4 Nov
2013
4 Nov
'13
2:29 p.m.
If a receive is picking up signals from 2 transmitters on the same
frequency they tend to interfere with each other. If the 2 transmitters are
sending exactly the same signal, and the arrive at the receivers antenna at
exactly the same time, they add and reinforce each other. If one arrives a
half cycle delayed (about 0.01 microseconds for FM band), they cancel each
other out and you receive nothing. If the delay is longer you hear a messy
mix of the signal mixed with a delayed version of itself. If the delay
changes with time you hear the signal fading in and out and distorted.
If there a multiple receivers in different places the the delay will be
different at each location because the are at different distances from the
2 transmitters.
So, basically, multiple transmitters on the same frequency doesn't work
unless the receiver can figure out how to separate the signals and either
throw one away, or compensate for the delay. You can sometimes do that with
directional antennas if the delay is constant and the transmitters are in
different directions from the receivers point of view.
Steve
On Monday, November 4, 2013, David Keenan wrote:
Anthony - I almost labelled the idea an LPFM mesh but
then I thought it
wouldn't have to mesh, ie the nodes would not need to talk to each other
(provided each host had an internet uplink) so maybe its a somewhat
different topology
Hol - In principle I totally agree with you about such a distribution of
radios simply blocking spectrum from being used for other purposen -- but
along the same sort of moral-political lines that justify pirate radio in
the first place, the spectrum of sidebands in question here (FM/LPFM) is,
from what I infer, already not technically-legally available to the
public..? So I don't see how in this particular case, how engaging in an
experiment like this cannabalizing a frequency etc would be that
deprivational for the public at large.. again, just as with 'normal'
pirate/community radio
Also - this is a really dumb question but in terms of interference, I
actually have no idea what sort of interference results when two
coverage-adjacent radios are broadcasting the exact same signal? Does it
make any difference if they'd both be broadcasting the same signal? I
should remember this, since I actually took one of those AARL tests wayyy
back when (and I think I am technically FCC licensed, at least for certain
spectrums like SSB? Can't exactly remember..i should have a certificate
somewhere) but:
Seems like one should be able to rewire existing off-the-shelf audio-in FM
transmitters for whatever frequency we want, and somehow make the signal a
bit stronger to cover a block or whatever, instead of only one's house..
Prolly a dumb idea, just a lark - I know nothing of 'cognitive radio' but
I'd love to talk to you more about it, anthony!
David
On Mon, Nov 4, 2013 at 1:32 PM, Anthony Di Franco
<di.franco@gmail.com<javascript:_e({}, 'cvml',
'di.franco(a)gmail.com');>
--
-steve
wrote:
Software-defined radio and the broader embracing
paradigm of cognitive
radio are still topics of active research on fundamental issues and they
are both built out of the practice of negotiation among participants in a
communication about the means of communication (what part of spectrum to
use and what encoding mainly).
They were in part motivated by desire to work around existing fixed uses
of spectrum (like FM audio broadcasts) in a non-interfering way, but
wouldn't really be useful to transmit to existing receivers that can't
participate in the negotiations they involve.
Good stuff to build into a mesh architecture but heavy-duty to implement
or even play with without hardware and software tools that are currently
mostly ad-hoc and specialized and usually fairly obscure and expensive.
This may be changing a lot fairly soon because imminent generations of cell
phones are due to incorporate pretty good software-defined radio, I recall
hearing somewhere.
The only simple hack along the same lines I can think of is to choose a
frequency to transmit FM audio on, detect interference from other
transmitters on that frequency, and stop transmitting in that case. I don't
see the usefulness of doing that in this context though.
On Mon, Nov 4, 2013 at 1:17 PM, Eddan Katz <eddan(a)sudoroom.tv> wrote:
I've read about software-defined radio making interference problems
negligible (can't find anything in particular at the moment - but most
coming from the IEEE publications).
I'd be interested whether others (a) understood if this is true; (b) knew
of affordable SDR equipment; and/or (c) thought this would solve the
problem.
Sidenote: While streaming-only radio stations do not have to deal with
spectrum licensing issues, their Internet presence make broadcasting anyone
else's copyrighted content a complicated and either expensive or risky
endeavor.
sent from eddan.com
On 2013-11-04 12:52, Hol Gaskill wrote:
it does seem philosophically better to provide content on an opt-in
basis via existing RF links than to simply radiate it in every
direction and block that portion of the spectrum from other uses
on Nov 04, 2013, ANTHONY DI FRANCO <di.franco(a)gmail.com> wrote:
This sounds a lot like the mesh networking projects, which move away
from broadcasting as fundamental and rebase broadcasting in a
peer-to-peer context, and are already oriented the right ways
technically and with respect to regulations for those goals.
On Nov 4, 2013 11:31 AM, "David Keenan" <dkeenan44(a)gmail.com [23]>
wrote:
I find myself most sympathetic to Naomi's position - although I do
still think FM as a medium has some romance and cool left in it, I
don't know that it's actually worth it, given the cost and effort.
Completely naiive riffing follows, but -- since decentralizing
information and the means of production are (for me) integral to
freeing information / culture.. if one wanted to recolonize the
airwaves, I wonder if it might be possible to simply distribute
LPFM?
Ie, give people a small appliance that transceives internet radio
into LPFM or way lower-power radio, ie just for their block /
neighborhood / whathaveyou.. A device that doesn't take a whole
lot of power, that is innately not geographically bounded, and can
become a diaspora of signal. And not necessarily legal but
decentralized and dispersed.. if enough folks did this in
aggregate in a given neighborhood or community, could that
collectively function coverage-wise as a single relatively strong
broadcast / antenna?
Has anyone tried anything s
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