Digital noise on Bela Mini audio output when powered with power supply

count_the_rings

I'm experiencing some significant digital noise (sounds a bit like R2D2) on the line out and headphone outputs of my Bela Mini, when powered using an external supply.

The noise sounds very much like that detailed in this separate forum post (though a bit less constant, with a few more stutters/blips etc) - but unfortunately this post did not seem to reach a solution: https://forum.bela.io/d/1244-digital-noise-on-audio-out

As in the above post, the noise goes away when the program is stopped, so does not appear to be a ground issue. It also takes approx 20 seconds from when the program boots up for the noise to start.

The one thing that makes the noise go away is plugging in the USB cable to a computer/laptop or power supply (in addition to the external power supply being used to power the Bela Mini on my custom board). This makes me wonder if the noise could be to do with some internal PMIC on the PocketBeagle, which somehow isn't noisy when it is operating as a USB device..? I've tried powering the Bela Mini directly on the V_USB pin but that doesn't change anything...

Any suggestions much appreciated!

giuliomoro

It sounds like an issue with your power supply? Can you confirm the output voltage you get from it, its current rating and how it's connected to the BelaMini? Also, are you using any of the analog inputs and how are these used in the code?

count_the_rings

Thanks for the reply. I've tried it with several different power supplies, ranging from 9V-12V and up to 2.5A. There is a voltage regulator on the board which outputs 5V and is rated at 1.5A; the MPN is OKI-78SR-5/1.5-W36H-C.

I initially wondered if the noise could be caused by this voltage regulator (it is a switching regulator which I know can be noisy), but I tested it with a linear regulator (MC78M05CDTRKG) and the noise persisted.

I then tried removing the regulator entirely, hacking a USB cable and soldering the power line directly to VIN_USB (P1.07) on the PocketBeagle, both with and without a ferrite bead - the latter of which I assumed should be exactly the same as plugging in a USB cable to the PocketBeagle, but again the noise persists.

I have also tried powering the Bela from both the regular VIN (P1.01) and the VIN_USB (P1.07) pins, which doesn't change anything.

In answer to your other question, yes I am using all 8 of the analogue inputs to read sensors, although when I comment this out the noise is still present.

giuliomoro

The headphone out should not have noise issues regardless of how noisy the power supply is, as long as it remains comfortably above 3.3V. The analogs would be a bit more sensitive.

although when I comment this out the noise is still present.

To make sure, try an empty project, so that all that goes to the audio DAC is zeros.

count_the_rings I then tried removing the regulator entirely, hacking a USB cable and soldering the power line directly to VIN_USB (P1.07) on the PocketBeagle, both with and without a ferrite bead - the latter of which I assumed should be exactly the same as plugging in a USB cable to the PocketBeagle, but again the noise persists.

Does this mean you got the same laptop or USB power supply that worked above (The one thing that makes the noise go away...) , cut open the USB cable, and connected GND and 5V to the PB and the noise was there, while with the same power supply going into the USB port there is no noise?

What ground pins are you using for bringing power into the board? Any other cables connected to the board?