"BobC" wrote in message
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"Frank Gilliland" wrote in message
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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.
But you're not explaining why I can eat up 2 out of 4 hi end audio cards.

Most often, even wahat you consider "high end" cards do not have the audio
sheild grounded properly. If you are running into problems try running the
sheild to case ground or better yet use transformers.

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

Prove yourself wrong and go see. There are countless DAC's sitting under
big power AM/FM transmitters with much more RF saturation than the common
splatter box can produce from a neighbor's house. AND if RF creeps into the
front end of a DAC you will NOT hear it (program audio) over the speakers,
it will simply raise the error rate and cause the audio to be glitched or
muted.

You may also notice that modems don't always use transformers anymore.

Most do, motherboard integrated ones are starting not to. Which sucks IMHO,
if something goes wrong "mother nature" it's new mobo time!

Modems are just as susceptible to rf on the lines as anything else.

No kidding, but their design dictates that unless their audio output is run
to the speakers, audio will not get ino the sound card.

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

If the input is not being used it is shorted to "ground" as stated earlier
this is not always a trusty ground, Shorting it to true ground or shutting
that input off in the control panel will solve the problem. if one input or
output sheild is grounded to a trustworthy ground it will all be good as
they share ground with no buffering. If this input is tied to an output
device the impedance of the output device is usually low enough to negate
problems

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

HEH there are no +/- rails in consumer audio cards, they are single ended.
Until you get into the external pro cards you will see this.

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

Ground the sheild and the problem will go away.

Tell us about that phone lead and why.

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



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

To what?

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

All not associated with audio, concentrate on the audio card. Hell, my cell
phone drives them nuts till they are properly taken care of. one ground and
the problem goes away.

Another question to raise is is there even a ground? how old is the house
and has the ground rod rotted away, better yet, is there even a ground rod?

Chad

_____________________
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?
_____________________
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.
_____________________
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.

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