Hello. I'm in the process of building a control system for a suitcase of motors that will excite various objects.
I'm fairly new to Bela, but somewhat experienced in Pure Data.

Right now I can control the motor just fine, but the whine of the motor is something I wish to get rid off. When using Arduino, I could move the motorwhine above 20khz, outisde the realm of human hearing, with a little bit of code at the cost of some of the timers being a bit off.

How would I achieve this in PD?

Thank you.
MH

Pd processes the digital signals at 44.1Hz, so the maximum frequency of your PWM carrier is 22.05kHz, and there is not much room there to move it outside the audio range, also because as you increase the carrier frequency you lose in resolution. There are - however - hardware PWM controllers on board that could help with your task.
What Bela board are you using? How many motors are you trying to control?

I'm using the regular Bela cape on top of a Beaglebone black board. I've borrowed this one for testing to see what I need to invest in myself when finishing the project. I think I'll be controlling 5-8 motors, alongside using the Bela to use some pots and switches for speed.

A PWM signal at 22kHz sounds more or less fine. How would I go about changing that via PD code? I tried changing the frequency of the [phasor~] object, which is being used to create a PWM signal, but nothing of note happened.

    mhasselbalch at 22khz your resolution will be lost, hence you cannot control the speed of the motor...

    you really want the BBB PWM as @giuliomoro suggested for this.

      Unfortunately there are only 4 PWM channels available on the BBB on pins that are not busy with Bela, so those wouldn't be enough for your needs unless you can drive several motors at the same speed (i.e.: off the same channel). There would be the possibility to use the spare PRU (one of the on-board microcontrollers) to generate more PWM signals, however that is fairly complicated to do on the software side, but it's still an option that could give you I think up to 11 channels. An easier, more flexible solution could be an external PWM controller, such as the PCA9685, to which you could communicate by sending i2c commands from Pd.

      I was under the impression that I had as many PWM channels as there are digital outs on the Bela. For the project it's crucial that there is individual control over the motors. So that limits my use of the Bela quite a lot without an additional PWM controller. Thank you for the link and the help so far. Would I be able to alter the PWM frequency using the PCA9685 controller?

        mhasselbalch I had as many PWM channels as there are digital outs on the Bela.

        Technically you do, but with a very limited bitclock frequency. So they are mostly good for non-noisy things (e.g.: LEDs).

        mhasselbalch Would I be able to alter the PWM frequency using the PCA9685 controller?

        I would have assumed so, but actually maybe not as much as you would have wanted. The datasheet says:

        The maximum PWM frequency is 1526 Hz if the PRE_SCALE register is set "0x03h".
        The minimum PWM frequency is 24 Hz if the PRE_SCALE register is set "0xFFh".

        Which, if I understand it correctly, means that this is not actually good for you.

            Let me attempt to write a PRU program that does this.

            Right I got 8 channels with a 24038Hz carrier with 8 bit precision. Is that good enough? https://github.com/BelaPlatform/Bela/tree/dev/examples/Extras/pru-pwm

            Interestingly, with this implementation you can increase the precision by "fractional bits" e.g.: to obtain 320 different levels, you get a carrier frequency of 24038*256/320 = 19230.4Hz

            Note: this only works on Bela and not on BelaMini. Or better, on the Mini it will have fewer channels because there are fewer pins available.

              Using the PRU as giuliomoro has presented seems like the best option to me, but I thought I would chime in with another way that might work out. Posted here for brevity 😉

              The trick is using pulse-density modulation rather than PWM. This has the effect of generating something like white noise rather than a well-correlated whine. The control input signal increases the noise power density, which averages out the same in the motor. Anything that bleeds through sounds like low-level noise rather than whine -- and this is typically less objectionable.

              I implemented something like this in C in another thread:
              https://forum.bela.io/d/807-controlling-voice-coils-with-bela/12
              and direct link to github code:
              https://github.com/transmogrifox/Bela_Misc/tree/master/PDM_digital_IO

              Unfortunately I have not attempted this in PD, but the algorithm is simple:
              The structure of the algorithm is this:

              
rand()->[high-pass-filter]---->[+]----->[+]-------{SAT: 1/0}--------o--->[OUTPUT]
                                ^       -^                          |
                                |        |                          |
[Input Signal]------------------o        o-----[low-pass-filter]<---o

                Thank you very much for the response, I will see what I can do with it. As of now, I will most likely use Arduino for the job, simply because it's what I own. The budget doesn't cover one of two Belas as of now, unfortunately. But it's wonderful to know what's possible with Bela, for future reference. Thank you again.