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uk.telecom.broadband (UK broadband) (uk.telecom.broadband) Discussion of broadband services, technology and equipment as provided in the UK. Discussions of specific services based on ADSL, cable modems or other broadband technology are also on-topic. Advertising is not allowed.

Ofcom to use free airwaves for rural broadband



 
 
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  #1  
Old July 11th 11, 11:20 PM posted to uk.telecom.broadband
Brian Gregory [UK]
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Posts: 208
Default Ofcom to use free airwaves for rural broadband

"The Natural Philosopher" wrote in message
...
Brian Gaff wrote:
Can you imagine what lift conditions would do to such a system on the FM
band? Gawd.

Besides, if common sense prevails, the FM system will be with us for
some time yet. Just because its an old systm does not make it out of
date. The aircraft still use AM for their comms, they do so because it
works.


And more importantly, it degrades gradually. Digital tens to be either
brilliant or nonexsistent with a region of 'braakup' in between.

FM really likes a continuous carrier as well. That makes it a bit of a pig
if several people try to use the same channel..at least on AM its just two
people on the same channel..on FM you are likely to herer at best one, and
at worst none.


Actually two AM signals on the same channel sounds pretty bad unless the two
transmitter frequencies are within, maybe a few tens of Hz of each other.


Horses for courses.

you can probably send a signal round the world with a few glass jars some
tinfoil, a 100 meters of cable and a car battery. And a switch.


--

Brian Gregory. (In the UK)

To email me remove the letter vee.


  #2  
Old July 12th 11, 09:21 PM posted to uk.telecom.broadband
alexd
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Posts: 1,765
Default Ofcom to use free airwaves for rural broadband

Meanwhile, at the uk.telecom.broadband Job Justification Hearings, Brian
Gregory [UK] chose the tried and tested strategy of:

Actually two AM signals on the same channel sounds pretty bad unless the
two transmitter frequencies are within, maybe a few tens of Hz of each
other.


If that were the case then surely the doppler shift would make it even less
likely that you'd hear two signals dead on the same frequency.

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  #3  
Old July 12th 11, 10:12 PM posted to uk.telecom.broadband
Brian Gregory [UK]
external usenet poster
 
Posts: 208
Default Ofcom to use free airwaves for rural broadband

"alexd" wrote in message
...
Meanwhile, at the uk.telecom.broadband Job Justification Hearings, Brian
Gregory [UK] chose the tried and tested strategy of:

Actually two AM signals on the same channel sounds pretty bad unless the
two transmitter frequencies are within, maybe a few tens of Hz of each
other.


If that were the case then surely the doppler shift would make it even
less
likely that you'd hear two signals dead on the same frequency.


It is the case.

Just listen around on airband.

I don't know how much it's due to the doppler effect though, they just don't
waste money building the transmitters with unnecessarily accurate frequency
references in them.

But AM is better than FM in that you're more likely to hear the squeak from
a weaker AM signal under a strong one than with FM where it can be
completely masked.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.


  #4  
Old July 12th 11, 10:51 PM posted to uk.telecom.broadband
The Natural Philosopher
external usenet poster
 
Posts: 2,728
Default Ofcom to use free airwaves for rural broadband

Brian Gregory [UK] wrote:
"alexd" wrote in message
...
Meanwhile, at the uk.telecom.broadband Job Justification Hearings, Brian
Gregory [UK] chose the tried and tested strategy of:

Actually two AM signals on the same channel sounds pretty bad unless the
two transmitter frequencies are within, maybe a few tens of Hz of each
other.

If that were the case then surely the doppler shift would make it even
less
likely that you'd hear two signals dead on the same frequency.


It is the case.

Just listen around on airband.

I don't know how much it's due to the doppler effect though, they just don't
waste money building the transmitters with unnecessarily accurate frequency
references in them.

But AM is better than FM in that you're more likely to hear the squeak from
a weaker AM signal under a strong one than with FM where it can be
completely masked.

Its worse than that. two added FM signals become in the limit utter
garbage. If one is very strong it captures the cachannel and the other
is totally suppressed.

