MOTM

You are observing a "hidden" HPF (high pass filter), caused by the reactance of the AC coupling
network on the sound card.

Try increasing the VCO frequency to like '7' tick mark.

In order not to see 'tilt' of a square wave, the 3dB point of an AC couping stage has be be about
7 *times* lower than the frequency of interest.

The sound card probably has it's lower 3dB point set for voice, about 200Hz. This is why MOTM
demos recorded on SoundBlasters sound terrible.

If you aren't convinced, I'll fire up my 2Gs/sec digital scope with the 500Mhz probes :)

Paul S.