XO-1.5 audio input DC mode

[Walter Bender]

On Wed, Nov 17, 2010 at 1:38 PM, Sascha Silbe
<sascha-ml-reply-to-2010-3 at silbe.org> wrote:
> 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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>
>

With whatever you learn, maybe you can help me sort out the remaining
issues with the Sugar Oscilloscpe (Measure) activity
(http://activities.sugarlabs.org/en-US/sugar/addon/4197).

Meanwhile, this thread may be of interest:

http://lists.laptop.org/pipermail/devel/2010-November/030326.html

-walter

-- 
Walter Bender
Sugar Labs
http://www.sugarlabs.org