Two AM actually add, plus the heterodyne between the carriers

IIRC aircraft have Xtal controls so they ought to be at least stable if
not totally spot on.
  #5  
Old July 13th 11, 10:08 AM posted to uk.telecom.broadband
Brian Gregory [UK]
external usenet poster
 
Posts: 208
Default Ofcom to use free airwaves for rural broadband

"The Natural Philosopher" wrote in message
...
Brian Gregory [UK] wrote:
"alexd" wrote in message
...
Meanwhile, at the uk.telecom.broadband Job Justification Hearings, Brian
Gregory [UK] chose the tried and tested strategy of:

Actually two AM signals on the same channel sounds pretty bad unless
the
two transmitter frequencies are within, maybe a few tens of Hz of each
other.
If that were the case then surely the doppler shift would make it even
less
likely that you'd hear two signals dead on the same frequency.


It is the case.

Just listen around on airband.

I don't know how much it's due to the doppler effect though, they just
don't waste money building the transmitters with unnecessarily accurate
frequency references in them.

But AM is better than FM in that you're more likely to hear the squeak
from a weaker AM signal under a strong one than with FM where it can be
completely masked.

Its worse than that. two added FM signals become in the limit utter
garbage. If one is very strong it captures the cachannel and the other is
totally suppressed.

Two AM actually add, plus the heterodyne between the carriers

IIRC aircraft have Xtal controls so they ought to be at least stable if
not totally spot on.


Not sure exactly what point you're making.

It's probably actually better for AM airband that a weak heterodyne is the
usual result of simultaneous transmission, weak speech underneath could be
mistaken for background noise (someone else in the cabin speaking) from one
transmitter.

What would be bad with an FM airband would be the increased chance that a
weak signal underneath would be much more likely to go totally unnoticed.

Exactly how bad two near equal strength signals sound is much less relevant.
The ATC operator will say something to try to get just one of them to
transmit on their own no matter what it sounded like.
Though I guess it might help speed things up if he can identify one of the
signals.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.


  #6  
Old July 13th 11, 03:52 PM posted to uk.telecom.broadband
The Natural Philosopher
external usenet poster
 
Posts: 2,728
Default Ofcom to use free airwaves for rural broadband

Brian Gregory [UK] wrote:
"The Natural Philosopher" wrote in message
...
Brian Gregory [UK] wrote:
"alexd" wrote in message
...
Meanwhile, at the uk.telecom.broadband Job Justification Hearings, Brian
Gregory [UK] chose the tried and tested strategy of:

Actually two AM signals on the same channel sounds pretty bad unless
the
two transmitter frequencies are within, maybe a few tens of Hz of each
other.
If that were the case then surely the doppler shift would make it even
less
likely that you'd hear two signals dead on the same frequency.
It is the case.

Just listen around on airband.

I don't know how much it's due to the doppler effect though, they just
don't waste money building the transmitters with unnecessarily accurate
frequency references in them.

But AM is better than FM in that you're more likely to hear the squeak
from a weaker AM signal under a strong one than with FM where it can be
completely masked.

Its worse than that. two added FM signals become in the limit utter
garbage. If one is very strong it captures the cachannel and the other is
totally suppressed.

Two AM actually add, plus the heterodyne between the carriers

IIRC aircraft have Xtal controls so they ought to be at least stable if
not totally spot on.


Not sure exactly what point you're making.


That two AM carriers can in principle and in practice sound like one AM
carrier with two people talking on it (pus the heterodyne whistle),
which two FM carriers never ever can.



It's probably actually better for AM airband that a weak heterodyne is the
usual result of simultaneous transmission, weak speech underneath could be
mistaken for background noise (someone else in the cabin speaking) from one
transmitter.

What would be bad with an FM airband would be the increased chance that a
weak signal underneath would be much more likely to go totally unnoticed.


Exactly.

Exactly how bad two near equal strength signals sound is much less relevant.
The ATC operator will say something to try to get just one of them to
transmit on their own no matter what it sounded like.
Though I guess it might help speed things up if he can identify one of the
signals.


The progression from AM-FM-'digital' modulation schemas is marked by
an increasing tendency of strong signals to get cleaner, and weak
signals to get lost altogether.

To the point where digital transmissions are either 'perfect' on a
packet by packet basis or 'absent'. The breakup and pixellation on DTB
is really a matter of just some of the packets getting thorough (albeit
perfectly) and others being significantly 'lost'

Understand that, and the case for AM in weak signal or high interference
situations becomes understandable.





 




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