groundloopnoise-like sound coming from outputs of Bela Audio Expander

minnowahaw

Hello there,

During the making of a prototype for an outdoor installation I am experiencing some groundloop noise like sound.
I am running a Bela with an Audio Expander and an infrared sensor connected to Analog Pin 0 getting its 5v from P9.07. I am using all 8 audio channels of the Expander and these are amplified by four 50watt Douk NS-04G Stereo amplifiers, which are themselves connected to 2 exciters, 2 resonant woofers and 4 passive speakers. All the channels have the groundloopnoise-like sound.... I just recorded the noise coming from the passive speaker (with the volume of the amp at max) with a zoom recorder:
[(http://sjoerdleijten.nl/audio/Bela_groundloopnoise.mp3)]

The sound is there when I power the Bela with a 5v adaptor, or via the usb from a power pack or via a usb cable from my laptop (all the 4 amps are powered with a single 12v battery). The volume of the sound gets higher when the sketch is loaded. I can get rid of the noise by using a DI box with the ground lift. But since I am using 8 channels and have 6 different installations in which I am using this setup - it would be space wise and financially nice to have a more compact and elegant option than the DI box (I'd rather not have to buy 24 DI boxes). Anyway I can solve this by using the GND from the Bela?
I checked some other forum posts about people having similar issues, but I couldn't find the exact same problem. But maybe I missed it.
Thanks in advance!

Kind regards,
Sjoerd

giuliomoro

Does the issue persist if you disconnect the IR sensor?
Does it apply only to outputs from the audio expander capelet or also to audio outs?
What's the revision of your Bela cape?
Does it persist when sending out only zeros from the Bela program (using the code below)?

bool setup(BelaContext *context, void *userData)
{
    return true;
}

void render(BelaContext *context, void *userData)
{
    for(unsigned int n = 0; n < context->audioFrames; n++) {
        audioWrite(context, n, 0, 0);
        audioWrite(context, n, 1, 0);
    }
    for(unsigned int n = 0; n < context->analogFrames; n++) {
        analogWriteOnce(context, n, 0, 0);
        audioWriteOnce(context, n, 1, 0);
    }
}

void cleanup(BelaContext *context, void *userData)
{
}

minnowahaw

giuliomoro
When I disconnect the 5v or GND of the IR sensor the noise becomes a lot, lot less, but there's still this typical noise present that seems to be influenced by the processes that are running on the Bela. All 8 expander channels have this same sound.

The audio out of the Bela capelet itself is perfectly quiet! When I remove the expander capelet and then connect the IR sensor, the audio out stays quiet.

The revision I am on is: Bela image, v0.3.8h, 5 January 2023. I actually tested the same setup with another Bela that still had an older v 0.3.6b image on it and another audio expander: it had exactly the same behaviour concerning the noise coming from the expander capelet.

When I use your zero sending patch, the sound changes in a more continuous noise sound. I made a recording of a resonance speaker, first you hear the rhythmic noise and then when you hear the plop you're patch has started and it changes into this other noise.

Maybe it has something to do with my expander settings?

giuliomoro

Are you using the Bela cape Rev C? This one has a way of bypassing the audio expander outputs by taking the analog outs from the unpopulated pads on the Bela cape itself. These are 0V-centered audio-quality outputs which should be significantly less noisy. Highlighted below:

To reduce the noise from that sensor, try using pin P8.01 for its ground.