Wireless Network and Speaker Interference

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Grinder

I've been trying to attach an older (Win 98se era) laptop to a wireless
network (802.11b). The CardBus card installs properly, but I'm getting
a lot of fart noises from the built-in speakers.

Is there any way to eliminate that, short of physically removing the
speakers?
 
Grinder said:
I've been trying to attach an older (Win 98se era) laptop to a wireless
network (802.11b). The CardBus card installs properly, but I'm getting
a lot of fart noises from the built-in speakers.

Is there any way to eliminate that, short of physically removing the
speakers?
Click to expand...

I notice that if I leave headphones plugged into the laptop, that the
interference (reasonably) is gone. Is there some sort of "null
headphone" that I could plug into the headphone jack to short circuit
the interference without having to leave a full set of headphones
dangling?
 
Grinder said:
I notice that if I leave headphones plugged into the laptop, that the
interference (reasonably) is gone. Is there some sort of "null
headphone" that I could plug into the headphone jack to short circuit
the interference without having to leave a full set of headphones dangling?
Click to expand...

Radio Shack has audio cables, and one of those might fit the 1/8" stereo
jack. You can also buy just the plug part itself.

Paul
 
Paul said:
Radio Shack has audio cables, and one of those might fit the 1/8" stereo
jack. You can also buy just the plug part itself.
Click to expand...

Yeah, I cut one of those down and it doesn't look so bad. I was sort of
hoping for something more finished looking, but I can live with what I have.

I have since opened up the laptop and taken a look at the speakers. On
the front edge of laptop there's a mousepad, headphone and microphone
jacks, a half a dozen LEDs and about a dozen buttons. All of that stuff
is directly connected to a small circuit board that is attached to the
mainboard by a ribbon cable.

From the mainboard are 4 heavier gauge wires that run to the speakers.
They look to be left, right and a couple of ground wires. Is there
any way for me to add shielding to these components?
 
Grinder said:
Yeah, I cut one of those down and it doesn't look so bad. I was sort of
hoping for something more finished looking, but I can live with what I
have.

I have since opened up the laptop and taken a look at the speakers. On
the front edge of laptop there's a mousepad, headphone and microphone
jacks, a half a dozen LEDs and about a dozen buttons. All of that stuff
is directly connected to a small circuit board that is attached to the
mainboard by a ribbon cable.

From the mainboard are 4 heavier gauge wires that run to the speakers.
They look to be left, right and a couple of ground wires. Is there any
way for me to add shielding to these components?
Click to expand...

I'm not sure I believe the problem is with the speaker wires. The RF coupling
is probably into a sensitive signal with is an input to the audio chip.
Sometimes, muting items in the Mixer, as inputs, will stop the noises.

There are several ways to upset a sound chip.

1) A sound chip, when in playback, needs a steady diet of data. If
it doesn't get to DMA data in time, or an interrupt handler is slow,
sound distortion can occur. Sometimes a trivial hardware addition can
upset the bus or interrupt structure enough to disturb playback.

2) If there is no playback currently occurring, it is also possible
for the analog CD input, the aux input, mic in, line in, etc.,
to travel through the mixer and to the Line Out and the speakers.
Muting all the input paths can sometimes improve things. RF coupling
and diode rectification, in an input path, is what would make the noise.

(PDF page 43, the "sigma" symbol, shows all the inputs to the mixer.)
http://download.intel.com/support/motherboards/desktop/sb/ac97_r23.pdf

3) "Farting" sounds can be caused by low frequency oscillation in
audio output amplifiers. For example, placing lower than a 50 ohm
load on an operational amplifier, will make the "farting" or
"motor boat" sound. But in this case, I don't see how adding a
wireless card, would place an actual electrical load on the
speaker wires.

HTH,
Paul
 
How creative do you want to get? You might just turn down
the speakers, or if there's space nearby on the case to do
it, put in a DPST switch that opens the positive lead to
each speaker. It needs be the positive instead of trying to
use a common ground through a SPST switch, or you could
instead use a DPDT switch and keep the speaker grounds
isolated until after the switch (between it and the
mainboar).



