

Why Do FM Frequencies End in an Odd Decimal? - shoeless
http://transition.fcc.gov/mb/audio/bickel/oddno.html

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kps
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.
    

¹[http://en.wikipedia.org/wiki/FM_broadcasting_in_the_United_S...](http://en.wikipedia.org/wiki/FM_broadcasting_in_the_United_States#History_of_FM_radio_in_the_U.S).

~~~
ef47d35620c1
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.

~~~
twoodfin
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/](http://www.pbs.org/kenburns/empire/)

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blueblob
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
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...](https://en.wikipedia.org/wiki/Frequency-
hopping_spread_spectrum#Multiple_inventors)

~~~
bnegreve
> _[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?

~~~
TallGuyShort
As they say, "The center frequency is located at 1/2 the bandwidth of the FM
Channel, or 100 kHz (0.1 MHz) up from the lower end of the channel."

It's natural to pick a range between round numbers. so 88.0 MHz - 108.0 MHz
doesn't seem so strange a range to be allocated. Now if one band starts at
88.0 MHz and needs 200 kHz of bandwidth, it's going to go from 88.0 MHz to
88.2 MHz. The carrier frequency will be in the middle of this band, so it's
represented as 88.1 MHz.

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icodestuff
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.

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miahi
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
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
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.

~~~
laacz
Oh, that servicing position. That's because you can't lift wipers from their
resting position since bommet is too close to windscreen. You have to turn on
the ingition, turn it off and then hold wiper stalk in the lowest position for
2 seconds. Same goes for 2013 Skoda Octavia. Had to learn that yesterday :)

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__david__
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.

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smoyer
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...](http://en.wikipedia.org/wiki/Single-
sideband_modulation#Vestigial_sideband_.28VSB.29)

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lutorm
They mean "Why do _U.S. assigned radio-station_ FM frequencies end in an odd
decimal"...

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mayonnaise23
I imported a car from Japan and it would get audio from TV stations as the
radio went down to lower frequencies =)

~~~
aestra
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](http://en.wikipedia.org/wiki/87.7_FM)

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zwieback
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.

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

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nathan_f77
Auckland, New Zealand seems to have mostly "even decimal" FM stations. 95.0,
90.2, 99.8, to list a few.

