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

[Matt Flax]

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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