[AudioI] OCTO soundcard DC input (+ some output measurements)

[Staš Vrenko]

 Thanks for fast response & succinct infos!

You're right about the ADC High-pass switch -
I managed to turn it off within alsamixer (already installed when using
Patchbox OS) -
took me a while to figure out "m" is used to toggle this (for future
reference/newbs like me)
The ADC channels differential vs. single-ended conversion type can also be
set in there.

Toggling the HP switch & using single-ended setting (/wo RCA breakouts)
we've measured that the card can capture
both AC and DC signals between 0-5V (cca. -200mV and +5.1V to be precise)
before clipping/distorting.
For future readers I'd emphasize what has already been mentioned - it makes
sense to be cautious when
handling the card (esp. around header pins) in order to avoid introducing
static electricity into it - I think we managed to destroy 2 inputs
by not being careful enough so they now only read a very noisy signal
(self-noise probably) and don't respond to external signals.

I will report with more on this when we unscramble the inputs - maybe this
pin-death turns out to be software related and they really aren't fried and
if not, it's
collateral for lessons learned :)

V V ned., 9. maj 2021 ob 23:32 je oseba Matt Flax <matt at audioinjector.net>
napisala:

> Hi Staš,
> On 8/5/21 3:10 am, Staš Vrenko via People wrote:
>
> Hey,
>
> I'm working on a project in which I'll use the Octo soundcard for
> capturing, processing, and outputting DC voltages.
>
> Sounds like an interesting use for the Octo.
>
>
> Using PD (Pure Data) for signal generation and a digital o-scope for
> measurement, me and my colleague managed to output DC voltages (directly
> from positive pins, not using breakout RCA boards) of approx. 0.77V
> (@amplitude of -1), 2.44V(@ 0) and 4.2V(@ +1). The negative pins output an
> inverted signal (same p-p though).
>
> The DC INPUT testing (not using breakout RCAs) was a bit confusing.
>
> There's a channel number re-assignment problem when Pure Data gets opened
> and closed or when RPi gets rebooted. I've read it can be related to a
> driver problem?
>
> It isn't too hard to unscramble input channels if they are scrambled. This
> code does it :
>
>
> https://github.com/flatmax/gtkiostream/blob/master/test/ALSAFullDuplexMinScan.C#L40-L72
>
> One way to use it with PD is to create an ALSA plugin which applies this
> code on input channels. Then you could start PD with that plugin.
>
> Getting the AC signals into PD was easy but there was no such luck with DC
> - so I'm
> wondering whether DC capturing has been tested/proven to work?
> If it has - could it be that there's a DC filter somewhere in RPi signal
> path (JACK, etc.)?
>
> Both input and output should be capapble of DC coupled signals. Is it
> possible that alsamixer has the setting for a high pass filter or DC
> removal set ?
>
> I notice in the ALSA mixer there is the high pass filter switch :
>
>
> https://github.com/torvalds/linux/blob/master/sound/soc/codecs/cs42xx8.c#L92
>
>
> Otherwise, it's a great card & hats down for developing a custom IC!
>
> thank you
>
> Thank you for your answer, best,
> Staš :)
>
> --
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>
>
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