guitar sounds funny after 3 sec of recording

[reidiq]

I am recording myself playing guitar and I am using the a cable 1/4 to 1/8 plugged into the mic slot on my sound card. I am using a distortion pedal and when I record everything sounds good until after 3 seconds then it sounds like its almost turning into a wah effect. Any help would be appreciated.

Even with the distortion off it sounds funny after 3 seconds of recording.

[alatham]

After months of hearing about this kind of thing, I've finally figured out the problem (though I can't prove it since I no longer have a sound card with a Mic In that I can run tests on). This problem crops up when people plug a mono source into a computer's Mic In port. That port isn't designed to have a mono plug inserted into it.

The problem is that computer mics have a 3 conductor plug and they require a bias voltage to be sent along one of the conductors. They're a mono source, but they need that third wire to carry the bias voltage. The upshot of this is that when you plug a mono plug into a computer's Mic In you end up creating a short circuit between the bias voltage and analog ground. Some sound cards seem to be able to deal with that and just go on without any problems, but not all of them. Once you start shorting that circuit out the rest of the sound card starts acting funny and (apparently) this causes the frequency response of the input to change (this last sentence is what I'd need to test in order to prove my theory, I'm certain that you're shorting out that bias voltage with a mono plug).

The only thing you should be plugging into the Mic In port on a cheap sound card is a computer mic with a 3 conductor plug (generally the plug is colored green). Nothing else you plug into that port will work properly (though you may be able to get some audio out of it).

Note that this doesn't apply to XLR plugs, they work more or less universally.

[kozikowski]

<<<Note that this doesn't apply to XLR plugs, they work more or less universally.>>>

Except for 12v and 48v Phantom Power and that messy Microphone Level/Line Level thing.

http://www.kozco.com/tech/audioconnecto ... ctors.html

The DC bias on the ring connection is isolated--almost always--by a 2k2 resistor in the power supply. It's designed to directly run the field effect transistor inside some simple electret microphones. In other words, you can short it to ground all day long and it won't make the slightest difference.

My money is on beating the crap out of the PC laptop microphone input electronics by feeding it line level (1000 to one mismatch) and doing it long enough. That can't be good.

Koz

[alatham]

Good point about the XLR plugs, I'd forgotten that they sometimes carry line level signals as well (even though my sound card has two XLR line level inputs, woops).

Also, I didn't know the bias voltage was run through a 2k2 resistor. If that's true, then you're completely right, that's not much current at all being dumped to ground. You may well be right about the level mismatch though, that's a good theory.

This is one of those annoying issues that pops up from time to time. I wish we had a definitive answer.

[steve]

Also need to check that there's no "closed loop" going on in the signal chain causing a delayed feedback (for example if "Software Playthrough" is selected in Edit->Preferences->Audio I/O and this is then being re-recorded). This would sound like a gradually increasing echo could cause strange phasing effects as the echo interfered with the original signal.

BTW, Some old amplifiers use XLR for the speaker outputs - so you need to be a bit careful with XLR leads. (modern professional amplifiers use binding posts or blue Speakon connectors)