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Trouble streaming from an audio microphone


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#1 gyc

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Posted 30 November 2006 - 11:43 AM

Hi there. I need to aquire an audio signal from a microphone. Mine is a normal balanced output mic, supplied with +48 V on both leads (relative to ground). I connected the mic's leads to AIN2 and AIN3 with a capacitor in series with both leads to keep the 48 V from the inputs. I am streaming at 10k scans/sec, 12 bits and computed the difference in software. However, when I start my program, what I see is signals from AIN2 and 3 drifting from +5V to -5V. The difference signal is only about 4 bits. Eventually, the sistem will "saturate" on -5V and the diference is then perfectly null (obviously). If I stop my software and allow enough time, when I restart I observe the same behaviour. If I stop and immediately restart, it seems to pick up drift where it had left off. I connected an oscilloscope right at the AINx inputs and I observe my signal around 0V while not aquiring (if I clap my hands I can sen the bursts). As soon as I start my program a see the signal drifting to -5V. The practical result is that I have been unable to acquire my audio signal. I have used this very same configuration with different data acquisition HW's and never had any problems. I have gone through the manual a number of times, but found no answers (it's possible that my brain is no longer reading it!!!) What am I missing or doing wrong here? Can it be this complicated to get audio from a mic? ! ! ! Please help ! ! !

#2 LabJack Support

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Posted 30 November 2006 - 12:36 PM

Sounds like both signals are floating compared to UE9 ground. Try putting your scope from AIN2 to GND (or AIN3 to GND), as that is what the analog inputs are measuring. Try putting resistors from both AIN2 and AIN3 to GND. Looking at some microphone circuits, I saw 47k being used, but you might experiment to find the biggest resistor that works.

#3 gyc

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Posted 01 December 2006 - 04:13 AM

Sounds like both signals are floating compared to UE9 ground. Try putting your scope from AIN2 to GND (or AIN3 to GND), as that is what the analog inputs are measuring.


This is precisely where I had the probes.


Try putting resistors from both AIN2 and AIN3 to GND. Looking at some microphone circuits, I saw 47k being used, but you might experiment to find the biggest resistor that works.


This solved the problem.

I couldnt find the electric diagram in the manual. Is it available somewhere?
If I had had it, I would have understood immediately the problem and wouldn't have wasted so much time!


LabJack was cheap as compared to other competitors, but I'm wasting 10 times more time in getting around many of these "details"...

#4 LabJack Support

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Posted 01 December 2006 - 12:52 PM

Are you talking about a diagram to connect a microphone? We don't have an exact diagram for that connection to the UE9. Rather I just looked around on the Internet for general information about microphone signals. The suggestion I gave you above came from Figure 4 of the PGA204 datasheet from ti.com:

http://focus.ti.com/...link/pga204.pdf

The PGA204 is not used in the UE9, but seeing the connection suggestions in that datasheet made me think it might be the solution you needed.

There are so many possible connections to a generic measuring device like the UE9, that we can't provide diagrams for everything. That is really where something like this forum fits in. Next time someone tries to do the same thing, they can search this forum for "microphone" and find this topic. Some of the most related information from the user's guide in this particular case is in Section 2.7.3.8, which discusses floating channels.

#5 gyc

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Posted 04 December 2006 - 09:34 AM

Are you talking about a diagram to connect a microphone?


No, actually I meant the schematics of the of UE9.
If I had those, I wouldn't fall in so many traps and wouldn't need to ask so many "dumb" questions (and waste so much time)!


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