How finished does it need be? The "plug" doesn't actually
have to be a headphone plug at all, just something of
roughly the same length and same diameter, and
non-conductive (at least not conductive across the output
pins & ground. For example a nylon flathead screw from the
hardware store.

I'm not sure I believe the problem is with the speaker wires. The RF coupling
is probably into a sensitive signal with is an input to the audio chip.
Sometimes, muting items in the Mixer, as inputs, will stop the noises.

There are several ways to upset a sound chip.

1) A sound chip, when in playback, needs a steady diet of data. If
it doesn't get to DMA data in time, or an interrupt handler is slow,
sound distortion can occur. Sometimes a trivial hardware addition can
upset the bus or interrupt structure enough to disturb playback.
Click to expand...

It seems less likely this would cause the motorboating
sound.

2) If there is no playback currently occurring, it is also possible
for the analog CD input, the aux input, mic in, line in, etc.,
to travel through the mixer and to the Line Out and the speakers.
Muting all the input paths can sometimes improve things. RF coupling
and diode rectification, in an input path, is what would make the noise.

(PDF page 43, the "sigma" symbol, shows all the inputs to the mixer.)
http://download.intel.com/support/motherboards/desktop/sb/ac97_r23.pdf

3) "Farting" sounds can be caused by low frequency oscillation in
audio output amplifiers. For example, placing lower than a 50 ohm
load on an operational amplifier, will make the "farting" or
"motor boat" sound. But in this case, I don't see how adding a
wireless card, would place an actual electrical load on the
speaker wires.

HTH,
Paul
Click to expand...

Agreed, anything picked up by the speaker leads won't
reduce the SNR by much.

I'm wondering if the notebook just has poor power filter
circuitry and so the addition of the card produces the
oscillation from dirtying up the power rail too much. A few
dozen pF cap across the +- analog amp stage imputs might
help (hard to say how integrated an old notebook is), or at
least tacked down across the power rails immediately
adjacent to the chip, though I'd have thought it already had
this decoupling.
 
Paul said:
I'm not sure I believe the problem is with the speaker wires. The RF
coupling
is probably into a sensitive signal with is an input to the audio chip.
Sometimes, muting items in the Mixer, as inputs, will stop the noises.
Click to expand...

In this case, muting seems to have no effect on the burps at all.
There are several ways to upset a sound chip.

1) A sound chip, when in playback, needs a steady diet of data. If
it doesn't get to DMA data in time, or an interrupt handler is slow,
sound distortion can occur. Sometimes a trivial hardware addition can
upset the bus or interrupt structure enough to disturb playback.

2) If there is no playback currently occurring, it is also possible
for the analog CD input, the aux input, mic in, line in, etc.,
to travel through the mixer and to the Line Out and the speakers.
Muting all the input paths can sometimes improve things. RF coupling
and diode rectification, in an input path, is what would make the noise.

(PDF page 43, the "sigma" symbol, shows all the inputs to the mixer.)
http://download.intel.com/support/motherboards/desktop/sb/ac97_r23.pdf

3) "Farting" sounds can be caused by low frequency oscillation in
audio output amplifiers. For example, placing lower than a 50 ohm
load on an operational amplifier, will make the "farting" or
"motor boat" sound. But in this case, I don't see how adding a
wireless card, would place an actual electrical load on the
speaker wires.
Click to expand...

The noises are very strongly correlated to activity on the wireless
connection. At startup, when the card is looking for connections I get
the aforementioned flatulence in bursts every 10 seconds or so. Once I
have established a connection, I get shorter static bursts while loading
web pages. If the web pages is already in the cache--no static. Also,
if I turn on the radio on the card, all the interference goes completely
away.
 
