Why do FM radio frequencies end with odd decimals only? You often see 94.5 or some such, by why never 94.6?
Is this a FCC thing or is it just so one station wouldn’t take, say, 94.5 and another take 94.6 and the signals overlap?
Or is there a bit of science I’m totally missing about radio waves…
http://www.fcc.gov/mb/audio/bickel/oddno.html
Apparently the stations are allocated a 00.2 MHz band aligned on even decimals, but you tune in to the middle of this band, i.e., 100.1 for the 100.0-100.2 band.
It’s nothing more than a need to keep broadcast signals far enough apart in order to avoid interference. That’s both the science and the policy reason. It’s no more mysterious than the fact that all FM stations (in the U.S.) broadcast between 87 and 108 MHz.
The F.C.C. was established in order to manage the broadcast spectrum. Radio wouldn’t work if just anyone started broadcasting on whatever frequency they liked. So the F.C.C. designated a certain band of the spectrum for each service, such as FM radio broadcasts, and designated the frequencies that would be available. Then the F.C.C. parcelled out the available frequencies on a city-by-city basis (giving more stations to more densely populated areas) and allowed private entities to apply for licenses for those frequencies.
Also, it’s my understanding that FM stereo broadcast signals don’t actually broadcast on just one, steady numerical frequency, but rather on a narrow band (channel) of frequencies that is merely referred to for convenience’s sake by the midpoint figure. In essence, any such system for frequency allocation would be arbitrary. It just so happened that the scheme chosen by the F.C.C. resulted in assignments that end with odd numbers in the tenths place.
Your first idea is correct.
US FCC regs dating from the early commercialization of broadcast FM require the 0.2 Mhz spacing between stations. It was due to the limited precision of the transmitters and receivers of the day.
Using your example numbers, if broadcasters used both 94.5 and 94.6 in the same area, the 1940s-technology receivers would pick up too much of the adjacent broadcast.
They also assumed that the spectrum from 88.1 to 104.9 giving 85 channels would be plenty; no need to require vastly more expensive transmitters and receivers in the early days just to permit 170 channels.
Nowadays, making transmitters and receivers with tighter frequency spacing is trivial and I’m almost certain they use 0.1 Mhz spacing in Europe these days.
I have a home stereo digital tuner (Yamaha T-70) which I bought overseas in 1982 which can tune to 0.05 Mhz spacing (i.e. 94.50, 94.55, 94.60, 94.65, 94.70 etc). I’m not sure where in the world that capability is used; Japan maybe? But I’m sure Yamaha wouldn’t have built it into the tuner unless it was needed in some part of the world. And that was over 20 years ago.
Since the demand for more than 85 channels in any US market just isn’t there, there’s no push to amend the regs now to permit tighter spacing.
The Feds get into a chicken-and-egg problem if they try to change the rules. If they permit 0.1 Mhz spacing transmissions while folks still have old receivers, that may create interference for those listeners. The FCC’s guiding principle is non-interference with existing users (sorta like the original Star Trek’s Prime Directive).
Fcc regulations require FM broadcast band stations to maintain 200 kHz spacing. Otherwise, one station could interfere with another if they operate too close in frequency.
Just to be sure I understand this - the radio station frequency as advertised (such as 101.1 MHz) is the frequency of the carrier wave, which shouldn’t very, correct? The carrier frequency is then modulated by superposition of the broadcast signal - my question: does the extent of this modulation correspond exactly to the frequencies being heard on the radio (i.e. the audible frequencies are about 20 Hz to 20 kHz, so the 101.1 radio station would be broadcasting 101.1 MHz, plus or minus 20 kHz at maximum)?
Just to be sure I understand this - the radio station frequency as advertised (such as 101.1 MHz) is the frequency of the carrier wave, which shouldn’t vary, correct? The carrier frequency is then modulated by superposition of the broadcast signal - my question: does the extent of this modulation correspond exactly to the frequencies being heard on the radio (i.e. the audible frequencies are about 20 Hz to 20 kHz, so the 101.1 radio station would be broadcasting 101.1 MHz, plus or minus 20 kHz at maximum)?
Local and national FM stations here in the UK seem to use both odd and even decimal figures. The band runs from approx 88-108 MHZ for commercial and BBC stations.
(I know you have had an answer but this was just for interest!)
V
Everyone here (and the cite) is talking about the interval - but the question is actually “why are the midpoints odd not even”? It just doesn’t seem to be a ‘typical’ thing for humans to do - we like stuff neat. It’s also the case here in Ireland.
Is it because the start and endpoints are even, so the first and last frequencies have to be at least 0.1 MHz from these points?
Well, thanks very much for the information, guys. Question asked and answered. And in less than an hour!
What you suggest is true for AM. For FM, the frequency range is greater. This is part of the advantage of FM (though it comes at a cost): greater channel bandwith allows greater robustness to noise. See http://en.wikipedia.org/wiki/FM for more.
That’s pretty much it.
In order to maximize broadcast spectrum usage, given that you have what amounts to a 15 kHz source with a standard of +/- 75 kHz deviation, you’re looking at approximately 90kHz of deviation from the center frequency. That leaves about 10 kHz for fudging so there’s no overlap, for a total FM bandwidth per station of 200 kHz.
Since the band is 88-108 MHz in the US, you have to start at 88.1 in order to account for the 100 kHz of deviation on both sides of the center frequency, and therefore it stands to reason that the center frequency of the following station will be 200 kHz up in frequency, since they’re using the same standards.
The whole point of FM is Frequency Modulation. In other words, in order to convey the information in the signal, they vary the frequency of the carrier back and forth. The ratio of how much they let the carrier vary in frequency (in this case 0.2 MHz) compared to the minimum shift in frequency that the receiver can discriminate determines the dynamic range of the system.
By varying the frequency, the receiver needs to only track the frequency to recover the signal. Since receivers tend to lock onto only the strongest signal, this means that there is very little interference from other signals (like other radio stations, background noise from space, etc). This is in sharp contrast to AM. Since AM only varies the amplitude (kind of like the volume) of the wave, any interference is added into the signal and there is no way for the receiver to get rid of it.
Some stations used to quote the even number. In Atlanta Ga. we still have Z93 (Which is 92.9) and years and years ago, we had Peach 95. (which is now Lite 94.9). Then again, the other stations quote just the standard number… Star 94 (94.1) and what used to be Fox 97 (97.1) Perhaps the idea that the odd numbers are the midpoint brings truth to this. Yet that would thow out the first two.
Meeko, that was done just for convenience. Remember, digital tuners are a relatively recent development and tuners were not and did not have to be labelled very precisely. Additionally, channels are allocated by region so that two stations aren’t close to each other. In Dayton there was a 107.7 MHz that called itself 108. It might have been considered easier to remember and easier to market and it didn’t make much difference when you were dialling in.