[maemo-users] Audio connection noise - NOT SOLVED! [was: Audio connection problem]

From: Mark wolfmane at gmail.com
Date: Fri Mar 14 22:35:28 EET 2008
On Fri, Mar 14, 2008 at 12:21 PM, Chuck Robey <chuckr at chuckr.org> wrote:
>  If you were a reliable judge of noise, then you'd be right if it didn't get
>  better/worse when charging, but being in/out of a car hasn't the least to
>  do with it, noise from a charger system doesn't need to come from outside
>  itself (from the car) it can be self-generated by any switching power
>  supply, and they've gotten cheaper and cheaper, which means more and more
>  likely.  Someone commented that car stereos don't have this problem;
>  that;'s because those designers KNOW they must operate in that environment,
>  and reasonably (because anyone could tell if they didn't) they design for
>  quiet in such environments.  Sometimes, as in computer environments, it's
>  not always so that a key spec is audio level noise, so that can get thru.
>  Ask any engineer, what I'm saying is reasonable.

It's *obviously not* charging, if it's not connected to any power
source whatsoever other than its own battery. It's just as obvious
that the source of the noise cannot be fixed with any
power-source-related device for the same reason. How do you connect a
GLI or anything else to thin air? Everybody seems to be focused on the
car audio/power input aspects, when I've repeatedly said that this
problem exists regardless of what line-in I try to connect to; car,
home audio receiver, boom box, or wireless headphone transmitter, and
that the presence or absence of external power input makes absolutely
no difference. The only two situations where I don't get the noise are
from the built-in speakers or any set of headphones.

... And *I'm* the one who said that *factory* car stereos don't have
audio noise problems but aftermarket car audio components frequently
do, and that there's clearly something other than ground loop at work
because using the same wiring hookups should give the same result. If
ground loop is really such a common problem and so easily/cheaply
fixed, then why isn't an isolator built into every aftermarket unit?
(Obvious answer: that's not really the problem...)

>  There are two reasons that you say (below) that the ground loop isolator
>  might help noise, and they're both equally likely.  One would indeed be
>  ground loop problems.  The other would be impedance mismatch problems.  You
>  might not care which fixes your problem, but if you jump at the easiest
>  guess, it's going to cost others who hit this problem (and rely upon your
>  assumtions) some woe, if their problem isn't fixed by such a solution, by
>  (for example) a ground-loop-isolation system that didn't incidentally fix
>  the impedance mismatch, something easily possible.

...and the "easiest guess" would be ground loop problems, because
that's every car audio engineer/seller/installer's pet solution. It's
both easy and cheap to remedy, never mind the fact that it's
frequently *not* the problem and always has limited success...

So how does one go about fixing an audio impedance mismatch with a
GLI? Does than mean I'm going to have to rig up a special audio cable
that is permanently wired into the vehicle via a GLI? What would the
schematic look like? How does it translate to my home audio gear? Am I
going to have to crack those open and hack their wiring?

(Rigging it into the Nokia's power supply wouldn't work, because
frequently the Nokia isn't plugged into it.)

>  I can't tell you which it was, without further testing, but I CAN tell you,
>  from bitter experience, jumping to conclusions that aren't truly justified
>  can really burn the hell out of you.  Just don't want to spread FUD.  When
>  dealing with noise, it;'s ALWAYS a complicated subject, because there are
>  multiple sources that usually all contribute together.  Sometimes, fixing
>  the worst one is enough, but if you're a real stereo enthusiast, well, you
>  get my implication?

Who's jumping to conclusions? You're the one trying to apply a pet
solution to a situation to which it doesn't apply!

>  I'm pleased that your problem is fixed, though, and maybe that's sufficient
>  for you.

My problem is *not* fixed - no one has come up with anything that
applies, much less works!

>From the sound of it, though, I'm the only one having this problem,
which leads me to believe that the problem is more serious, and that
it's a defect in my particular unit. Maybe the circuitry that turns
off the the loudspeakers and switches the output to the headphone jack
when a plug is inserted isn't fully switching over, and the hanging
output is what's causing the problem. The load & impedance
characteristics of the speakers/headphones as opposed to line-inputs,
combined with the fact that there is no additional downstream
amplification, might explain the symptoms.

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