Bela Gem Multi DC-Coupled In/Outs

MIZI

Hello, I am trying to understand the DC-coupled capabilities of the Bela Gem multi, I am trying to build a matrix mixer for Behringer Neutron, I got some Y minijack TRS to female mono 3.5mm cables and I am doing a simple test of a RNBO~ patch that connects audio in 2-9 to audio out 2-9 and then I just connect the Neutron's LFO to Osc1 pitch through Bela, it kinda works but the range is not the same as when connecting it abridged.

I found the following info on the crowdsupply page:

"DC-coupled Audio Inputs: we have added support for DC-coupled connections to audio inputs 2-9 on the Bela Gem Multi. When coupled with suitable level-shifting circuitry, this will let you use these eight audio inputs for CVs or other low-frequency signals. DC-coupled inputs will be available when connecting directly to the pin headers on the Multi cape; the 3.5 mm TRS audio jacks will be AC-coupled as before, as this configuration provides the best noise performance for most audio applications.

Using DC coupling with the audio inputs requires some changes in software settings, which will be selectable via the Bela IDE. It also requires an external preamplifier circuit to shift the input levels to the right range (strictly between 0 and 3.3 V, and centered around a 1.375 V reference which is required by the ADC). We will publish schematics for a level shifter after the campaign."

So how would I go about achieving what I am trying to do ? The wings are actually NOT dc-coupled? They kinda feel like they're DC-coupled their range is not the same as the straight connection, but it is not similar to non-dc coupled outputs that I've tried in the past. Also, has the level shifter schematics been published yet? Can't find anything. Also can't see any software settings regarding DC-coupling in bela IDE.

Any help to steer me in the right direction is greatly appreciated, what I'd like to have is simple; DC-coupled in/outs and the Bela working as a transparent mixer between the Neutron connections.

giuliomoro

The jacks and pin sockets on the wing are AC coupled. They have a 1u capacitor in series that makes them suitable for connecting audio signals.

If you want to connect DC coupled signals you need to remove the wing and connect to the 2x12 sockets directly. There, you'll have differential inputs, biased at 1.375V; you can use them single-ended if you want. These are the raw inputs into the TLV320ADC3140, so you can check its datasheet for more details. We'll publish a reference circuit soon.

Long story short, you'll need to remove the ~~cape~~ wing and use signal conditioning to make sure your signals are in the correct range.

MIZI

Okay so the things written at the crowdsupply page hold, so for someone who is not well adversed in hardware, just to summarize, I need to remove the wings, make my own terminals by connecting directly to the socket, then what would I need to do to make the signal CV compatible ?

"you'll need to remove the cape and use signal conditioning to make sure your signals are in the correct range. "

What kind of signal conditioning? Any leads?

giuliomoro

MIZI remove the cape and

I meant remove the wing.

The broad-strokes description of the circuit is what you posted above:

preamplifier circuit to shift the input levels to the right range (strictly between 0 and 3.3 V, and centered around a 1.375 V reference which is required by the ADC).

We'll post a sample schematic soon.