• Audio

  • Noise from Audio Output - Troubleshooting

Hi all,

I've been getting on well with the Bela, using it roughly 5 hours a day, 6 days a week!

However, I've been getting a variety of noises from the audio output recently, and I don't know their cause.

I've uploaded recordings of the audio output of the Bela in several different configurations:

I suspect the problem might have been caused by briefly (for a few seconds) powering the board with an RS powerbank via the barrel connector, while it was already being powered via Mini USB by a mac.

I've confirmed that the noise doesn't originate from anything external to the Bela. The Bela has been updated to the latest version, and I ran the tests above with a blank Pd patch in a fresh project.

Where do you think the noise problems originate? Do you think the BBB is damaged?

Many thanks for your help!

    Could you describe the setup you used to make these recordings?

    Yes, of course - It wasn't anything elaborate:

    I ran the audio output of the Bela through a 10cm jack cable, into my audio interface (Apogee Duet 2), then into Logic. The audio interface was connected to the Mac via USB, and the sample rate was 44100. I used the audio interface preamp to add 40db of gain, to make the noises easily audible.

    I didn't normalise the files when I bounced them out, but I think Soundcloud may normalise or increase the level of files when you upload them.

    I alternated between powering the Bela without any program running, and running a blank Pd patch (by selecting 'run project on boot'. The worst noises occur when a program is running.

    The Pd patch is a blank Pd file, in a fresh, blank project that I created in the IDE after updating the Bela to the latest version and then clearing out all of my existing projects.

      papa_sangre I suspect the problem might have been caused by briefly (for a few seconds) powering the board with an RS powerbank via the barrel connector, while it was already being powered via Mini USB by a mac.

      This should not be an issue: when a 5V supply is available at the barrel jack, it will override the 5V coming over USB.

      papa_sangre I didn't normalise the files when I bounced them out, but I think Soundcloud may normalise or increase the level of files when you upload them.

      "thanks" Soundcloud. This makes it a bit difficult to grasp the difference. Maybe you can put them all in a single track and add markers/comments to tell which is which. This way, any amplification introduced by Soundcloud would apply to all the recordings equally.

      papa_sangre I ran the audio output of the Bela through a 10cm jack cable, into my audio interface (Apogee Duet 2), then into Logic. The audio interface was connected to the Mac via USB,

      You are not really meant to connect the audio from Bela back into the laptop if the laptop is connected to Bela via USB. This includes the setup where Bela is connected to the laptop over USB and the audio from Bela goes to a soundcard that is in turn connected to Bela. This may cause a ground loop that generates some nasty noise (see, e.g.: here).
      When there is no Bela program running, the audio codec is powered off therefore the headphone output is basically floating and if you plug it into a high-impedance input then you are likely to hear a loud buzz.
      For the above reasons, the following :

      2794987 - Bela Not Powered
2794987 - Bela > Mini USB > Mac, Mac Connected To Mains, No Program Running On Bela
2794987 - Bela > Mini USB > Mac, Mac Connected To Mains, Blank Pd Patch Running On Bela
2794987 - Bela > Mini USB > Mac, Mac powered by battery, no program running on Bela
2794987 - Bela > Mini USB > Mac, Mac Powered By Battery, Blank Pd Patch Running On Bela
2794987 - Bela > Barrel To USB Cable > RS Powerbank, No Program Running On Bela
2794987 - Bela > Mini USB > RS Powerbank, No Program Running On Bela

      As for the two remaining ones:

      2794987 - Bela > Mini USB > RS Powerbank, Blank Pd Patch Running On Bela
2794987 - Bela > Barrel To USB Cable > RS Powerbank, Blank Pd Patch Running On Bela

      the difference between the two is in the way the 5V supply is handled on the board. I am not 100% sure what the difference is, but @LiamDonovan should know more.

            Nice one, thanks very much for the in-depth analysis Giulio. Aside from the earth hum, which I'd uploaded as a reference, I hadn't understood the source of each of the noises, so this sheds some light on their cause.

            In fact, the difference between the two final recordings you mentioned was what I was most interested in:

            2794987 - Bela > Mini USB > RS Powerbank, Blank Pd Patch Running On Bela
2794987 - Bela > Barrel To USB Cable > RS Powerbank, Blank Pd Patch Running On Bela

            I'll pass it over to @LiamDonovan - are you able to judge what might be the cause of the nasty noise that occurs when the Bela is powered via the Barrel jack? The two recordings in question are linked above.

            Thanks so much both of you for your help!

            (P.S. just to avoid confusion - the two recordings in question are the 3rd-to-last and the last recording in the list.)

            can you concatenate the two recordings in a single file so we get rid of the difference due to Soundcloud's (alleged?) normalization?

            Yes of course, sorry for not doing that before. Here they are:

            Both sounds are from the Bela, powered by an RS power bank, running a blank Pd patch:

            The first sound is with the power bank plugged into the mini USB port of the bela (pretty clean);

            The second sound is with the power bank plugged into the barrel connector (noise).

            Any idea why it might be making the second sound?

            Hmm, not sure why that might be. Looking at the schematics of the BBB the power signal from the barrel jack goes straight to the PMIC, while the USB has some extra protection circuitry first, and goes to a different input. Perhaps the signal from the USB port has extra filtering or regulation compared with the barrel jack?

            Thanks Liam, that makes sense - I'll just favour the Mini USB over the Barrel jack from now on in that case. I'll report back if I notice anything else unusual. Thanks both for your help!