XO-1.5 audio input DC mode

[Sascha Silbe]

Excerpts from Sascha Silbe's message of Wed Nov 17 19:38:20 +0100 2010:

> I'm trying to use the XO-1.5 as an oscilloscope, using xoscope (*) [1].
[...]

Thanks to everybody who replied and helped me figure this out. I still
haven't succeeded using the XO-1.5 as an oscilloscope, but made some
progress:

1. The totally strange values I see in xoscope are a software issue.
   If I run it using alsa-oss (instead of relying on the kernel-level
   OSS compatibility layer), the values are sane. However it now keeps
   hanging for long periods of time (many seconds).

2. The voltage I see with bias off is probably generated internally by
   the codec chip. I don't think the chip designers envisioned it to be
   used without a decoupling capacitor. I've been told the (non-public)
   data sheet is rather sparse on details about the analog part of the
   chip, so this is a good an understanding as we're going to get.
   Unless someone finds a magic way to disable this from the digital
   side of the chip (which I doubt), we'll have to cope with it. This
   means that we need an external buffer circuit in order to prevent the
   XO-1.5 from feeding voltage back into the measured circuit.

3. I have managed to build osqoop [3,4] and fix the most obvious bug
   that prevents its usage on high-resolution displays. Like xoscope,
   it still uses OSS (instead of ALSA), but apparently in a different
   enough way not to trigger the same bugs (or cope better with whatever
   OSS does differently than xoscope expects). The sampling rate and
   resolution is fixed to 16bit @ 44.1kHz (the hardware claims to do
   up to 24bit @ 192kHz).
   Like Measure, there is no support for calibration. The UI is based
   entirely on voltage levels, so you'll get fake voltages that have no
   relationship to the input voltage.

4. The problem with the right channel probably was a bad cable. I'll
   check this once I got osqoop running well enough.

The schematic of the analog audio circuitry on the XO-1.5 is now
available on the wiki [1,2].

Sascha

[1] http://wiki.laptop.org/go/Hardware_specification_1.5#Integrated_peripherals
[2] http://wiki.laptop.org/go/File:Ext_audio_1.5.png
[3] http://www.eig.ch/fr/laboratoires/systemes-numeriques/projets/osqoop-l-oscilloscope-libre/index.html
[4] http://gitorious.org/osqoop/pages/Home
-- 
http://sascha.silbe.org/
http://www.infra-silbe.de/
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