I’m curious about the difference between AM and FM radio…
Why do most of the stations on AM have a talk format while most of the stations on FM follow a music format? (This is true here, at least, I don’t know about other areas)
Why can you get AM broadcasts late at night far away, but not FM? (I often used to listen to KCBS, an AM news station form San Francisco, while in L.A.)
Which will give better reception into tunnels, on the other side of hills & such?
FM is a better choice for music, because of the better fidelity.
Well, here’s a starter.
FM stations are set up to use more bandwidth than AM stations. That means each FM frequency is a ‘thicker’ chunk, so to speak, of the full radio spectrum and can carry more information. Thus, FM stations get better sound reproductions, especially for high and low tones. This difference is much more apparent for music than for speech, which dictates some of the specialization you have noticed. (Telephone lines generally have even less bandwidth than AM radio, and though you don’t notice that too much when talking to people, it really does tend to jump out at you if someone plays music at you over the phone.) On preview – hi, revtim!
AM radio, on the other hand, has a strong tendency to ‘bounce’ off of certain media differentials, (media in the sense of what the radio waves are travelling through, one medium, several media.) For instance, they will often bounce off of the ground and back into the air, or off of a particular level of the atmosphere (the heaviside layer??) and back down. This is at least one factor that gives AM radio extra range… you might be getting a signal that has ‘bounced’ several times without noticing much of a loss in quality. On the other hand FM needs to have a mostly-clear line of sight from the transmitter (usually on top of a high tower) to your stereo set.
Hope that helps.
You neglected to mention the critical factor that accounts for this difference: wavelength. AM-band signals tend to propagate farther because they are longer waves, and longer waves have a larger angle of reflectance from atmospheric layers than shorter-wave FM transmissions. Your post implies that the difference is due to whether the signal is AM- or FM-modulated, but this has nothing to do with it. A transmission on 700 kHz from a given antenna at a given power level, for example, will travel the same distance regardless of whether it is FM, AM or phase modulated. However, such an FM signal may be able to be received clearly from a slightly greater distance, since FM is less susceptible to being lost in the background noise, all else being equal.
Yes, that’s an important factor, and someone might get that impression from my post. I didn’t mean to imply it that way, but was trying not to overwhelm the OP with too much detail. Thanks for the catch.
I heard that AM waves go farther at night because they bounce off of the “e-layer,” something in the atmosphere at night.
As proof that FM has more immunity to interference like Q.E.D. rightly writes, listen during a thunderstorm. You can usually hear the lightning on an AM station but not FM.
I’m not sure about this. Since FM has more bandwidth, that equal power level is spread over the FM signals bandwidth while the AM wattage is concentrated over it’s carrier frequency and two sidebands, no?
Also, I think in actual commercial broadcast applications, AM stations transmit with more power to begin with.
AM frequencies bounce off the ionosphere, down to earth, and back after sundown. This is why you can receive stations far, far from where you live. The reception will be fine for a period, then fade out completely and come back.
There are many hobbyists who like this pursuit. They call it ‘DX-ing’, DX being an abbreviation for Distance. They take a radio to a place far from electrical wires and transformers, with a sensitive AM tuner, and see how many remote stations they can pick up long enough to identify the call letters. Then they contact the station saying they were received, and usually get a card back with the station’s call letters and logo.
DX is the only AM I can get down here. The closest station I can pick up is Jacksonville, but only if the weather is right. Otherwise, Atlanta and Charlotte, NC are the only two stations on the whole AM dial. The rest is noise.
FM is, as mentioned, line-of-sight. If you’re driving away from a city with an FM station, it will eventually get all fuzzy, then die out as you round the curvature of the earth from where its signal trails off.
AM does talk because it’s suited to the frequency response limitations of AM. It covers a wide area. FM does mainly music because of the improvement in frequency response, although it has a smaller transmission contour. It’d be a waste to use all that potential for talk only.
AM is inherently a worse modulation method. AM is Amplitude Modulation, which means that to encode an audio signal, you use the audio to vary the amplitude of the carrier signal, or in other words you vary how loud the carrier signal is. The disadvantage of this is that if you have two AM signals they add together, and there is no way to seperate them out from each other. Any noise on the same frequency also gets added in and you can’t get it out either. If the signal fades a bit due to atmosphere conditions, the audio signal also fades.
FM is better because instead of varying how loud the signal is, the audio signal is used to vary the frequency of the carrier wave back and forth. To decode the signal, all the receiver has to do is track the frequency back and forth. This means that weaker signals and noise are completely rejected. Additionally, because the signal just tracks the frequency, any fading of the signal only affects the amplitude and not the frequency, which makes FM immune to fading as well.
To make things worse, as chrisk mentioned, FM signals are set up by the FCC to have more bandwidth allocated to them. AM signals have a bandwidth of about 10 kHz (IIRC) while FM signals have a bandwidth of 15 kHz. Your ear can hear up to about 20 kHz, so while FM signals won’t sound as good as a CD, they will sound a lot better than AM to the average ear. Note that it is possible to make AM signals with a wider bandwidth. The FCC just chose not to, because back when the standard was put into place, you wouldn’t have noticed the difference on equipment at the time, and all that extra bandwidth means you can have less radio stations in the same amount of frequency space. Also, FM signals are set up to be stereo, while AM is only one channel (mono).
