM stations.

One other reason, by my observation, is that most radio stations today use FM. Therefore, when you tune in to an AM station, chances are that station is a long distance away from you. It's signal will have deteriorated a bit by the time it reached you.

There are all sorts of explanations like depth of modulation...but it is much simpler to reverse your question and ask why on earth would you expect two entirely different methods to be the same?

Most radio reception is abysmal quality...we keep being sold 'improvemnets' which are not much better and sometimes worse... ho hum... digital anyone? It will be perfect reception/sound quality (not).

hello

AM means amplitude modulation, and amplitude of an electromagnetic wave decay with distance and weather conditions, thus the information carried get poluted and the sound cannot be correctly replayed.

with FM, frequency modulation, it is a matter of signal processing , once you have a fixed amplitude , for a fixed distance , and the information can be more eficiently restored with higher degree of signal perfection, thus the sound comes more clear!

Thank you

your post was informative and it helped me

--

Srikanth AD

AM - amplitude modulation depends on the amplitude of the received signal. Noise is additive and affects the amplitude of radio signals. The amplitude is where the signal lies. Thus, noise is received along with the desired signal.

FM - frequency modulation depends on the frequency of the signal. Noise doesn't effect the frequency of a signal (unless it's really BIG noise) since it rides on the "top" of the waves. Noise added to the amplitude doesn't affect the frequency content, and the desired signal can be recovered.

This is it!

Go no further.

Modulating VOLTAGE(Amplitude) at transmitter causes Carrier phase/frequency to shift+-50KHz.

Same max. 50KHz WHETHER moduating Signal is 300Hz or 12KHz.

At reciever, you receive(due to Automatic Gain control) a constant Amplitude CarrierSignal-with frequency -as modulated at the transmitter. You get none of White Noise, Adjacent Channels( I.F.= 10.7MHz is a sharp Band-pass Unit)and atmospherics.

You can even use peak clipping to get constant Amplitude of IF output (harmonics won't dare to pass)

So next your Discriminator(detector )merely hands over the original Audio Signal sans noise.

You Amplify,play and enjoy the Original HI-FI.

It is not just the modulation, it is also the bandwidth (sampling rate & # of bits):

FM radio stations broadcast with a bandwidth of approximately 20 kHz, whereas AM radio stations are limited to about 3.2 kHz of bandwidth.

"High fidelity" music requires a bandwidth of 20 kHz, but to reproduce (fairly natural sounding)human speech only requires about 3.2 kHz.

Even though the frequency range has been reduced to only 16% (3.2 kHz out of 20 kHz), the signal still contains 80% of the original sound information (8 out of 10 octaves). More than sufficient for satisfactory voice reproduction...

Well, technically AM/FM radio are analog media (not counting the new digital channels), so there is no sampling rate / # of bits issue per se, but you are correct about the bandwidth - although I think the numbers are closer to 10KHz for AM and 15KHz for FM).

Also, much of AM radio is obnoxious "talk radio" and they typically use compression to make it as obnoxious and "in your face' as possible.

Another issue is the receiver. Most radio designs have more fidelity in the FM section than the AM section, since that's where the hifi broadcasts are.

And then there's the fact that FM is usually broadcast in stereo, while AM is mono.

Interesting. These above answers do help make sense of this. I'm going to be trying an experiment on my commute home from work today- listen to FM but NOT on the 'stereo' setting on my car stereo and then AM, and see if they still sound so different in mono.

I do see why there's a difference- and don't really expect them to be the same, but didn't have a good understanding of WHAT exactly the difference is. thanks.

Ah, but now you're talking about the difference between mono and stereo. That's a different topic altogether. AM radio broadcasted in mono because the bandwidth for each station did not allow for stereo broadcast. There was an attempt several years (or is it decades) ago to upgrade AM to stereo and a few stations even went on line. I don't know if it ever took hold.

The difference in mono and stereo is like night and day. Personally, I can't say that I know exactly why. If you lived close to an AM radio station (so that the signal quality is good) and tried to compare an AM broadcast in mono to an FM broadcast in mono, you'll be hardpressed to tell the difference.

