XO-1.5 audio input DC mode

[forster at ozonline.com.au]

Sascha

This matches what Walter and I are observing with TurtleArt 103.

TA uses commands issued to amixer, amixer seems to be behaving inconsistently. Biases of 5k and 15k have been observed but unpredictably. The amixer commands that should switch the bias off just change the gain. With 0dB gain, bias off commands, the output is +26000 0V, 0, 1.7V, -26000 3.4V. Try to switch off the bias and the gain changes to +-14000. I am using the tip of a stereo plug which I thought was the right channel?

Tony

> Hi!
> 
> I'm trying to use the XO-1.5 as an oscilloscope, using xoscope (*) [1].
> I thought that should be easy to do given that the audio input seems to
> be specifically designed to do something like this. However my experiences
> so far are strange at best...
> 
> For a start there always is a bias voltage. I can reduce its impedance
> from ~ 15kΩ to ~ 5kΩ and increase it from ~1.75V to ~2.5V, but never turn
> it off.
> 
> I also don't understand the values I am seeing: It's always oscillating
> with half the sampling rate. The Capture level seems to affect both the
> base line and a partitioning into several value ranges. E.g. for level
> 39 (-43dB), I get a waveform growing downward from 100 and apparently
> proportional to the input voltage for 0-1.65V, a jump to an waveform
> that starts at 100, but gets smaller as I further increase the voltage
> and at about 1.9V it grows downward again (from 100) until at about
> 3.3V it reaches a maximum (values 103 and -104).
> 
> This is for the left channel (hooked up to a 5kΩ potentiometer acting as
> a voltage divider for 5V from a PC power supply) - the right one behaves
> even more strangely. Maybe it's been left floating (no pun intended)?
> 
> It would be nice if somebody could explain how the XO-1.5 audio input
> is wired up exactly. A simplified schematic would be awesome (AIUI Quanta
> has the "IP rights" for the schematic, but a simplified version should
> be fine I think).
> 
> Sascha
> 
> (*) X11 OscilloSCOPE. No relationship with the hardware. :)
> [1] http://xoscope.sourceforge.net/
> --
> http://sascha.silbe.org/
> http://www.infra-silbe.de/
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