Re: Nigel's gratuitous troll

[Raymond <Reb.barnett@xxxxxxx>]

Speaking as someone who doesn't understand that much about splatting
signals about (radio or otherwise) but who can apply logic...

How well do DECT telephones work in practice? If everyone in the street
has one do they run out of available channels, or do they implement code
division as you mention?

Presumably if you transmit your signal digitally, using a DECT like
protocol, it will be superior to an analogue signal over a basic cable.
Assuming that the average person doesn't use balanced audio cables 'cos
they're not that bothered about quality/the distances are too small, and
assuming that the digital transmission has sufficient bandwith, what is
the problem?

Cabled solutions seem fundamentally flawed to me from a practical level:
whilst people are prepared to accept a second lead (second to power)
going into the back of the TV, few would be happy with the idea of
plugging two leads into the back of a kettle: one for power, one for
control/feedback. This is why I like USB because of the convenience
factor. This is also why a radio network would be superior to cabled
purely for the convenience of 'installing' an automatable light simply
by placing it in the house (and plugging into the mains), or simpling
plugging in an MP3 amplifier in a bedroom for it to instantly work
(rather than worry about having a CAT5 socket near enough, in addition
to the existing worry as to whether there is power socket close by).

I'm not really articulating myself properly, but my basic point is that
for domestic home automation, you are never going to sell a system to
ordinary people that needs a re-wire of their house and everything
plugged in twice. I'm sure the ideal would be to use a 4-pin plug to
transmit your data down, but I can't see that happening for a long
while. And if you don't sell the system to ordinary people, then volumes
will always be low, and we'll moan about high prices in the UK.

Radio transmissions do seem to be fairly well regulated, so I don't see
why a standard for DECT like transmissions of audio and control
information isn't possible within a domestic environment. If the
transmissions via radio are digital and have proper collision detection
etc then I don't see what the problem is.

I accept that cabled protocols will always be better in principal, but
then I also accept that Betamax was better in principal...

Ray.

