On Tue, 26 Apr 2005 22:20:28 -0400, "BobC"
wrote in :


"Frank Gilliland" wrote in message
.. .
On Mon, 25 Apr 2005 23:24:49 -0400, "BobC"
wrote in :


Ok, let's consider the circuitry in front of the speakers: Assuming
the speakers do not have their own power amps, the amps are enclosed
in a fully (or almost fully) shielded case. The only unshielded lines
into the case are pairs; i.e, only common mode currents can enter the
case. Excluding the power line (filtered by the power supply) and the
phone line (filtered by the modem transformer), the only means of
entry is through the speaker wires connected to a very low impedance
power amp. And assuming the power amp uses feedback (and that
rectification of the RF occurs at the power amp stage), it would take
just as much power to distort the amps' intended output (and therefore
cause audio feedback of the demodulated RF) as it would to drive the
speakers directly. That also assumes an efficient antenna -- resonant
lengths of speaker wire.

Now if the speakers are amplified externally (amp in the speakers),
they are more than likely driven by a shielded cable. Regardless,
almost all low-level preamps are designed for high common-mode
rejection (we're talking 60-90 dB+ at each stage); and since the low
impedance of the power amp is no less suseptible to RFI than if the
amp was enclosed in the computer case, it would -still- take a
considerable amount of RF power on the lines to drive the speakers.

This is not just theory but fact -- they are designed this way for the
specific purpose of eliminating that annoying AC hum and digital RFI
that permeates most houses, -especially- the horrific noise generated
by computer monitors and light dimmers. "Overloaded DAC's"? I don't
think so, Bob. How do you overload a DAC? YOU will have to do better
than THAT.

Common mode shmommen mode.
In a perfect world your model works.


It's not -my- model, and it works in the real world just fine. If it
didn't there would be so much noise coming from the speakers that they
would be almost useless.


But you're not explaining why I can eat up 2 out of 4 hi end audio cards.


I have my suspicions.....


And if you're not cognizent of how you eat a DAC's lunch, go read.


Read what? A DAC datasheet? Heck, I get those as junk mail every month
(and I wish they would quit sending them). Maybe you should learn what
a DAC actually does before you start spouting off about subjects you
know very little about. DAC stands for "Digital to Analog Converter",
and the only way to overload them (aside from blowing them up with too
much Vcc) is to push all the inputs to the same logic level, in which
case you will not get audio from the output but a steady DC signal. So
how is it that you think that you can overload a soundcard's DAC with
AM (analog) RF and get demodulated audio from the output? If you can
then you got some serious voodoo happening.


You may also notice that modems don't always use transformers anymore.
Modems are just as susceptible to rf on the lines as anything else.


You might have noticed yourself that the modem is usually a seperate
and isolated card, that the line inputs always include RFI protection
by law (transformer or chokes), that they have excellent common-mode
signal rejection, and that the line impedance is quite low when it's
off-hook. Any RF on the phone line stops at the modem for the same
reason that I explained about the speakers -- because they are
designed to reject environmental RF hash. The only way a stray RF
signal can hop the phone line, skip past the modem, infiltrate the
power supply and drive the sound card, with a demodulation stage
happening somewhere in that path, is if the RF has some significant
power.


Sound cards have fairly high imp, non-balanced inputs and hi gain.


Unbalanced lines use shielded cables.


Your model assumes no (-) or (+) supply rail changes from rf.


Your criticizm assumes no power supply regulation, no bypass caps on
the chips, no capacitance between power traces and the ground plane
layer, no inductive losses from the straight traces on the bus, etc.
IOW, you are reaching.


Your model only survives within the puter itself.
Add rf on a mic, spkr or phone lead and you have a great receiver.


Not even close for the reasons I already explained (and apparently you
couldn't understand). Yet I forgot to mention that the only way for
the impedance of an RF signal to be low enough to force it's way past
the hardware, the line must be both resonant -and- terminate at the
computer at a low-impedance node, -and- provide a signal with
sufficient strength to defeat the protections and/or output power.
That's a pretty tall order for a speaker line -- especially when you
realize that most of the time the excess line is wound up and tied,
making a pretty good RF choke.


It doesn't need to overload the main audio amp, just a prior stage.


I didn't say "overload", I said "distort". There is a difference. And
while I have no problem with RFI interfering with a stage prior to the
output, the most obvious route is the feedback loop which I already
addressed. Or weren't you paying attention?