If you have a radio receiver with two speakers, a mono signal will tend to be concentrated towards the middle. If your listening position is off-center, part of the sound is attenuated.

A stereo signal (assuming they broadcast different sounds for each channel), doesn't suffer from the same phenomena.

And what a difference! "stereo" comes from "stereos", Greek for "solid". It really is solid sound. Experimental stereo music recordings had been made in the 1920's and 1930's. In the 1950's many AM radio stations operated FM affiliates over which they often broadcast classical music just to keep their licenses active before FM receivers became cheap and widely distributed.

Experimental stereo music broadcasts were a fairly frequent result of this situation, employing the AM station to broadcast one stereophonic channel and the FM station to broadcast the other. The enhanced sense of "presence" on the receiving end more than made up for any deficiencies in the audio quality of the AM channel, although knowledgeable engineers and well-maintained equipment were capable of producing some pretty impressive sound, even over the somewhat restricted AM bandwidth.

Electrical noise can easily be added to the signals of both systems, causing a change in the amplitude of the signal. This shows up in AM but is ignored in FM. Also, the perception that AM isn't for serious listening may mean that AM receivers are designed with less care.

Of course, modern error correcting techniques can fix the noise problem and high speed data communications systems (that must be ultra reliable) use both FM and AM simultaneously. Jeff

What I know of the subject is that FM can carry way more information than AM, and, as has been mentioned already, is not so subject to amplitude noise. This is why your TV uses an FM signal (on an antenna connection) rather than AM.

That extra information payload carries more sound info, as in frequency of sounds, as well as stereo information. AM just doesn't have the same bandwidth for this information payload - unless they do some really weird signal compression stuff that really doesn't pay off with more fidelity. It was more sort of an effort to keep little teenie-boppers tuned into AM.

There are two different problems:

Bandwidth: As a result of AM there will be two sidebands. For example, if you have a 500 kc carrier and 1 kc modulating sound, the sidebands will be 499 and 501 kc. According to the international regulations the lowest frequency distance between two AM stations can be 10 kc (9 kHz in Europe). That means that in AM you must limit the maximal modulating frequency to or below 10 (9) kc, so you will loose the higher components. In FM the frequency step is 100 kc so you can use higher modulating frequency.

The other thing is the noise immunity: in FM it's much better.

Think of the following analogy with regard to radio wave modulation.

Imagine a light bulb that is on, and not at full brilliance. A light bulb that is simply 'on' carries no information other than that it is there, which can be called the 'carrier' signal in radio terms. In order to transmit a sound signal, one needs to add that signal in some way to the carrier, the light coming from the bulb, to convey that information. The process is called modulation and there are several ways of doing it, the simplest of which is called amplitude modulation [AM] and the next simplest, frequency modulation [FM]. Back to the light bulb quickly: AM varies the brightness of the light. FM varies the colour of the light. The speed of the variation is a direct translation of the audio information, which is now superimposed onto the light coming from the light bulb to add meaning to the carrier signal. What the receiver does is to take the meaning and convert it back to sound.

Now substitute a radio wave for the light from the light bulb and there it is in a nutshell...

One doesn't need a lot of 'room' for amplitude modulation, so it is easy to pack a lot of stations into a relatively small space on the radio dial. In fact, they are usually packed so close together that unless the sound quality is limited in some way, the receiver will 'hear' the station it is tuned to and also the next station above and below that point on the dial, which would sound awful. So the sound is limited in quality by chopping off the upper end of its range; think of it as a piano with the top eight or nine keys missing - the receiver just can't play those high notes. Sound that is limited in this way can be easily crammed into the limited space around the 150-250kHz ("Low Frequency" or "Long Wave") and 550kHz-1500kHz ("Medium Frequency" or "Medium Wave") parts of the radio spectrum, and that is where AM stations can usually be found.

