Why do major radio stations broadcast on odd frequencies?

[BigToque]

Why do major radio stations broadcast on odd frequencies like XX(X).[1,3,5,7,9] and not [0, 2,4,6,8]?

 

[Joemonkey]

signal bleed

 

[spidey07]

harmonics?

 

[KLin]

Just to piss you off.

 

[MikeMike]

Originally posted by: Joemonkey
signal bleed

MIKE

 

[Oscar1613]

if they did broadcast on even frequencies, would you still be asking this question (in reverse)?

 

[BigToque]

Originally posted by: Oscar1613
if they did broadcast on even frequencies, would you still be asking this question (in reverse)?

Yes.

 

[chrisms]

101.5, 99.9, 102.5, 91.1, 107.7.. by god you are right

Another question is why do these use the decimals at all? Why not 101, 99, 102, 107?

 

[Bootprint]

"Because each channel is 0.2 MHz wide, the center frequencies of adjacent channels differ by 0.2 MHz. Because the lowest channel is centered on 87.9 MHz the tenths digit of the center frequency of any FM station in the United States is always an odd number."

 

[trmiv]

In Europe it's even numbers, right?

 

[Joemonkey]

Originally posted by: trmiv
In Europe it's even numbers, right?

i don't think so, but it wouldn't surprise me if they were even in freaking canada...

 

[beer]

Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

 

[EvilYoda]

Originally posted by: beer
Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

It's like EECS206 all over again.

 

[Yellow Dog]

Originally posted by: Stefan
Why do major radio stations broadcast on odd frequencies like XX(X).[1,3,5,7,9] and not [0, 2,4,6,8]?

It is an evil plot that your government is using for their own subervsive reasons.

 

[So]

Originally posted by: beer
Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

This is basically correct. :beer:

 

[trmiv]

Originally posted by: Joemonkey

Originally posted by: trmiv
In Europe it's even numbers, right?

i don't think so, but it wouldn't surprise me if they were even in freaking canada...

There are even numbered stations in the UK.

 

[beer]

Originally posted by: So
This is basically correct. :beer:

I am not exactly sure if the FCC uses '99% power' bandwidth. A quick reference to the Sklar text leads me to believe it might be 35 dB attentuation bandwidth. I'm not sure which, but for all intents and purposes I think it's close enough

And these FCC regulations were made a long time ago anyway. It is very easy to lock onto an FM signal using phase-locked loops or even varactor diodes, so if the standards were rewritten now, the bandwidth rolloffs wouild be more tightly defined. But back in the 30s and 40s, such technologies did not exist, and it doesn't make sense to change anything now due to all the grandfathering requirements of changing communications standards.

 

[Colt45]

odd in north america, even in europe, not sure about the rest of earth..

 

[So]

Originally posted by: beer

Originally posted by: So
This is basically correct. :beer:

I am not exactly sure if the FCC uses '99% power' bandwidth. A quick reference to the Sklar text leads me to believe it might be 35 dB attentuation bandwidth. I'm not sure which, but for all intents and purposes I think it's close enough

And these FCC regulations were made a long time ago anyway. It is very easy to lock onto an FM signal using phase-locked loops or even varactor diodes, so if the standards were rewritten now, the bandwidth rolloffs wouild be more tightly defined. But back in the 30s and 40s, such technologies did not exist, and it doesn't make sense to change anything now due to all the grandfathering requirements of changing communications standards.

Yeah, people would be VERY angry if their FM radios suddenly stopped working.

 

[RaynorWolfcastle]

Originally posted by: beer
Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

This is correct except for the fact that analog radio is probably lowpassed to 40 KHz or less before even being modulated so the "infinite bandwidth" part is purely theoretical.

As a side note to non-engineers, the modulation scheme makes a big difference in the necessary bandwidth. A simple AM modulated signal in a perfect environment would, in theory only need about 45 KHz bandwidth for "CD-quality" sound.

 

[Syringer]

How come there's no channel 1?

 

[RaynorWolfcastle]

Originally posted by: Syringer
How come there's no channel 1?

WTF are you talking about?

 

[JustAnAverageGuy]

Originally posted by: beer
Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

I barely understood anything you just said, but it was long and used alot of big words so you're probably right.

 

[beer]

Originally posted by: RaynorWolfcastle

Originally posted by: beer
Every realizable signal is time limited, therefore it has infinite bandwidth. All communications signals exist on all frequencies, but they are greatly, greatly attenuated depending on the design of the transmitter. Therefore, it comes down to filter design to attenuate the signal at the appropriate frequencies. I believe that with FM in the US, the FCC defines 200 KHz of bandwidth for each station, but the bandwidth is not 'true' bandwidth (which is infinite) but '99% signal power bandwidth', that is to say, the integral of the frequency response curve centered at X MHz, symmetric about each side with integral bounds of 100 KHz +/- the centered frequency would be 99% of the true infinite bandwidth of the signal. Therefore, if you tuned into 90.0 MHz, you would be halfway in the signal spectrum of 89.9 and 90.1, which would be garbage to a human ear.

This is correct except for the fact that analog radio is probably lowpassed to 40 KHz or less before even being modulated so the "infinite bandwidth" part is purely theoretical.

As a side note to non-engineers, the modulation scheme makes a big difference in the necessary bandwidth. A simple AM modulated signal in a perfect environment would, in theory only need about 45 KHz bandwidth for "CD-quality" sound.

Come on, speaking of theoretical, there can be no ideal filter that gives 100% attentuation anyways. So yes, the 'theoretical' aspect of infinite bandwidth is there, but so is your ideal LP filter that smooths off exactly at 45 KHz. It doesn't exist. And for what it's worth, you should know that the modulation scheme has nothing to do with the receiver architecture. Signals are modulated onto a certain carrier wave because of the channel's properties. The FCC used the frequency range of 87-108 MHz because it has a fairly direct line of sight, and nothing else. It allowed them to densly pack FM transmitters, instead of AM transmitters that at night can interfere hundreds of miles away due to ionosphere reflection.

 

[RaynorWolfcastle]

Originally posted by: beer
Come on, speaking of theoretical, there can be no ideal filter that gives 100% attentuation anyways. So yes, the 'theoretical' aspect of infinite bandwidth is there, but so is your ideal LP filter that smooths off exactly at 45 KHz. It doesn't exist. And for what it's worth, you should know that the modulation scheme has nothing to do with the receiver architecture. Signals are modulated onto a certain carrier wave because of the channel's properties. The FCC used the frequency range of 87-108 MHz because it has a fairly direct line of sight, and nothing else. It allowed them to densly pack FM transmitters, instead of AM transmitters that at night can interfere hundreds of miles away due to ionosphere reflection.

A) You're right there are no filters that can give you 100% attenuation, but if your filter attenuates your signal so that its stopband power is below channel noise then that really doesn't matter.

B) My side note was a general musing and not related to any of your comments specifically. I don't know why you feel I need to be informed about receiver architecture since I didn't say anything about it in my post.

C) As you surely know, the modulation scheme doesn't have much to do with the spectrum allocated to a given technology but has a lot to do with the spacing between carrier frequencies. This has a lot to do with how good your receiver is (though not its architecture per say) and how complex you're willing to make it.