Austin got an Air America affiliate, so I decided to switch my background noise from music to talk. But now I wonder a few things…
Why is AM radio so much more static-filled than FM? Why does AM seem to “drift”–I don’t have a digital tuner, and often have to retune to the station (which I never had to do on FM)? Are there any advantages to AM over FM that keep it in use? It seems like the FM dial would have to be completely saturated before anyone would bother with AM, unless they’re just an old station using the old technology. Am I missing something?
AM is amplitude modulation, FM is frequency modulation. Very different ways to change your wave, so that would account for the different ways the tuning behaves.
Why is AM still around?
1 – The signal can travel farther… you can get an AM signal for sometimes as much as 120 miles, where FM seem to only go about 30-60 (depending on the power of the station of course). AM signals tend to bounce around in the atmosphere more, therefore they travel further.
2 – It has a lower sound quality, but for things like talk, you might not need the fidelity of FM. I assume an AM station is a lot cheaper to operate, or at least the license is cheaper, so as long as there is a market, why not?
FM relies on the deviation from a frequency, which it turns into amplitude, which becomes what you hear. If the analog dial drifted at all, the carrier frequency you’re looking for would be a little off but you’d still get most of the band around the true carrier wave. There’d be some distortion (which you may or may not notice), but you wouldn’t necessarily lose the signal itself. AM, on the other hand, relies on varying amplitude of a specific carrier wave. If the dial drifts at all you quickly lose the ability to pick up the signal because you’re looking at entirely the wrong wave.
As far as static goes, consider that there is background radiation across all frequency ranges. Any bit extra on a particularly frequency would come across as noise on an AM broadcast (waves superimpose or otherwise interfere with each other). The amplitude of an FM signal is mostly irrelevant, however, so background radiation would not affect FM in that way. (Of course amplitude eventually matters when the signal is too weak to pick up at all, but otherwise it doesn’t matter to what you hear.)
Here’s the Wikipedia entry about am radio. It explains pretty much why am is not as clear as fm. One thing about am radio is that at night you can pick up stations from a long way away, especially some of the higher wattage stations. I can pick up stations from southern california in Mississippi sometimes, for example. I remember reading about a very high powered station in the thirties that was so high powered that barbed wire fencing a few miles away from the broadcast tower would play the signal out loud.
That’s not right. The noise is not due to signal drift, and it’s not harder to lock onto an AM signal than an FM one.
AM has more noise than FM because of two main reasons.
The simplest is that common noise sources (engine sparks, electric motor brushes, and lots more) put out less and less of their noise at the higher frequencies. If you looked at the spectrum of that noise, they’re larger at lower frequencies and lower at higher ones. The AM band is around 1 MHz - FM is around 100 MHz. Big difference right there.
FM is inherently less susceptible to noise because of the modulation technique. FM stations are 200 kHz apart, instead of what, 10 kHz (?), for AM stations, so FM gets to further increase their noise immunity by using more bandwidth. (The noise immunity on an FM signal is dependent on the ratio of the used bandwidth to the bandwidth of the unmodulated signal - I can’t recall if it’s directly proportional, or proportional to the square root of the ratio).
AM signals also bounce off the atmosphere, especially at night, while FM signals don’t. So a powerful AM signal can literally be heard across half the country, while FM has trouble being brought in 100 miles away.
Back in the olden days, the government allowed each frequency to have one and only one 50,000 watt station, that being the most powerful legally operable. These so-called “clear channel” stations became the core stations of entire regions of the country, listened to by people hundreds of miles away from the base. Many of them were affiliated with the networks but some were powerful independents.
When I was a kid - pre-FM - we used to lie away into the night with our transistor radios, logging the most distant stations we could draw. The clear channels were easy; it was the little 5000-watters that only came in when the conditions were right.
The cultural and economic power of the clear channel stations (now you know why Clear Channel Broadcasting took that name) lasted well into the 1970s, when the growth of stereo FM broadcasting took most of the music away. AM didn’t quite wither and die, though. The AM spectrum is perfectly well suited for the human voice, and so first all-news, then all-sports, all-weather, all-religious, all-politics, and other all-talk stations took over the AM band. (I wrote a major paper in grad school predicting the coming of this kind of “narrowcasting” as we called it then. Not bad for 1973.) These can still draw listeners from large areas and are fine for the nationally syndicated programs with 800 numbers and a central computer to keep everything straight.