There is more 'room' at Very High Frequencies [VHF], typically 88-110MHz, so the FM technique can be used here to great advantage to give a higher quality sound. In fact there is so much room that the top eight or nine keys can remain, as well as a few more keys off to the right where the piano keyboard usually ends. The thing with FM is that as well as the information on the extended piano keyboard, a few additional piano keyboards are needed further off to the right to convey the full sound image, which is why an FM signal needs more room. Further, a receiver can squash out ('squelch') a signal that is weak, allowing the listener to receive just the station selected and in high quality sound. Those additional keyboards to the right can be used to carry the additional information that is needed to produce stereo sound. Of course, if the receiver is switched to mono then it doesn't have to 'work so hard', so a weaker signal can still be received and the meaning converted to decent audio by disregarding those additional keyboards off to the right.

Tuning an FM receiver through a part of the band with the 'squelch' switched off will cause the white noise to be found there converted to unpleasant audio, until a station is found at which point the receiver 'locks on' to the station and the noise vanishes. An AM receiver doesn't have this capability, so repeating this procedure will cause all sorts of bits of radio sound from all over the place to be converted to various squeaks and whistles at the speaker, because that is all the (much simpler) AM audio circuits can do with it. Further, VHF has a relatively short range, whereas LF and MF will bounce off various things like the underside of the ionosphere, which explains the same-channel interference that can be had from other stations after dusk.

Phew! Not too technical. Hope that's OK?

Not too technical at all, PW!

That was masterful explaining of wave modulation. If you're a teacher, your students must enjoy your classes.

Aw, shucks!

Agree with Vulcan, a very nice explanation, the most correct & coherent so far (some of the other answers are rather wanting). Mr. Slack's analogy to lightbulbs, varying intensity vs. color is excellent, and one I hadn't thought of (in fact this subject is right down my alley but it's very difficult to answer such questions without a whiteboard in front of us, a cup of coffee, and about 2 hours). I hope I can help a little..

Staying with the analogy of light-bulb brightness-modulation for AM: the key point is, ANY light source – another station, or external noise (lightning being the equivalent, in the AM band, of a *massive* camera flash going off!) – will make it harder to discriminate between individual light bulbs and subtle changes in their brightness.

Slack is right, there is much more 'room' in the FM band. The stations stand more apart, whereas in AM they're closer together. This makes the AM receiver's job of 'tuning' – selecting one station while rejecting all other signals – much harder. The higher channel spacing in FM allows a cleaner, wider channel for each individual station.

If you've listened to AM radio at night (and its cousin shortwave), you've surely had the wonderful, remarkable experiences of receiving a station from thousands of miles away. This is because the much longer wavelength of AM radio allows it to bounce off the night-time ionosphere, thus allowing these signals to reflect around the earth. But even during the day, AM generally propagates further. FM is more or less line-of-site. Add this to the previous paragraph and you see that AM suffers a double-whammy: the receiver 'sees' many more signals, closer together, and they're all varying in their amplitude!

AM radio typically has a bandwidth of 300 – 5,000 Hz or so – slightly better than a good telephone line. FM is more like 30 – 15,000 Hz – more or less the frequency content of an LP record, which allows very fine reproduction indeed. FM also has significantly greater dynamic range (though this depends a lot on the cleverness of the detector designer).

Last but not least, thinking back to the light-bulb analogy: it's rather simple to devise a detector to convert amplitude modulation to sound, but much more complex to convert changes in frequency to sound. The FM demodulation process inherently rejects many of the noise sources that plaque AM.

By the way, in reply to #11: TV broadcast modulation (the existing analog systems, not the new digital) is a mixture of AM & FM modulation

And as I often end up these days, the ubiquitous Wiki articles are quite good:

http://en.wikipedia.org/wiki/AM_broadcasting

http://en.wikipedia.org/wiki/FM_broadcasting

Have fun listening

/rf_guy

AM (amplitude Modulation) is simple changing the size of the signal (the amplitude or brightness as was said of the light bulb). The noise comes from the fact that many things compete for that frequency and can increase or attenuate (cancel each other out) the amplitude at specific instances when they both arrive at the AM reciever at the same time.

This interprets to static when a lightening bolt strikes radiating radio waves all over randomly, causing unplanned rises in the amplitude arriving at the AM reciever. Other AM stations far enough away to have license to operate on same frequency also intermittantly intefere as they bounce off of large objects (especially at night). Also there is such as thing as secondary signals resulting when a specific signal reflects off of buildings and comibine with other radiation/radio signals that are being sent by many transmitters (including police, fire, aircraft, construction crews, road maintenance, county, city state workers, etc).

All this adds up to a glorious cacophony of mixed sigals , secondary signals, on all frequencies, bouncing off of everything, all of which easily interfere with the planned amplitude at any one instance ( of course only the signals or 'noise' that is on the same frequency approximately as your reciever is tuned actually interfere with your chosen frequency at any one time as you listen).

Since amplitude modulation is easily intefered with in this way, Jammers are easily constructed as well .. (just pick the same frequency as you want to jam such as the aircraft communications frequency of your enemy and then broadcast as big a signal as possible which causes constant 'noise' to intefere with the planned signals arriving the receivers tuned to the frequency. Squelching lower amplitude signals works well to block unplanned signals but its not easy to block higher amplitude signals since the signal you want to receive is mixed in with them (saturated and absorbed).

For FM the massive amplitude noise is not 'heard' because the reciever is looking for frequency (color) changes not voltage change and occasional frequency changes that correspond to your planned reception are much less frequent (it has to be the same color and the same changes of color as the light bulb so to speak). Of course a big enough amplitude signal (like a lightening bolt) will definitely put a 'popping' noise briefly thru your reciever because FM yes can also be jammed with a big enough amplitude signal but its much harder to do because somce Radio signals drop off exponentially with distance .

The higher FM signals also are better at bouncing off roadways buildings, and undersides of bridges which is why FM works under bridges as long as not a true tunnel of sufficient length. But the same shorter wavelength (higher frequency) which makes that bouncing great also penetrates the ionosphere easier and so they don't bounce off very much at all (long distance is less possible). You can hear chicago for a hour or so at certain times of night on AM but not FM for this reason (FM is really a straight line 'line of sight" deal which a the surface of the earth is only about 13miles. But they overcome this to an extent by using very tall towers to get more line of sight (like 50 -100 miles). AM is supposed to be predominantly but happily they bounce off mountains and ionosphere which gives them 'super' range of of a sort for limited times at night.

Ok, so how do they do that thing with radio signals (TV, too), where they can make it sound louder even though it's not really louder. Something to do with modulation?

The CB nuts of the 70's were all going crazy to get an upgrade for this. Any ideas?

To #25 (Triple): good stuff. To which I only add, in order to emphasize again: it is the nature of modulation which leads to a detector (or demodulator) type, which makes AM very susceptible to noise, and FM much less so.

To #27 (Vermin): They use 'compression', as nearly all broadcast services do, to some degree. You devise circuitry to make the difference between the softest and loudest sounds smaller, i.e. compressed, then modulate, using that maximum level, at 100%. Makes the whole thing 'sound louder', but it doesn't sound very good, because our ears hear the difference. Some level of compression is used almost universally, because the real world has background noise sources, not everyone has perfect equipment, etc. Otherwise quiet passages would just disappear into the background.

RF guy,

Thanks!

It may not sound very good, but it's enough to make you jump for the remote WHEN THE SOFT DRINK COMMERCIAL COMES ON!!!

You are right, it gets on your nerves having to switch the sound off each time for the Ads....if the advertisers knew this, they would probably desist.....but obviously they don't!!

I have a small cheap portable TV with DVD player, that I bought in Tescos UK a few years ago, this has a setting that stops this compression being noticeable! It works just great to keep volume stable.

It would be nice if that feature was available on all TVs! Or in the sound equipment and receivers! But its generally not!!! Sadly.

If anyone knows of a plug in unit that I could place between the Sat receiver and my TV to stabilize the volume changes of this compression, I would be most grateful.....

A great unit for someone to design and sell!!! It is also not affected by TV standards as the sound is international to a great degree! JUST FOR STEREO WOULD BE ENOUGH!!