The xformers you mentioned have enough cap across the windings to pass rf.


No they don't, simply because they either have electrostatic shielding
between the layers (somewhat old-fashioned) or use tandem windings on
the bobbin (much more common these days, and a whole lot cheaper). But
they do have enough -inductance- to choke any RF on the line.


The leads you mentioned are not the only ones connected to the puter.
There are monitors,


Shielded.


scanners,


Shielded, (except for USB, which is a balanced pair).


mice,


Shielded.


cameras


USB.


etc.


Keyboard: Shielded.
Power cable: Filtered.

And you should be aware that the same engineering standards used to
prevent RFI from -exiting- the computer case also serve to prevent RFI
from -entering- the computer case.

Got any more lame excuses?


_____________________
Authorization to transmit with a CB is automatically revoked when the
station is operating illegally. Shall I cite the code?

Not necessary. Just prove that the station is illegal.
It wouldn't hurt to have some decent field strength readings.
Maybe a witness attesting to the actual use of an amp?


How about just setting up a legal CB radio next to the computer and
see if it causes the same problems described by the OP? I have done so
many times and never experienced a problem. In fact, I have a Tram 60
sitting right beside my computer and use it frequently with no ill
effects to the computer, although the computer does tend to cause RFI
to the radio.....


_____________________
Which is all well and good but until you've satisfied the feds that
your consumer grade stuff is properly filtered, they aren't going to
bother
sending anyone out to check.

They won't send anyone out regardless. They don't care about the CB.
But they do suck up to the ham community, and if it turns out that
this guy has a license they may indeed take action if they find he is
operating illegally (i.e, using power on the CB).

Which brings us back to doing the necessary part of filtering before
calling.


A local AM station (KGA, I think) had a problem a few years back. They
were pumping so much power that you could hear the audio sounding from
the chain-link fence at the nearby school. Are you suggesting that it
is the school's responsibilty to filter the fence?


_____________________
So far, I've not read from Jade anything that remotely suggests he or she
has a serious interference complaint involving another radio service.

I read "fire service" radio and I invited Jade to tell me what he/she
considers a fire service radio.

Scanners don't count.

Actually, they do. If someone is causing interference to a scanner,
it's very possible..... nay, -likely-..... that he will also cause
interference to an emergency service radio that happens to be in close
proximity. This is a problem because while the cops chase the killer
with the gun running through the neighborhood, Andy the Amphead keys
up and the guy gets away (or worse) because the cops lose comm.

Are you trying to present that a "real radios" front end isn't any better
than a scanner?

I'm sure the folks paying for those $3000 MA/COM's & Motorolas
would love to know about that. Even the Kenwood & Icom users.


If the harmonic falls on the operating frequency then it doesn't
matter how well the front end is built -- interference is the
inevitable result. But even the best receivers are not immune to
overload.


You're also back to calling the guy guilty before you have proof.
Reread the part about innocent till proven guilty.


I'm convinced with the information that was given. I suspect that you
are also convinced but are simply making excuses. If you are not
convinced then either you have very little experience or are woefully
ignorant about the subject.


______________________
I haven't seen any real proof that the neighbor is running an illegal
station.
Jade has not indicated other neighbors are complaining of similar events.
Such complaints might induce me to think the cb'er is running power.

Then that would be a good question to ask. So ask it.

I believe I already did.


I looked back through the thread and I saw no such question. Perhaps
my news server didn't pick it up -- care to cite the post?


_______________________
Unless or until someone brings more convincing proof of the allegations,
I'm going with the notion it's a singular complaint due the quality of the
consumer electronics invloved.

You'll excuse me if I happen to believe in "reasonable doubt" &
"due process" instead of conjecture & unfounded finger pointing.
bc


How about "civic responsibility"?

How about Constitutional Law?


How about it? Although it has fallen by the wayside during the Bush
administration, there is nothing I suggested that would deprive anyone
of their Constitutional rights. I'm not a court and I don't execute
due process. But if I have good reason to believe that someone is
violating a law then I don't keep my mouth shut because of some
whacko's ultra-literal interpretation of the Constitution. Due process
is not pre-empted by a presumption of innocence -- OTOH, due process
must be initiated before it can occur, and reasonable suspicion is
enough to begin that process. That's the law. If you don't like it,
work to change it. If you don't then quit whining and learn to live
with the system the way it is.






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