Why Do FM Frequencies End in an Odd Decimal?

http://transition.fcc.gov/mb/audio/bickel/oddno.html

by shoeless • 13 years ago

99 54 13 years ago

transition.fcc.gov

blueblob• 13 years ago

Did I miss something, or did this not answer the question in it's heading? It basically says it's like this in the US.

datawander• 13 years ago

Yes, that's exactly why, because the FCC said to do it that way. There are countless examples of "why is it this way" being answered "because that is what some arbitrary person picked."

My favorite example of this I remember reading in Andrew Tanenbaum's Networking book that the first frequency-hopping protocols used 88 frequencies because that is how many keys there are on a piano:

https://en.wikipedia.org/wiki/Frequency-hopping_spread_spect...

bnegreve• 13 years ago

[Why] the first frequency-hopping protocols used 88 frequencies because that is how many keys there are on a piano

Well, it is a rather arbitrary reason but a valid explanation.

In the case of FM frequencies, the question remains ... why did they picked odd frequencies?

ds9• 13 years ago

Another example is the Fahrenheit scale. The eponymous originator calibrated his thermometer by the temperatures where he could obtain brine (0 F) and an inaccurate estimate of human body temperature (100 F). https://en.wikipedia.org/wiki/Fahrenheit

kkjkok• 13 years ago

The most interesting part of that is the WHO of spread spectrum... Hedy Lamarr (movie star) and George Antheil (composer). Very fascinating that two major Hollywood players were able to innovate in hard science/math. A different era.

I think the piano keys stuff was because the idea was semi-based around a player's piano (if I recall the legend from my DSP prof). The roll moves around and different bumps make the notes sound - a similar concept (visually at least) to coding a communications signal and having it jump all over frequency. 88 notes -> 88 frequencies. I wouldn't be surprised if the experimental equipment was a modified version of a player's piano. Very cool stuff, though I may be recollecting the story wrong. Wikipedia agrees about the movie star + composer bit at least.

baddox3• 13 years ago

Article with headlines like "Why X was chosen" are expected to actually provide explanations of human motivation, rather than the nearest proximate cause. For example, an article called "Why this stretch of highway has a speed limit of 30 mph" would be expected to say something like "because there is a history of accidents there due to the narrow shoulder and tight turns" rather than "because the transportation department, which is in charge of setting speed limits, posted 30 mph signs there."

pwg2• 13 years ago

It's the typical "write a lot, but don't really answer anything" style that so many politicians utilize, only substituting "talk" for "write".

The entire section could have been summed up as:

Because each broadcast band is identified by the frequency of the center of the band, the bands begin on an even number, and the bands are two "units" wide, making the center frequency an odd number.

Edit: for clarification.

dfc2• 13 years ago

The text was most likely written by a civil service employee. Civil servants are not politicians.

miahi• 13 years ago

This only happens in the US, probably. The fun fact is that some car manufacturers believe this is global, so they only allow odd frequencies on the car radio.

nkvoll• 13 years ago

We had a radio in our old Chrysler Voyager that required a pretty insane secret code being entered (turning it off and on again 5 times in a row or something like that was part of it) in order to enable to tune to regular EU frequencies...

richardw• 13 years ago

My 2013 VW Golf has a similarly mad method to move the windscreen wipers to a position that enables the window to be cleaned. I forget the trick - I think its turn the car off, then on, then move the wiper stalk to one of its four or five positions. Or vice versa. I really can't remember.

Edit: switch ignition on, then off. Briefly press the down the windscreen wiper lever.

talmand• 13 years ago

I recently watched an episode of Top Gear where Clarkson was driving a Ford Mustang and used the odd-number ending frequencies on the radio as a part of his never-ending "American cars are weird" joke.

steeve• 13 years ago

The first and only time I saw a radio with odd only decimal frequencies was in the US. Nowhere else.

kps• 13 years ago

Wikipedia¹ has a more interesting answer to "Why?" —

  After World War II, the FCC moved FM to the frequencies between 88
  and 108 MHz on June 27, 1945. The change in frequency was said to
  be for avoiding possible interference problems between stations in
  nearby cities and to make "room" for more FM radio channels. However,
  the FCC was influenced by RCA chairman David Sarnoff, who had the
  covert goal of disrupting the successful FM network that Edwin
  Armstrong had established on the old band.

¹http://en.wikipedia.org/wiki/FM_broadcasting_in_the_United_S....

ef47d35620c1• 13 years ago

RCA went after Armstrong. They knew that by convincing the FCC to change the frequencies they could seriously disrupt FM and make existing equipment useless. Armstrong later committed suicide. His wife won all of the on-going lawsuits. Basically, RCA broke him financially and mentally with multiple lawsuits and years of "discovery". Armstrong is the father of frequency modulation (FM) and some argue the father of modern radio.

jejones3141• 13 years ago

I'd be among them, since he also came up with the regenerative, superregenerative, and superheterodyne radios.

twoodfin• 13 years ago

Ken Burns did a terrific documentary on the RCA/Armstrong fight and the birth of radio generally, Empire of the Air.

http://www.pbs.org/kenburns/empire/

gorner• 13 years ago

In fairness that's more about the current range of FM frequencies (originally in the 42–50 MHz range), not why they all end in an odd number.

icodestuff• 13 years ago

I had an Aiwa stereo/receiver several years ago that could tune the FM radio in 50kHz increments instead of 200kHz. Most stations were weaker 50kHz off, but a few were stronger. More interestingly, there were one or two stations I could only get on the even decimals, usually between two empty odds. Now I wonder if someone's transmitter was configured incorrectly, or whether it was an illegal broadcast.

smoyer• 13 years ago

Yep ... the (unmodulated) carrier frequency is in the middle of the channel (as it is with most AM radio stations). What's more interesting is to look at the old analog television broadcast channels. They were transmitted using VSB AM (Vestigal Side-Band Amplitude Modulation) which effectively meant that the luminance information (brightness) was transmitted using both side-bands (regular AM) but the chrominance information (color) was transmitted using SSB (Single Side-Band).

http://en.wikipedia.org/wiki/Single-sideband_modulation#Vest...

zwieback• 13 years ago

Never noticed that it's like that here in the US. My radio station growing up was SDR3 on 92.2 and channels are spaced .1MHz apart -- apparently the channel assignment in Germany took a different path.

__david__• 13 years ago

What actually more interesting to me is that FM has assigned channel numbers. I don't think I've ever seen a consumer radio that used channels instead of the raw frequency. I never knew they existed until I read that.

dfc2• 13 years ago

PSA: If you 404 turn of httpseverywhere for fcc.gov.

lutorm• 13 years ago

They mean "Why do U.S. assigned radio-station FM frequencies end in an odd decimal"...

mayonnaise23• 13 years ago

I imported a car from Japan and it would get audio from TV stations as the radio went down to lower frequencies =)

aestra• 13 years ago

Before the North American conversion to a digital only TV broadcasts, the audio for all analog channel 6 television stations in the United States could be picked up on 87.7 FM

Some channel 6 stations have expressed interest in/have implemented also broadcasting the sound for their broadcasts back on radio again after moving solely to digital broadcasts.

http://en.wikipedia.org/wiki/87.7_FM

nathan_f77• 13 years ago

Auckland, New Zealand seems to have mostly "even decimal" FM stations. 95.0, 90.2, 99.8, to list a few.

Why Do FM Frequencies End in an Odd Decimal?

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