Re: geography nerds?

On Friday, in article
     <memo.20080222151009.556A@[EMAIL PROTECTED]
>
     prd@[EMAIL PROTECTED]
 "Paul Dormer" wrote:

> In article <62856lF21d16bU1@[EMAIL PROTECTED]
>,
> chatterbox@[EMAIL PROTECTED]
 (Paul Ian Harman) wrote:
> 
> > "Paul Dormer" <prd@[EMAIL PROTECTED]
> wrote in message 
> > news:memo.20080222113134.3260D@[EMAIL PROTECTED]
> > > Somebody thought our transmission towers would
> > > be good places to mount mobile phone aerials.
> > 
> > 
> > I'd've thought there would be interference problems, but then I 
> > did Comp Sci not Elec Eng.
> 
> Totally different frequencies.  UK mains is 50Hz, mobile frequencies
> much higher.  Furthermore, don't mobiles use PCM?  Probably wouldn't
> even get a 50Hz mains hum.  You don't get interference on a mobile when
> you put it next to an incandescent bulb, which would be about the same
> magnitude of interference.
> 
> However, I have found that if I switch my mobile on whilst listening to
> my iPod, I get a few seconds stuttering sounds from the iPod.  This can
> be a problem also if I've got the phone and the iPod in the same pocket
> when I'm listening to the iPod on a train.
> 
> Although I worked in computing all my working life, my degree was in
> physics.

There are people who can witter at length about RFI, wihout actually 
explaining agything, and they claim to be the experts.

Part 1: When a mobile phone is switched on, it has to get through an 
exchange of signals with the network, to let the network know where it 
is, and to find the best two-way signal. So that stuttering noise is the 
phone transmitting. And the phone has to do some transmitting to let the 
network know where it is when a call comes in.

Part 2: The iPod is a small computer and an audio amplifier. If it 
doesn't actually crash, the frequencies used by the cellphone, and the 
radiated signal, probably isn't messing with the computer side. But 
there's a general audio problem when you expose an audio circuit to a 
strong radio signal. If there's a diode somewhere in the circuit (or a 
lot of other components with the same effect) the induced AC from the 
radio signal becomes a lot of pulses, of varying strength. Because the 
circuits lose the radio frequencies--headphones will do this--the signal 
is sort of blurred, and you hear any audio frequency element.

This is, incidentally, how a crystal set works. A very strong radio 
signal (helped by a long aerial), a diode as detector (the original 
crystal), and the headphones which only responded to the audio. Oh, an a 
tuned circuit so you could listen to a particular transmitter.

-- 
David G. Bell -- SF Fan, Filker, and Punslinger.

On the horizon, a carrier task force of the Salvation Navy was 
turning into the wind, preparing to launch Zeppelins.