Impedance Measurement

[Simon Curran]

I feel like I'm missing something basic. I'm an Electrical Engineer with lots of technician and audio experience. But with the recommended setup, I'm not able to really get any readings from the Impedance measurements. Nothing shows up. I'm slightly suspicious it's related to different sampling rates between inputs and outputs? Basically it's looking like an open circuit in my measurements but I'm using a 2 ohm speaker.

Thanks everyone!

 

[John Mulcahy]

How are you able to have different sample rates on input and output? For impedance measurement the input and output should be on the same audio interface. What is your measurement setup?

 

[Simon Curran]

You're right, it's not different sampling rates, I confirmed by looking at the scope data on the scope panel. I'm using a UCA202 by behringer for input. For output, I've tried the headphone output on that audio card as well as the PC and Docking station. I can hear the speaker during the test and the "Check Levels" comes back good prior to the test. I've attached some data.

 

[John Mulcahy]

Please select the input device, input, output device and output you are using on the REW soundcard preferences. Use the same device for input and output, so either the UCA202 for both or the PC onboard audio for both. Check the connections are correct and that the channel selected as input in the soundcard preferences is the one that connects to the load. Should go through the impedance calibration process (different for V5.19 than V5.20, probably best using V5.20).

 

[Simon Curran]

Update: For the UCA202 the Windows 10 drivers do not actually create 2 input channels. It only uses one channel but repeats it on the other channel, but doesn't tell you. The work around is to install an older driver. Specifically: BEHRINGER_2902_X64_2.8.40
Now things are working much better. The main issue I'm having now is no matter what Output I'm using to drive the load (UCA202 or PC headphone) during calibration it is detecting a delay an offset of something like 1.06 samples or 22.1 uS. It looks like you've worked in the offset to the 5.20 beta (Thank you). But should I be worried about the results? The Impedance does seem to be shifted about 20-30 hz from where the lab that created the speaker tested each driver at.

 

[John Mulcahy]

Duplicate channels!? That's novel. Inter-channel timing offset would tend to affect phase rather than magnitude, it is compensated out so should be OK.