[sdiy] computer soundcard mic question

[Batz Goodfortune]

Y-ellow Dean 'n' ya'll.
         I'm in the, I should be in bed but I'm procrastinating stage. This 
is just so esoteric that I might be able to shed some light on it.


At 03:39 PM 5/29/02 -0700, Dean Stiglitz wrote:
>hi all,
>
>we had a client in the other day (my day job is 
>selling/installing/training speech recognition software), who was having a 
>very strange problem.
>
>as soon as a mic was plugged into the laptop (either into the analog mic 
>in, or via a usb audio converter), a bad hum would start up (audiable via 
>the speakers, and trashing any chance to get speech recognition to 
>work)...it is not uncomon to have problems with the built in sound on a 
>laptop, but the usb pod almost always clears it up.
>
>my theory was that there was some kind of grounding problem, and the mic 
>cable was acting as an antenna.  today i came in planning to lift the 
>ground, and save the day :)...not so.
>
>1.  there was no hum today, no matter what i tried...decided to make the 
>ground lift anyways in case the problem occurs again.
>
>2.  it's my understanding that on these computer mics, tip is signal (and 
>has a voltage riding on it), ring is unconnected, and sleeve is 
>ground.  when i try to lift the ground, i get terrible sound, until i 
>reconnect it.
>
>is it merely that the headset mic cable is too long for the shield not to 
>be grounded (it would be ideal to lift it right next to the microphone, 
>rather than at the plug end)?  or am i missing some fundimental things????


Sounds to me like your classic unsatisfied electret insert. Electrets need 
power. They get their power in one of two ways. Either the plug is TRS and 
the DC is supplied on the ring. Or it's a kind of phantom power. Where the 
line has DC on it but is blocked by a cap. All kinds of mis matches can 
happen here and I'm not sure what it is you're experiencing exactly but 
I'll give you an idea.

Creati-flabs, many years ago use to provide TRS and/or phantom power for 
their microphones as standard on their sound cards. If you plugged one of 
their microphones (Crappy as they are) into one of their sound cards, then 
in theory, it all functioned OK. (Yeah right) But if you plugged one of 
their microphones into some other device, the most you'd get is a wee bit 
of signal above whatever noise it could drag in with it.

Somehow, if you plugged a dynamic mike in, it either shorted the ring and 
sleeve or maybe it was just to low an impedance and it would drop the 
power. However sometimes, as is common with mini-jacks the plug would not 
quite make contact and you'd have problems with crackling and so fourth. 
Worse still it's possible to just kill the mic (if there's enough current 
there.) but I've not noticed this happen. And anyway, with the shitty 
quality of the mics you typically use with sound cards, who cares?

Headset mics are typically shitty and they often use electrets because 
they're small, cheap and lite-weight etc. They figure this is all voice 
quality stuff so who's gonna notice?

Of course, this may not be the problem at all but I've come across such 
things before and it sounds like what's going down here. Trying a known, 
good mic and getting that working on that particular config would be a good 
start. My guess is that a new, half decent head-set mic would be cheaper 
than a USB pod thingy.

I don't know if that helps but I'm just crazy enough to think that it might.

Tomorrow I go back on dialup for a week or two so I'll have to remember to 
try and type a bit slower. I think I've forgotten how to actually use a 
telephone.  But I'm sure someone could sell me some instructions on eBay.

Be absolutely Icebox.

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