Sideways topic - this is how reggae music started. The signals from an AM pop radio station reached islands in the Carribean - but they were distorted. There was an echo which caused a backbeat. As bands tried to imitate the sound of American Pop Radio, they invented reggae rhythms.
I’m not trying to be snarky, but this is GQ and that sounds an awful lot like an Urban Legend. I am in no way an expert on the origins of Reggae, but I’ve never heard this theory. It’s a great story if true, but … could you back it up with a source?
Ska and “rocksteady” are the forerunners to reggae. Ska developed in the late 50s/early 60s as an amalgamation of a wide range of styles, from early rock-n-roll, R&B, jazz, Afro-Cuban music, etc… In the mid-60s, the hyper stylings of ska lost popularity and gave way to the much slower rocksteady style. Take ska, play it twice as slow, swap the brass for a piano, throw in a prominant electric bass, and you basically have rocksteady. From there, it was only a short hop to reggae.
Yes, contrary to many people’s perceptions, reggae was an outgrowth of ska, not the other way around.
FM radio was developed by Edwin H. Armstrong specifically to eliminate the static and noise in AM. This hacked off his boss, David Sarnoff.
http://www.wsone.com/fecha/armstrong.htm
For more information you might check out the book “Empire of the Air” or watch the Ken Burns documentary of the same name.
To expand on this a little. The FM modulation and detection system supresses noise a great deal. The output voltage of an FM detector depends upon how far the received frequency deviates from the center, or carrier, frequency of the wave. A frequency that is 2*f Hz away from the center frequency produces twice as much output as one that as only f Hz away.
Let’s add together a wanted signal of 10 kHz and an unwanted noise signal of 9999 Hz. The two signals will thus go in and out of phase and the resultant will be a phase modulation superimposed on the frequency modulation of the desired signal. The maximum phase shift of the sum of the two signals is 45[sup]o[/sup] or 0.785 radians.
The equivalent frequency deviation of such a phase modulation is the phase shift in radians, 0.785, times the modulating frequency 9999 Hz. This frequency deviation is thus 7849 Hz. FM broadcast standards are such that the frequency deviation of the signal will be about 75000 Hz. The ratio between these two is 75000/7849 = 9.555.
This means that the FM detector described in paragraph 1 will produce 9.5 times as much signal voltage as noise voltage for a signal to noise improvement of nearly 20 dB.
For lower frequencies of noise of the same amplitude as the signal the improvement in signal to noise will be even greater.
On the same topic, I have a bass amp that sometimes picks up radio frequencies when it’s on and the chord is in the amp but not the bass. Is that AM or FM?
Some believe that the early jazz and rock ‘n’ roll broadcasts from American radio stations were misinterpreted by an eager Jamaican music audience, hence the off-beat rhythms that almost mimicked the break up of weak radio signals that hit the West Indies shores. Others consider ska not a misinterpretation but its own response to American music. The sound of ska was created at facilities like Studio One and WIRL Records in Kingston, Jamaica, by producers like Dodd, Reid, Prince Buster, and Edward Seaga (later Jamaica’s prime minister).
I guess I should have mentioned that my example was illustrative only. FM receivers include amplitude limiters that remove most of the input signal amplitude variations that are actually encountered in practice. However the sum of two signals of equal amplitude and nearly the same frequency would result in amplitude variations from two times the amplitude of either of them down to zero amplitude or the disappearance of all input signal. The limiters cannot remove the effect of this much amplitude variation. However a 2/1 signal to noise ratio could be within the limiter capability and the same computation as before results in an 18 dB (8/1) signal to noise improvement.
“This is how reggae music started” is miles away from “some [unnamed sources] believe that the early jazz and rock ‘n’ roll broadcasts from American radio stations were misinterpreted by an eager Jamaican music audience…” It’s not a very convincing explanation at all, and the more prosaic idea that American R&B was combined with native calypso and Afro-Cuban rhythms seems infinitely more plausible to me.
I wasn’t addressing noise in that part of the post. He was wondering why he seemed to lose the signal, whether not having a digital dial would account for that. It was the only thing I could think of. The noise issue, we both agree, is about modulation and background radiation.
In reference to a crystal radio. From an advertisement online:
The first form of modern radio, this simple to build AM radio does not need any battery to operate. It is a great introduction to a life time of electronic hobby building fun. Includes enclosure!
Anyway, the nature of AM radio can cause it to get picked up by things that aren’t radio receivers…or weren’t MENT to be. Every now and then you’ll read about someone picking up radio on a tooth filling, for example.