Voice communications

[superliner]

Guys, I was wondering why most, if not all, aeronuatical voice communication over radio is amplitude modulated rather than frequency modulated although from my own experience of tuning into (non-aeronatical) radio station tells me that FM stations carry lesser noise.
Just curious, please correct me if I'm wrong.

[Tomliner]

Superliner,casting my mind back to college days,I seem to recall that FM needs much more bandwidth per channel and considering that only voice frequencies need to be modulated as opposed to music etc FM would be wasteful.No doubt someone will be along soon who knows much more than I and can give a better answer.

[BelArgUSA]

Tomliner -
xxx
Among other things, you got it correct - bandwidth is a consideration.
With AM you can put more channels on a given frequency band.
And like in HF, even supress half of the "bandwidth" as in SSB (USB or LSB).
Aeronautical VHF is straight AM, but is basically static free.
xxx
From 90 VHF channels 60 years ago, how many channels now.
Cannot recall. Went to 180, then 360, then 720... And now...?
xxx

Happy contrails

[HEATHROW DIRECTOR]

<<Aeronautical VHF is straight AM, but is basically static free.
>>

Errr... not quite!! Have you ever used the airband during a thunderstorm? I've had my ears nearly blown apart a few times. AM is very susceptible to ignition noise too, which FM is not.

[Bye]

as far as i remember. the reason for AM is the doppler shift problem.

with FM the frequency is compressed by the movement of the aircraft due to doppler shift.

i look at it this way, FM is the wave varying/moving backwards and forwards wheras AM has the wave varying/going up and down.

So up and down is not affected by the forward movement of the aircraft.

In the old days, traffic police cars used to AM due to them primarily moving wheras, foot patrols would use FM as they couldn't run fast enough to effect the transmissions.

Not a very technical reply i'm afraid, but hope it helps.

Geoff

[Daveg4otu]

It is indeed all about bandwidth and quality .... or rather the lack of need for high quality audio .


AM provides adequate quality within a comparitively narrow bandwidth.
Even NBFM requires far greater spectrum space than AM ...and the extra quality is superfluous to the required standard of audio reproduction.

There were studies years ago that indicated that a comparitively narrow but relatively high audio freq signal gave much better readability in poor conditions ....you can hear this in action any time there is a lady controller/pilot on air - their voices with a relatively high AF are much easier to read than their male counterparts.

For the same reason - many Amatuer bands DXepeditions often include a lady operator as she will make far more contacts than the men- and not just because she is a female.

[forget]

In the old days, traffic police cars used to AM due to them primarily moving wheras, foot patrols would use FM as they couldn't run fast enough to effect the transmissions.

Where'd you get that from? How about a car doing warp 9 away from the station and listening to FM. Does it work?

[Bye]

well lets think about it.

the car is moving away from the transmitter at warp 9.

the transmitter is not moving.

so the signal will not be affected by doppler shift from the transmitter.

the car does not transmit back to the fm radio station does it. so no effect then.

so FM reception is ok in a moving car. it is just transmission that is affected.

Now if the transmitter was on a car doing warp 9 away from the listener it would matter of course.

hope that helps you understand.

Geoff

[HEATHROW DIRECTOR]

<<many Amatuer bands DXpeditions often include a lady operator as she will make far more contacts than the men- and not just because she is a female.>>

Interesting theory, Dave, but having been a fanatical DXer for 34 years my experience doesn't back that up although I know what you're getting at.

73 Bren G4DYO

[forget]

... the car is moving away from the transmitter at warp 9. The transmitter is not moving so the signal will not be affected by doppler shift from the transmitter.

Hope that helps you understand.

No. I already understand that Doppler shift doesn't need a moving transmitter to take effect.

[Bye]

well you better enlighten us then, its certainly a new one on me.

Geoff.

[forget]

Doppler effect is the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves.

It doesn't matter whether it's the transmitter OR the receiver which is moving.

See Doppler effect - Wikipedia, the free encyclopedia

[Daveg4otu]

with FM the frequency is compressed by the movement of the aircraft due to doppler shift.#


with all due respect this is total rubbish- given that radio waves travel at the speed of light (300,000,000 meter per sec 186000mps), the effect of an aircraft travelling at 500 knots would not have any noticable impact.

Bren - we proved it on a few occasions when operating special prefix stations on 7Mhz /3.5 Mhz SSB - our young lady consistently would get far more responses to a CQ in really poor condx than the rest of us would .... got better reports and seldom got asked for repeats....


......or maybe it was just that she had a real sexy voice!



73 G4OTU

[Bye]

i nearly concede, but not quite.

the difference is that when the observer is moving and the transmitter is stationary. the frequency transmitted doesn't change.

it is only perceived to be different due to the velocity of the receiver.

When the transmitter is moving the actual transmitted frequency does change.

but i do concede that the perceived frequency of transmission will be different to that transmitted if the observer is moving due to the doppler effect.

as for why it is ok for FM radio transmissions in your car travelling at warp 9 then someone with some radio knowledge better help with that one.

it is a fact about the police and fire radio frequencies i'm afraid.

i got it from a guy that designed the radio systems for emergency vehicles many years ago.

regards

Geoff

[Bye]

with all due respect this is total rubbish- given that radio waves travel at the speed of light (300,000,000 meter per sec 186000mps), the effect of an aircraft travelling at 500 knots would not have any noticable impact.

how can it be total rubbish when its true.

as you say would not have any noticeable effect therefore does have some effect therefore the statement is correct and not total rubbish at all.

so with all due respect dont say that something is total rubbish when its true.

regards

Geoff

[forget]

When the transmitter is moving the actual transmitted frequency does change.

as for why it is ok for FM radio transmissions in your car travelling at warp 9 then someone with some radio knowledge better help with that one.

Well the first aircraft radio system I fixed was on an Avro Lincoln, the last on a 767. I suppose I might fit the bill, but I'm giving up. Over, as they say, and out.

[Bye]

Quote:
When the transmitter is moving the actual transmitted frequency does change.

don't really understand what your problem is with that statement but hey this is PPRUNE i suppose.

i bow to your superior knowledge.



Geoff

[Daveg4otu]

how can it be total rubbish when its true.

I'm sure you know what I meant.


When the transmitter is moving the actual transmitted frequency does change.

No it doesn't ...only the perceived frequency from the POV of an observer ( receiver)

[canard68]

I seem to remember chatting to a lady on the space shuttle on 145mhz FM and doppler shift was not a factor on FM. Back to the original question I think aviation comms are on AM because of history it has been AM since WW2 and a change of mode would be a difficult and expensive operation. I hope the next move will be to a digital full duplex system with an embedded data transfer system.

[HEATHROW DIRECTOR]

<<Bren - we proved it on a few occasions when operating special prefix stations on 7Mhz /3.5 Mhz SSB - our young lady consistently would get far more responses to a CQ in really poor condx than the rest of us would .... got better reports and seldom got asked for repeats....>>

Ahh, OK. I meant real DXpeditions! Hi! It would be interesting to see the figures for the recent YL-only VP8 op.

I think canard68 is right about the reason being historic. Interesting that marine radio comms are on FM..