bob prohaska  wrote:
> Has anybody used a USB sound capture device for an audio frequency
> oscilloscope on a Raspberry Pi? xoscope is available using apt,
> but I can't find any reference to what sound devices, if any, it
> can use over USB.

Xoscope's webpage says it supports ALSA as an input source, so as
long as a USB audio adapter is working with ALSA (the cheap Chinese
ones seem to do so without issues) it should be fine.

It will have a series capacitor to block DC. You'll need to either
remove this (might be difficult if it's a small indistinct-looking
SMD part) or use a chopper circuit like this if you want to measure
DC signals:
http://lea.hamradio.si/~s57uuu/scdsp/CheapChop/cheapchop.htm

For more sensible input impedence (so the behaviour of the circuit
doesn't change when you probe it), the buffer circuit shown at the
xoscope homepage would also be recommended:
http://xoscope.sourceforge.net/hardware/hardware.html

Of course at this point you've got a significant amount of
front-end circuitry for your "simple" sound card oscilloscope. For
a data logging application that might be quite reasonable, but for
basic troubleshooting maybe it would be easier to just buy an old
CRO (Cathode Ray Oscilloscope), which has the added benefit of
working outside the audio frequency range.

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