I think the reason why they don't stop it is they believe, like a child, bad attention is better than no attention at all.

But your system actually came with a feature to cancel it! That's a really cool selling feature!

PW, where were you when I was growing up? You are the Master of Enlightenment

I the USA, I believe around 30 odd years ago, maybe a tad more, some experiments were made with AM Stereo. I do not believe it ever became popular, but perhaps someone from the US of A can enlighten us further.....

In the UK in the later 70's and early 80's you could experience surround sound from the BBC if you had two stereo FM radios, each tuned to a different station, one broadcasting the front and one the back!! It was amazing!!

It's really very simple. Most AM radio receivers offer limited bandwidth, and relatively low quality audio circuitry. Most FM radio receivers, on the other hand, offer much wider bandwidth, and much higher quality audio circuitry; hence, a difference in perceived sound quality.

Commercial AM broadcasting takes place in the designated AM Broadcast Band, from 550 Khz through 1,800 Khz. At these frequencies, in order to provide room for a reasonable number broadcast stations, bandwidth of the transmitted signal must be restricted. Allowing a 20 Khz audio bandwidth on the transmitted signal would result in 40Khz+ wide channel allocations, which wouldn't leave room for many stations. So audio bandwidth of AM broadcast program material is restricted to 5 Khz, yielding a 10 Khz signal bandwidth (4 times as many stations are now possible).

Commercial FM broadcasting takes place in the designated FM Broadcast Band, from 88 Mhz through 108 Mhz. Already, we see a huge difference in available spectrum. Where the entire AM band fits within1.25 Mhz of spectrum, the FM band is allocated a total of 20 Mhz (over ten times the spectrum). FM broadcasters are allocated a 100 Khz bandwidth per station assignment, permitting the transmission of 20-20Khz audio bandwidths in two separate program streams (at least, here in the USA they are. How the bandwidth is utilized elsewhere in the workld I do not know).

Hi!

differences belongs tothe early beginning (microphone) and the last outcoming (speakers and the room)

All the way are a lot of distrosions.

Any way FM signal can manage a broad band or spectrum, that means the signal you recive is close to the source signal.

Any way your hearing ability is also very important, for example I've a cut in the frecuencies I can handle in my hear, for me sounds different from you. for the same source at the very same time.

Have a nice sound system!!

So you're saying it's like the difference between m&m's and w&w's... Right?

What are w&w's?

Andy, think about it. If you've never had m&m's I'll explain it to you. Otherwise...

m&ms is the USA name for what we call Smarties .......??!!??

Explain further please....

I don't know whether Smarties are the same as m&m's - these are filled with chocolate. "Melts in you mouth, not in your hand." is their famous tag line.

Anyway, there is a joke based on the presumption that not only did you have a deprived childhood, but the Universe is somehow out to keep you down.

Take the m&m symbol and turn it upside down.... ww, right? So, "while all the other kids had m&m's all I had were ww's." Get it? I would have thrown in an upside down "&," but I couldn't find one.

Thanks for the explanation...

Then again, I can't expect it to translate well when "lukewarm" means homosexual, and "Three cheese guy" means he's short.

As you probably are already aware, about 40% of US citizens originally came from (or are descended from people from) Germany, and a typical German saying for a shorty, or even a small child is "Drei Käser Hoch", which is an almost direct translation of your "Three Cheese Guy", well almost! " Three Cheese high" would be even more accurate....

I guess they mean those huge round cheeses.....three of them, one on top of the other....

I have never heard it in the UK, so I guess it comes into the US from German immigrants....

I just got it from this really foxy German chick I was messing around with. She was telling me some of the weird stuff her first-generation German mother would say - oh, "spinach" was also somehow related to homosexuality. Beats me!

Please, please, please use the spelling checker on CR4's Editor as it can affect the readability of your posts sometimes. Nobody expects you to know each English word yourself, and the spell checker does it all for you in a very simple manner.

If you have any problems using it or even finding it, let me know and I will write you a simple description on its usage. It is very easy and even I need it from time to time.....!

Have a great day.