a7n8x sound <-> video interference

[jpyle]

Strange problem:
I am using my ASUS a7n8x to record music. The output of my bass guitar's preamp is plugged into the back panel (pink) microphone jack. My software processes the microphone input and outputs on the standard speaker output to two standard PC speakers.

After this audio setup has been in use for a few minutes, my monitor starts freaking out. The brightness fluctuates and the image zooms in and out slightly. The behavior is random and not seemingly correlated with anything the audio is doing. Even when there is no signal on the microphone input, it does this. When I turn off the monitor and turn it back on again, the screen is normal for a few minutes, but then it starts up again.

I thought it might be that I was overloading the microphone input. But I added an audio isolation transformer to the output of my preamp, and verified that the input level is less than 1 volt. The input levels are quite reasonable according to my software. It sounds fine. It's true that the bass guitar is prone to producing sharp peaks (slapping, strings touching pickups, etc.), and sometimes my software reports clipping on the input, but that does not seem to be correlated with the monitor freaking out.

Any idea what would cause this?

Jonathan

P.S. Other facts:
In my mixer settings the only non-muted levels are master and PCM, which are turned up 75%.
The PC speakers supply their own amplification, so they aren't presenting a big load on the line out.
I also have a midi keyboard controller plugged into the game port, and a software midi synth that outputs to the PCM out.
video card is Radeon 9200 (AGP).
monitor is 19 inch viewsonic ultrabrite, connected to video card through VGA connector.
I have a 400 watt power supply, which should be more than enough for my system.
The load I am putting on the microphone input is about 600 ohms.
I understand that the microphone jack consists of 5 volts on the ring and audio input on the tip. I leave the ring alone and only present a signal to the tip.
The cable to the microphone input is shielded.

[rpertusio]

Have you tried the Line-In connection (I believe it is blue) instead of the Microphone? If the bass is pre-amped, you shouldn't need the microphone to amplify it any further. (The Line-In is not amplified at all, as far as I know.)

- rp

[jpyle]

thanks, I tried line in and figured out that I had to change my mixer settings to get it to capture sound. It worked well, and I thought I was golden but then it started acting up again. One thing that appeared to trigger it once was when my software crashed and produced a loud continuous square wave.

I forgot to add in my last post that my speakers are nowhere near my monitor.