Simultaneous measurement problem in a PXI-4772 device

[Keunhwa]

Hi, I'd like to measure acoustic signal with PXI-4772 and acoustic sensors.

I made data acquisition VI like the attached figure. I use 3 channel in a device.

Channel 0 is connected directly to function generator and used for analog trigger.

Channel 1 and 2 are connected to acoustic sensors and acquire the acoustic data.

Function generator is also connected to acoustic source sensor and generates source burst signal.

My problem is that all signals measured channel 0,1 and 2 arrive at a same time.

This is not physical because acoustic source stays away from two receivers .

If trigger signal's measured from 0 sec, other signals must be measured with positive time

delay. But, in waveform graph or data, all signals arrive at the same time.

What am I doing wrong? Please give me advice. Thank you.

Ken,

[Brooks_C]

Hello Keunhwa,

It sounds like you're actually a lot closer than you think.  From what I can tell you're doing a stimulus-response test using the function generator to generate a tone that you then measure with two acoustic sensors (microphones) connected to channels 1 and 2.  If that is the case then you've already configured the hardware and written the code exactly like I would.

I think what is throwing you off is that the NI DAQmx Read will return a single timestamp for all channels included in the read.  Additionally, this timestamp is just software timed based off when the data is transferred.  It will provide a decent estimate of when the data was collected, but since it is software timed, it will only be accurate to 10's or 100's of milliseconds.

The correct way to measure the delay between when you generate the tone with the function generator and when each of your acoustic sensors receive the signal is actually exactly what you've written in your code.  By connecting the function generator to channel 0 and configuring an analog start trigger you ensure that all channels begin collecting data when the tone is first generated by the function generator.  This means that you can look at the data you see on channels 1 and 2 to measure the delay between when the function generator started and when the sensors got the signal.

Best Regards,