To start with I must appreciate acknowledge and thank ‘Christian Zeitnitz’ for the fantastic Sound-card Oscilloscope & signal generator software available at his website
on which this Instructable is based.
While working with the PC sound-card oscilloscope and signal generator I found the following limitations:
1. A low Input Impedance of the order 10 Kilo Ohms
2. Input voltage range limited to 2.8 V p-p
3. AC coupling of the I/O signals
4. Output voltage limited to 2V p-p
5. Sampling rate limited to 44kbps
Not much can be done about the sampling rate as it is a limitation of the sound card but the scope interface presented here attempts to improve the other factors.
Most significant is the addition of circuitry to estimate the positive and negative peak DC values of the signal and use this to offset the waveforms providing a realistic DC-coupled scope display.
Let me illustrate this with an two Examples