Audio Measurements: XLR - I/O - Labview (or Python)

[dougl122]

Hello,

Hoping for some help in setting up a system to record some audio using a hydrophone (underwater microphone).

My hydrophone has a 3 pin XLR, and I have an NI USB-6210 to interface with my laptop. The hydrophone also has a built in preamp that I want to send 5V to. I could just use an off the shelf audio interface, but I want to work through the I/O card and hopefully also figure out how to send a square wave to a function generator and collect GPS data (but those are less critical) simultaneously with a single script/program.

I have very little experience with this type of thing, so I'm hoping for some help with the setup. Mainly - what pins do I wire each XLR lead to? Do I need to do anything to define what these pins are, or are they predefined by the hardware? How do I send the 5V to the preamp? Having never used LabView, is there a simple program that I can reference for collecting the data (which ultimately should just be voltage data)?

I appreciate any help or references that can send me in the right direction. Thank you in advance!

Alex

[NIquist]

We can't tell you how to wire your microphone since you didn't even tell us what make/model it is.  XLR connections are generally "Balanced" which means that the original single-ended signal is copied and inverted, then sent down two physical lines.  This means the noise that gets generated on the long wires can be cancelled out on the other end with an instrumentation amplifier.  If this is the way your Mic/Preamp as set up you can connect the + and - lines directly to two different inputs on your DAQ (Analog In) and set the DAQ for differential input.  The analog front end of the DAQ will then convert the differential +/- voltages back into one audio signal to be digitized and processed.  The other line in the XLR is the ground/shield and should be connected to the AIGND on your DAQ.

Look at the manual for your mic and see if it shows the pinout.  If not it's probably 1 : GND, 2 : POS, 3 : NEG.  

You can power your preamp with the +5V output on you DAQ but it will only source 50mA or so.  You're better off with a separate 5V supply.  Again, check the manual and see if the current requirements are spec'd.

Look in the Example Finder for DAQmx Analog Input and start with one that has differential inputs.  It should be pretty easy to get a visible signal if your preamp is putting out a decent voltage level.  Experiment from there. 🤓

[dougl122]

Thank you! I didn't think the mic actually mattered as long as it was known to be an XLR connector. It is an HTI-96-MIN.

That mostly makes sense to me. The unclear part is how to supply the 5V. The preamp is built into the hydrophone, so it needs to be supplied via the XLR cable (like phantom power, but only 5V). Should I have two additional wires connecting a 5V pin to the + and - lines (i.e. using 5 pins on the DAQ side, connected to my 3 pins on the XLR)? So:

Input 0 & 5V -> '+' XLR pin

Input 1 & 5V -> '-' XLR pin

Ground -> Ground XLR pin

Does this seem right? Worst case, I can also use the hydrophone without the 5V power, it will just reduce the sensitivity.

[NIquist]

If your mic is phantom powered you would connect the +5 supply to both signal lines and the power supply GND to the mic ground.  That's assuming that HTI follows standard audio wiring.  Since your DAQ will be reading +5V with the audio signal added to one line and subtracted (since it's inverted) to the other line, the 5V power will be ignored and you will digitize only the audio signals that are not in common.  5 volts is in the differential range of the 6210 so this should work.  You should see a signal of a volt or two depending on the gain of the pre-amp (assuming you are recording some loud fish).

I didn't see any specific details on controlling the preamp at the HTI website so you may need to contact them for more info if you have problems.  On the bright side all the voltages are in the safe range for your DAQ so the worst that could happen is your signal would be offset by a DC voltage.  You could subtract that out with software if need be.

In addition to the Example Finder and even simpler way to test your connections is to use NI-MAX.  Once you plug in your DAQ you should see it in the Devices and Interfaces section and be able to open test panels where you can view signals.  It also has pages for device pinouts so you know where to connect your wires.  Channel one is usually paired on lines 1 and 8 for a differential input.  Channel two is 2 and 9, etc.  Don't forget that since in programming we use zero-based numbering so the channels will be listed as AI0 and AI7 (Analog Input).  This will all be diagrammed in NI-MAX pinout page.

NI-MAX comes with the NI System Config Utility:  System Configuration Download - NI