[sdiy] computer soundcard mic question

[Dean Stiglitz]

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????

any help (as always) is appreciated.

deknow