• Hardware

  • Radio Noise Coming Through Audio out

Hello,

I am using a Bela with the Audio Expander Capelet. The setup involves 4 XLR inputs and 4 audio headphone outputs. I am finding that I am picking up a fair amount of radio noise through each of the headphone outputs when the Bela is powered. I tried to isolate the source of the noise and have found that I can still hear quite a bit of radio noise when only a single headphone out is plugged into the capelet. I was wondering if anyone had similar experiences with radio noise coming through the Bela or capelet.

Some things I've tried that haven't affected the amount of noise:

  • Different usb power cable.
  • Trying the barrel input instead of usb input.
  • Trying various power sources, isolated power bank, computer, wall socket.
  • Trying different headphones.
  • Unplugging everything from the Bela except a single headphone out, so the only IO connect are usb power and headphone out connected to capelet.

The only thing that seems to make a difference for the radio noise is moving to a more isolated/windowless room. This may serve as a temporary solution but it would be nice to get to the source of the error.

Are you plugging headphones directly to the audio outputs of the audio expander capelet? They are not designed to drive headphones, though this is not necessarily the reason you are picking up this noise.

Does this happen while the Bela program is running or when it is stopped?


Here is a picture of my setup (sorry a lot of wires probably makes it hard to decipher). The wire I am holding goes from the Bela to a headphone output.

The noise can be heard when the Bela is stopped and running.

Can you try with a different power supply ?

What happens if you send a sinewave out of that output? Does the radio noise mix with the sinewave?

I've tried a few different power supplies with no difference between them.

When sending a sinewave out of each of the 4 headphone outputs, it does mix with the radio noise.

what do you get when running board_detect in the console at the bottom of the IDE?

try removing the audio expander capelet and try to connect directly to the unpopulated pads marked GND 0 2 ... these should be roughly equivalent to the capelet's outputs 0 and 2, hopefully without the radio noise?

5 days later

Sorry for the delayed reply, I didn't get another chance to look at this until today.

Due to how the setup was put together I am unable to remove the capelet, however I do have a headphone output connected to the layer below the capelet and get no radio noise through that. Can I infer from this that it is somehow the capelet that is picking up the radio signal?

Thanks.

    kaim however I do have a headphone output connected to the layer below the capelet

    I don't understand. Where are the headphones connected exactly?

      Sorry for the confusing terminology, here is a picture showing where the headphone out that doesn't pick up radio noise is located on the Bela. My understanding is this is the layer below the capelet but perhaps that is wrong.

      right, that's the Bela stereo headphone out, that's entirely a different signal from the one that goes to the Audio Expander capelet, whereas the one I recommend is the same signal that goes to the audio expander, but earlier in the signal chain. You can try to get the signal from the 10x1 analog out header on the audio expander capelet, but as that has a 2.5V DC offset, plugging headphone into it will most likely distort it beyond recognition. You can try to apply a large capacitor in series between that output and the headphones and hope to get something out of it, or you can follow my recommendations above.

      • kaim replied to this.

        giuliomoro I see thank you for clarifying. Is your hypothesis that the capelet expander is the culprit of the radio noise?

          Well that's what I am trying to figure out by asking you to perform that test.

          I tested connecting headphones directly to the unpopulated pads marked GND 0 2, and I believe that has eliminated the radio noise. I still hear a periodic low volume buzzing which I'm not sure can be fixed or not.

          Do you have any thoughts on how to eliminate the radio noise given that it is likely the top capelet that is causing it?

          I don't have a remedy at hand for the audio expander capelet ... that design has been unchanged for 8 years and it's the first time we have such issue. If you are in a hurry, a possibility would be to take the analog outputs from the Bela Rev C cape , using those unpopulated pads, but that may require a lot of hand wiring. We could send you a replacement for the audio expander instead, but that will take a week or more to reach you.