 
Grinder said:
The noises are very strongly correlated to activity on the wireless
connection. At startup, when the card is looking for connections I get
the aforementioned flatulence in bursts every 10 seconds or so. Once I
have established a connection, I get shorter static bursts while loading
web pages. If the web pages is already in the cache--no static. Also,
if I turn on the radio on the card, all the interference goes completely
away.
Click to expand...

Well, try playing with the mixer first. Mute some inputs and see if it
stops.

A remote possibility, is the place where the Wireless device draws power,
is near to where the sound chip has its three pin linear regulator. Some
motherboards provide a separate regulator for the sound chip, to try to
remove noise on the power rail. But I've been told, that spectrally,
a linear is not really good for doing that kind of cleanup. High frequency
noise might get passed through.

And if the Wireless device drew so much power, that the linear regulator
dropped out, that would also be a good reason for some farts.

You still didn't mention whether a sound file was playing at the time.
If the sound chip is not being used, and yet there are farts, then
at least we know it isn't a problem with DMA data getting there in time.

Paul
 
Paul said:
Well, try playing with the mixer first. Mute some inputs and see if it
stops.
Click to expand...

It doesn't.
A remote possibility, is the place where the Wireless device draws power,
is near to where the sound chip has its three pin linear regulator. Some
motherboards provide a separate regulator for the sound chip, to try to
remove noise on the power rail. But I've been told, that spectrally,
a linear is not really good for doing that kind of cleanup. High frequency
noise might get passed through.

And if the Wireless device drew so much power, that the linear regulator
dropped out, that would also be a good reason for some farts.

You still didn't mention whether a sound file was playing at the time.
Click to expand...

No, no other output is necessary. The interference appears (apparently)
unaffected by any intentional signal on the line.
If the sound chip is not being used, and yet there are farts, then
at least we know it isn't a problem with DMA data getting there in time.
Click to expand...

DMA = Direct Memory Access?
 
Grinder said:
Oh, I found this article from microsoft that appears relevant, if not
particularly helpful:

http://support.microsoft.com/kb/837123
Click to expand...

Agree absolutely. There are ways to design the PCB in the laptop
(or in a desktop computer for that matter), to make it more
resistant to interference. For example, one desktop board had
the Ethernet diff pairs routed right next to the audio signals,
which is a no-no. (Any time there was network activity, there was
noise in the audio, which could have been caught during product
verification.) Normally, there are clearance rules you are
supposed to use, for the audio with respect to other things.
Desktop motherboards go through two or three design cycles, so
there is ample opportunity to catch mistakes.

While soldering a Faraday cage above the audio circuit may seem
like the perfect solution, there are still ways for interference
to get in (i.e. all the signals on the PCB itself). If the designer
did a crappy job, you may find that "bandaid" solutions just won't
fix it. Perhaps a USB wireless solution would be a compromise.

Some design companies, have a series of tests they do on a product,
to determine it is fit to ship to customers. One of the tests
is for emissions (signals leaving the PCB, that might violate
FCC part 15) and for susceptibility (signals entering the PCB
from outside). A rule of thumb is, if signals can enter the
board and upset operation, it is also possible that garbage is
being emitted by the product. So you might even see herringbone
patterns on TV sets while that laptop is working.

Regarding FCC part 15, my company was quite paranoid about it. If
a product failed to meet it, it could mean stopped shipments at
the border etc. So it is pretty important to attend to stuff like
that, even if the enforcement system seems lax. No company wants
increased surveillance of their activities.

Paul
 
I've been trying to attach an older (Win 98se era) laptop to a wireless
network (802.11b). The CardBus card installs properly, but I'm getting
a lot of fart noises from the built-in speakers.

Is there any way to eliminate that, short of physically removing the
speakers?
Click to expand...


Given that network cards are fairly inexpensive, you might
just try a different make/model.
 
kony said:
Given that network cards are fairly inexpensive, you might
just try a different make/model.
Click to expand...

I've tried a couple of different cards in addition to the original Asus
card. Although the interference is substantially different, none of the
cards are appreciably better or worse than the others.

Thanks for your input guys.
 
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