Gem Multi: Enabling DC Coupling

rozling

I'm trying to make a basic CV -> MIDI converter using an Intellijel Planar 2 joystick to test.

When I move the joystick I can see activity on the audio input (inputs 2&3), however the level is high just for a moment + goes back to zero which indicates that the inputs aren't DC coupled.

The pinout images describe the inputs as "DC-coupling capable" - is there something we need to add to the sketch to enable this?

Also, what's the recommended voltage range? Are negative voltages ok, or could they damage the inputs?

giuliomoro

It's supported but we don't have command line options for it yet. You'll need to open the file on the board /root/Bela/core/Tlv320Adc3140_Codec.cpp and find line 191 which looks like this

setInputDcCoupling(-1, false, false);

And replace it with

setInputDcCoupling(-1, true, false);

Note that this applies to all input channels on the adc; if you need more fine grained control, the code is a bit more complicated. Also note that to obtain electrical DC-coupling you should remove the input wing and access the pins on the 2x10 header directly.

rozling

Thanks Giulio!

So that's for the differential audio inputs right?

In the meantime I was able to get something going with the analog inputs and a few resistors.

This circuit makes a voltage divider which brings my 0-10V output down to about 0-3.75V

I think 15K at the start would be closer to 0-4V but I didn't have one of those to hand.

The purple board with the euro jacks was particularly handy for this, it's a Mutated Breadboard Friends board which can be found from a few sources.

I'd like to remix all this into some kind of hat or cape though.

I guess the Gem Multi schematics are here so I'll try and base something off that

@giuliomoro just in case, are there any publicly available hat / cape designs out there that fit the Gem Multi?

I'm particularly interested in the analog ins + GPIO (i.e. the 'middle bit') at the moment, but the audio expander schematic would be cool as well

If I get time I'll post a little demo of this working, once I got things scaled for my voltage range it's surprisingly accurate and expressive!