MIC input through AIC23 on EVMDM642

I am having some trouble receiving an audio signal through the MIC
input of the AIC23 part of the EVMDM642 board?

I am trying to use the "audio echo" example and have changed the INSEL
register in evmdm642_aic23.h to select the MIC input instead of Line
in, but still I get no signal from the MIC (and the LINE-IN still works
as an input!). Does this indicate that I need to do something else to
enable the MIC?

(I am using an electret microphone connected to MICIN, MICBIAS, GND.
When I connect the mic to a scope, I see the sound waveforms clearly.)

Do I need to use a pre-amplifier circuit as well? I was expecting the
amplifier stages of the AIC23 to remove this requirement.
Any thoughts most welcome,

Stephen.

smo59 wrote:

> I am having some trouble receiving an audio signal through the MIC
> input of the AIC23 part of the EVMDM642 board?
> 
> I am trying to use the "audio echo" example and have changed the INSEL
> register in evmdm642_aic23.h to select the MIC input instead of Line
> in, but still I get no signal from the MIC (and the LINE-IN still works
> as an input!). Does this indicate that I need to do something else to
> enable the MIC?
> 
> (I am using an electret microphone connected to MICIN, MICBIAS, GND.
> When I connect the mic to a scope, I see the sound waveforms clearly.)
> 
> Do I need to use a pre-amplifier circuit as well? I was expecting the
> amplifier stages of the AIC23 to remove this requirement.
> Any thoughts most welcome,
> 
> Stephen.

If your electret has no bias battery, you need to be sure that the board
you're using it with provides bias. (Often, you need to install a
resistor or jumper on the board.)

You see the signal on a scope. What is its amplitude? What amplitude
ought you be feeding to the codec? A properly biased electret out to
work into a properly configured AIC23. Check the configuration.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Thanks for your answers Jerry,

It turns out the that the code I was modifying to change the INSEL
register was not the code being used to actually setup the AIC23. (I
had picked up loads of code from CodeComposer but not reviewed it
properly.)

The input I get is pretty basic, but it does prove the concept of what
I am trying to do.

FWIW, the EVMDM642 does provide the MICBIAS signal OK so the electret
is powered.
Also, I probably do need to try and match the mic output amplitude to
the input of the AIC23.

Thanks again,

Stephen.