PC sound card oscilloscope

[Leuven]

I am trying to put together a MAC based oscilloscope for one purpose only: to detect maximum signal level before clipping in car audio components (headunit, processors, amplifiers etc.), and ONLY working with 3 test tones - 40Hz, 1KHz and 4KHz.

So far I have acquired the software (Electroacoustic toolbox) with a suitable oscilloscope function and I have an external M-Audio Firewire Solo sound card with a max. unbalanced input of 1.3Vrms and 150?Ohm line in impedance. As probes I am using some 18AWG wire with a mono 1/4 jack at the sound card's end and croc clips/RCA plug at the other. Knowing that I will routinely work with voltages higher than 1.3, the lead goes through a project box with a 1kohm variable resistor used as a voltage divider. That helps to tune down the voltage to 1V no matter what goes in.

In a very basic way, it actually works - or at least it appears to work. I get correct sine waves representation, clipping is shown pretty much where I expect it to show - including sound card clipping if I leave the voltage go past 1.5V on the line in.

What am I expecting from this thing in terms of accuracy compared to an old school analog scope for example? Also any general advice of what to watch for?

Thank you in advance,
Adrian

[jahonen]

I suggest you build a front-end which protects the sound card from blowing up (I have seen some project but can't remember the URL), in event if you inadvertently connect it to something nasty, something like 100 volts DC. Or if channels have common ground (no balanced input), then a differential amplifier which again prevents magic smoke escaping if you connect it to speaker output which is bridged (common stuff in car amplifiers). Same happens if you connect to two non-bridged amplifier outputs and put the other channel with leads reversed.

At low frequencies, the sound card has better dynamic range and lower distortion figures than normal oscilloscope, but DC measurements are not usually possible, and even they could be done with modifications, the stability and accuracy might not be very good (naturally, DC accuracy is not required in audio applications).

Regards,
Janne

[Leuven]

Thanks Janne, I've got a rms multimeter plugged into the project box at all times so I will be monitoring any voltage very closely before it gets into the sound card.  So I take it it's good as long as I'm being careful.

I looked over a few diagrams for front end protection but they were above my head to be perfectly honest.

[johnmx]

Check out this article: “Front End Turns PC Sound Card into High-Speed Sampling Oscilloscope”

http://www.analog.com/library/analogdialogue/archives/45-11/soundcard.pdf