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Filtering / Second PCB Design

A project log for DrumKid: aleatoric drum machine

A lo-fi digital drum machine which uses randomness to create unpredictable beats. Runs on Arduino, with audio provided by the Mozzi library.

Matt BradshawMatt Bradshaw 05/09/2019 at 22:380 Comments

I feel like I have a handle on the noise issues now. Part of the issue was the PWM noise (Mozzi generates audio via pulse width modulation rather than a "proper" digital-to-analogue converter). The Mozzi website gives a useful low-pass filter circuit for filtering out this noise, but I found that it wasn't sufficient once I started amplifying the signal. Although the signal is only just within my range of hearing, I could tell from recording the signal into my computer that there was a lot of high-frequency oscillation that I wanted to get rid of.

I played around with different resistor and capacitor values, but found that in order to filter out enough of the noise, I was also having to filter out the higher frequencies of the drums, leaving an unwanted muffled sound. I searched the web for "make steeper low-pass filter", and suddenly felt like I was back at university again. I came across this great explanation of passive low-pass filters, which had a really useful section on building a second-order filter. This is basically a chain of filters, and gives a steeper "roll-off", cutting out more of the unwanted frequencies (about 15kHz and above) and keeping more of the frequencies that actually contain drum sounds. I played around with a few different components (why are ceramic capacitors so hard to read?) and found a sweet spot, meaning I was ready for my next PCB...

There are a few key differences from the first version:

I'm still learning about board layout, and should probably get some professional feedback at some point, but for now I've got my fingers crossed that this will be a good improvement on the previous PCB. Now I've got about three weeks before the boards turn up - I could have shelled out the extra money for fast shipping, but I need some time to work on the code anyway, so I'm happy to wait.

Next steps: more work on the code, 3D-print a new case, try DrumKid with my band.

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