> -----Original Message-----
> From: nigel.orr@xxxxxxx [mailto:nigel.orr@xxxxxxx]
> Sent: 01 September 1999 16:46
> To: REB.Barnett@xxxxxxx
> Subject: [ukha_d] Re: Nigel's gratuitous troll
>
>
> At 13:18 01/09/99 +0100, you wrote:
> >Well, my gut feelings tell me that I agree, but I'd be interested
to
> >hear some science to back them up...
>
> Gratuitous troll indeed- hrumph.
>
> There's not a great deal of science behind it, but I'll summarise the
> boring bits...
>
> Basically, you have a source signal, which you want to transmit to a
> destination.  You need to cause some sort of perturbation or
> disturbance
> that will propagate so, ignoring acoustics (if only I
> could...), you are
> left with sending something electromagnetic.  Now your basic
> choices are to
> send the signal down some sort of guide (eg cable for most practical
> purposes, fibre optics or waveguide for higher frequencies),
> or just put it
> into the air.
>
> If you choose cable, the signal is transmitted as its own little
> electromagnetic field surrounding the wires (technically, it
> doesn't travel
> along the wires but around them).  This field can affect other nearby
> cables if it is sufficiently close or strong to interfere
> with their field.
>  However, for most practical purposes, where the signals are
> in the same
> frequency band, have similar drive capability (eg not one
> really weedy high
> impedance signal and one low impedance one), and are at a
> similar level
> (not 240V and 10mV!), the resulting interference will be
> insignificant.
>
> If it does cause a problem, you can shield the cable (which
> you can think
> of as 'soaking up' the unwanted field) and/or use
> impedance-balanced pairs,
> where the wanted signal and its electrical inverse are sent down the 2
> wires, which are close enough to be considered colinear
> (mostly), and the
> interference will be equally picked up, with equal polarity,
> on each.  At
> the other end, take the signal on one wire, subtract the signal on the
> other 'et voila' 2x signal and 0x noise!
>
> Then there's the option of just transmitting it through the
> air.  Now all
> of your signals are in the same place as all of everyone
> else's signals.
> You are all trying to get a good range, and good signal
> quality, so you all
> make sure you have the highest power transmitter permitted in
> your band,
> and the most sensitive receiver to your transmitted signal.
>
> The problem comes when you have to grab your signal from all
> the unwanted
> mush that everyone else is transmitting out of your control.
> You can't
> balance it, you can't shield it, in fact you can't do
> anything to it once
> it has been let loose from your antenna until it arrives at
> the other end.
> There are various techniques (almost said bodges, but this is
> supposed to
> be informative, not argumentative!) for separating the
> signals which come
> down to 4 commonly used.
>
> 1- Frequency division
> 2- Time division
> 3- Code division
> 4- Not sure what to call this one, maybe power division ;-)
>
> The first is pretty effective- each broadcast radio station
> has its own
> frequency, allocated regionally, with permitted bandwidths
> etc to make sure
> they don't interfere with others.  It does need fairly strict
> regulation to
> stop someone else using your frequency in your area, so isn't really
> practical for unregulated systems (as you would want your
> home network etc
> to be).  Receivers are fairly easy, with tight filters in analogue
> circuits, or, increasingly, using DSPs.
>
> Time division (everyone gets their own time slot to transmit)
> is next to
> useless in wireless systems without complete regulation, so
> you can ignore
> it.  It is used is request-response systems, with one 'master' sending
> requests, and the appropriate 'slave' responding, but it's hard to
> co-ordinate when it gets more complex than that...
>
> Code division (AKA spread spectrum etc) has great potential.
> Every user
> can be allocated a unique 'code' from a large "bucket o'
> codes", and it is
> practical to unscramble their wanted signal using their code,
> and reject
> other people's unwanted signals.  It currently needs rather a lot of
> electronics to cope with that, and it is important that all
> the users are
> received with similar power levels, but it could work very well.
>
> Power division (which doubtless has a better name) might
> refer to things
> like car alarms, garage door openers (and my wireless
> doorbell!), operating
> on 418 / 433/ 900MHz, at strictly regulated powers and
> frequencies, where
> lots of users have them, but the power is supposed to be low enough to
> avoid interference.  Add a simple coding system to try to avoid false
> alarms/commands and off you go.  Basically the same idea as
> X-10, try to
> produce just enough power to cover your target area but not enough to
> interfere with someone else's area, and add a little bit of
> coding (house
> codes and unit codes) to avoid false triggering.  It sort of
> works, right
> up until the time (like now) when someone decides to
> reallocate your band,
> or something near it, to a PMR radio system, and all your carefully
> designed low power systems stop working reliably (or, in the
> case of X10,
> someone decides that noise on the power line is a "Bad
Thing" and
> encourages everyone to do their best to suppress it...
>
> Basically, wireless transmission is like sharing the same cable with
> absolutely everyone else in the world, so there will have to
> be concessions
> and additional complexity for everything to work smoothly.
> Whether the
> system is digital or analogue, it still shares the same
> 'wireless' cable,
> and still has the same problems.
>
> For that reason, I do use wireless for necessarily portable
> systems (eg
> radio mics) and 'for fun(?)'- like my wireless doorbell, but
> I'd rather
> wire the rest- it doesn't take _that_ much extra effort, and
> saves a lot of
> faffing about afterwards...
>
> This is a fairly fast summary 'as I see it', mostly compiled as I was
> having my lunch... feel free to add, subtract or ask questions, on the
> group or email...
>
> Nigel
> --
> Nigel Orr                  Research Associate   O   ______
>         Underwater Acoustics Group,              o / o    \_/(
> Dept of Electrical and Electronic Engineering     (_   <   _ (
>      University of Newcastle Upon Tyne             \______/ \(
>
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