Built a beam sensor using photo transistor and a voltage divider to cut voltage to ~1.96 volts as input to "line in" on SB-16 soundcard. The objective is to count swings of a pendulum.

My problem is that 1) when the beam is broken occassionally the response is a high digitized value rather than low as expected and the norm.
2) If sampling at high rates >10 s/s, a single beam break responds with values both above and below zero (126-129) band without a significant delay at the zero band to separate the beam breaks.

Note the pendulum swings as beam blocked-unblocked-blocked-unblocked and finaly blocked for a single cycle. So I want digitized output to be lo-hi-lo-hi-lo.

Can someone confirm for me that this is caused by "ringing" in the soundcard?
Is the voltage possibly too high thus causing the problem. I can see the digitized output swinging as it returns to zero band circa 128.

I do not have an oscillascope so that is not an option.

i'm not sure what your trying to do but I don't think there is any ringing in your sound card
I use my sound card for an oscilloscope .With no external circuitry just two inputs and a ground directly to line in using a program called xoscope.hope this helps

Oh yeah you have to be careful with that .I use it on an old computer with a sound card ,not built in sound,and I always keep my voltage around 5v.

Have you got all the ambient light blocked off?