Turtle Art sensor xo 1.75 range voltage

[James Cameron]

On Sat, Jun 02, 2012 at 12:36:08PM +1000, forster at ozonline.com.au wrote:
> > > Testing on SKU199 and SKU204, the impedance has gone from 1k to 4k
> > > and the calibration is all wrong on 204
> > 
> > Possibility of damage?  How are you measuring this, so that I can
> > reproduce on units that I believe are undamaged?
> > 
> > Are you measuring DC impedance with laptop power off, or on?  With the
> > microphone bias off or on?  With the DC mode off or on?
> 
> Thanks James
> 
> Testing is with Turtle Art V140, ie I am letting TA do the ALSA stuff, setting up bias, gain etc
> 
> The turtle code is forever print voltage

Could you please explain how to do this?  I've not used Turtle Art, so
don't presume any foreknowledge.

Or, could you use "alsactl store" in the test state, and provide the
file it generates.  I can then use it on "alsactl restore" and be sure
of the same ALSA mixer configuration.

> 
> To check voltage calibration for +ve voltages and measure impedance I use the existing bias (however the turtle code and ALSA have set that up) and a variable resistor on the mic input.
> 
> I estimate input impedance by setting the variable resistor that results in half voltage (1.5V) as measured with a multimeter.
> 
> I check voltage calibration by measuring V across the variable resistor with the multimeter.
> 
> I can see a possible error in my method, SKU204 OS12 may be changing the bias conditions briefly during the measurement. This does not have any effect however on the calibration for 0V which is changed - in this case the source impedance is zero.
> 
> > 
> > If the kernel is running, are you measuring with the same kernel?
> 
> Quite possibly not. SKU199 is running OS32 and SKU204 is running
> OS12 which I think has kernel version issues. I updated OS12 with
> olpc-update, I presume that does the kernel too.

Okay, please compare with identical kernels.  There has been a bit of
change in the audio driver.  Use uname -a to check the kernel
revision.

> 
> 
> > 
> > There are some differences against 1.5, in that there are two
> > channels, on the tip and ring of the connector respectively.  Please
> > make sure you are using a stereo plug with correct manufacturing
> > tolerances.  Test for shorting between the left and right channel pins
> > after the plug is inserted.
> 
> I thought the 1.5 was stereo too? Just the XO-1 mono?

My mistake.  I've been working on XO-1 all week.  ;-}

> I am not getting channel shorting, different results on the 2 channels.
> 
> > 
> > After the socket, there is a 470 ohm series resistor on each channel.
> > 
> > After that, reverse diodes to ground and +3.3.
> > 
> 
> Thanks for confirming

I'm not sure about the wattage of the 470 ohm resistor though, I'll
leave that to someone else to confirm.

-- 
James Cameron
http://quozl.linux.org.au/