For all of these reasons (plus everything mentioned by previous posters), AM radio sucks for music when compared to FM. In the late 70’s and early 80’s, the quality of the typical radio in homes and cars got a lot better, and people really started to hear the difference. In the 70’s there were still quite a few radio stations on AM that were still playing music. By the 80’s, almost all of these had either switched to FM or had just gone away. Some car stereos started being made that didn’t even have an AM band on them. It was really starting to look like AM was a dead radio band, and there was a lot of talk about what should be done to either revive it or use the frequency space for something useful. There was a push for a while to have AM stereo, and to increase the bandwidth allocations for each channel. This doesn’t get you around the fade and noise problems, but otherwise it would allow AM to sound almost as good as FM.
In the late 80’s and early 90’s, talk radio started taking off. If you wanted a talk radio station, you had two choices. You could fight for space on the overcrowded FM band, or you could use some of the empty space on the dead AM band. Since voice signals don’t sound bad on AM’s narrow bandwidth, the lesser quality of AM wasn’t really an issue for talk radio. The next thing you know, the AM band is hopping with talk radio stations, and all of the talk about AM stereo and other methods of rejuvinating the AM band pretty much disappeared.
In Colombia I listened to AM radio at night, and received a station in Ohio, once, and another time New Jersey.
Getting off topic, but DX-ing is even more fun if you have a shortwave receiver. I can get the BBC World News and stations in various European languages out here on a clear night via shortwave.
Anyway, I presume shortwave travels better than AM for the same reasons AM travels better than FM (frequency and, to some extent, the power the long-range stations operate at).
Now, something really sideways: Is there any scheme that modulates both frequency and amplitude? Would that even be useful?
A couple of different layers (of the ionosphere): the Heaviside layer bounces “long wave” radio and the Appleton layer bounces “medium wave”. The practical effect is much the same in either case.
Sure is. It’s called “television”. 
I don’t what the situation is in the US , but here in the UK we now have a third system of terrestrial sound broadcasting called DAB (digital audio broadcasting) . As the name implies it uses digital as opposed to analogue technology which broadcasts on a new frequency band of around 200 MHz. The results are crystal clear sound with less interference than even on FM broadcasts. There are some down-sides . For instance, not all of the country is covered at present, but new transmitters are being built. On some stations the sound quality is not as good as it could be, because the bit-rate has been reduced to cram in more stations on a given frequency (or multiplex). Finally the radio sets are more expensive. On this latter point things are getting better . Prices used to be over £200 , now you can get several models for £50.
With these sets do do get a few extras . This includes a LCD screen which gives information such as play-lists, weather and traffic reports . Also the sets are self tuning and most a pre-set buttons for your favourite stations. Because of this new service, several broadcasters (including the BBC) have been able to offer several new minority stations which are not available on the the old analogue services because of lack of space on the broadcast bands.
I didn’t know that, but I meant using AM+FM to add extra information to a pure audio broadcast.
It’s interesting to know and I’m glad you told me, but I was expecting something different. The SDMB challenges my expectations once again. 
I don’t know that you could. Commercial AM has too narrow a bandwidth and Commercial FM relies on strength of signal. If you were to mess with either you’d be unable to recieve it for any sort of distance with the current equipment, and I can’t see how adopting some new standard would be an advantage over what we have now.
Back in the early 1930s, a professor at the University of Iowa conducted experiments with a primitive scanning disc television set to determine changes in the altitude of the Heaviside. There were 2 shortwave experiment stations in Chicago at the time. He tuned in, measured the distance between any ghost images and the real image on the screen, then worked that value through a formula. I guess it was the best they could do at that time.
Anyway, this led to the founding in 1932 of the world’s first educational TV station.
Television uses AM for the video channel and FM for the voice channel. It’s not using both modulation techniques on the same signal.
Combining AM and FM on the same signal isn’t going to buy you anything and it’s going to chew up a lot of bandwidth. If you want to make a real improvement on the signal, go to a digital encoding method. Digital signals have the advantage that unless you flip or lose a bit, the signal coming out the receiver is exactly the same as what went into the transmitter. You can also use error correcting schemes to handle the occasional loss and corruption of data.
To be really picky, in NTSC the video info is transmitted Vestigial (sp?) Side Band, not full AM.
If you look at an AM signal with a spectrum anaylizer, you will see the carrier frequency (what you tune the dial to) and two side frequencies (which happen to be the carrier + and - the frequency of the audio). The carrier and one of the side frequencies aren’t stricly needed to get the audio back. If you strip off both you have Single Side Band (SSB) – this is often used by amateur radio on shortwave. It uses less bandwidth and is more efficient (at the expense of more complicated circuitry and losing some redundancy and fidelity)
Without getting too complicated, in TV the video brightness signal has one side fequency reduced. This reduces bandwidth somewhat without the minuses of SSB.
(For completeness, I’ll color info is transmitted double side band with a reduced carrier)
http://cnyack.homestead.com/files/modulation/ntsc_sig.htm
Brian
AM can sound very good if you have a strong signal and a broadcaster that cares about the audio quality of their transmission system. You also need a decent AM receiver. Unfortunately, audio quality is not a priority for most broadcasters. They would rather be “loud” than “clean”. The FCC’s rules (Part 73.44) allow for a flat audio frequency response out to approximately 10 kHz.
Most people would be amazed at how good AM can sound.