I'm trying to acquire an audio signal from the output jack of a continuous wave ultrasound machine to do spectral analysis. I can't figure out how to set up the hardware and get sensible numbers. If I hook one of the ear piece and ground terminals from the audio jack output into the terminals of a cdaq-9178 using a 9215 module, I get this -10V measurement like an open circuit, which I suppose makes sense if the circuit is designed to have an output load. I then tried to measure across a 50ohm resister, which is a typical headphone resistance, and I get zero.
I tried the continuous sound input vi, which utilizes the sound card and mic input and it works, but it drops samples when running a while loop. It the audio inputs do not have the same DAQmx features that allow you to search the buffer and whatnot to ensure you don't drop samples. I put a "high resolution seconds" to verify the timing and indeed it seems to be refreshing at an inconsistent rate (dropping samples between iterations of the loop). For my spectral analysis I need a continuous measurement that could span between consecutive acquisitions, so I need a continuously sampled waveform.
I'm kind of at my wits' end. Is there a better way to ensure I don't drop samples using the built in audio functions? Is there a better way to set up the hardware acquisition to use DAQmx? I can't find any information about using a daq with the output audio jack of a device anywhere. Anything that mentions acquiring audio uses the sound input VI's (that use the sound card) and drop samples between acquisitions...
