Commonplace Thoughts

I’ve recently started to learn about and practice with Teensy’s audio library, and their companion shield.  The reason that I have gone this route is that the Mozzi library uses lower fidelity DACs that result in too much noise, even though they promise 12 bits.  There are just too many artifacts.

I have built a simple drum synthesizer using the amazing audio design tool from PJRC – which you have to check out!  https://www.pjrc.com/teensy/gui/

Here is the project I’v built using brass strips as keys – simple on/off but responsive keys to the digital inputs of a Teensy 3.2 digital inputs (4-8).

And here is the input prototype, made from yardsticks, thin hobby plywood and brass strips:

The audio design tool is a simple front end for arduino sketches to enable hooking up audio components/signal processing blocks quickly and integrate into a sketch.  Here is the block diagram for this project:

Way cool, isn’t it!  Finally, I coded it into the Arduino IDE with a polling mechansim for the 5 switches, and the source file is “sD_tAd_419.ino” and it creates incredible sounds, especially for 5 second note times.

Recently completed is a step sequencer that combines many different subsystems to yield a nice sounding and playful platform.  We hope that this provides enough documentation so that when I come back to this project, I can actually get up to speed quickly and accurately.  I have decided to document projects both on line via 47meters & via a paper file to keep at my home office.  Here is what the project looks like – the whole system:

Below is the schematic for the project.  Basically it consists of a filter module and a rail splitter block.  The filter module consists of a first order LPF at 7 KHz, followed by a Sallen-Key second order filter at at 8 KHz, followed by another first order LPF at 7 KHz.

Here is the wiring on the white board:

[Software Architecture] – [ Digital Realization] – [ Analog System] – [ Audio Output ]

The project looks like the following two images.  Note, that I think that the so called ‘white boards’, or ‘poke boards’, or ‘solder-less breadboards’ provides an amazing almost permanent construction medium.  I really believe that unless you plan mass production, that you stick with white boards.

I will start with the important Software Architecture (lifted from the Mozzi playbook):

Update Control

The system design consists of a sequence array that contains the notes and their relative position (all notes in MIDI integer values), which is part of a loop that tests a ‘note ready’ condidiont.  If this is satisfied, the the note frequencies are updated, and the next note cycle is initiated.

And the table for the Digital Pins:

Update Audio

This cycle, run at very high speed is responsible for the DAC response to the note(s) instantaneous value, the envelop value, and an overall mixer value to balance the sound between multiple oscillators.

Here is the filter used to reconstruct the audio and filter out excessive (out of band noise):

Just to make sure it was working properly, here is the frequency response, showing approximately a 4th order Low Pass Filter: