Build Guide

Firmware

• Design
• Bill of materials
• Construction
• Top

The firmware and hardware source files are hosted on GitHub, and can be found here.

The code will build with a standard install of Teensyduino, since the required libraries: ADC, Audio, Bounce2, Encoder, MIDI and USB MIDI support are all included out of the box.

The FM library code is in the src subdirectory.

Important! Do not attempt to connect the Teensy's micro-usb port to your computer while the module is being powered by the 10 pin header. It is quite handy to be able to use the usb Serial port while the module is in a rack, but to do so you need to disconnect the usb power — either by using a special usb cable with the power wire cut (usually coloured red), or by cutting the jumper on the Teensy board that connects VUSB and VIN.

Calibration of the V/Oct CV input is required. Easiest is to adjust the following section of code in the main .ino file:

		...
		// Serial.println(saveraw);
		/*
			Calibration data
			C2 input is 33
			C7 input is 3489
			scaling factor f ~ 60 / (3489 - 33)
			pitch_cv = (raw - 33) * f;
		*/

		const float fbase = 33;
		const float f = 60.0 / (3489 - fbase);
		...
		

Uncomment the saveraw output and see the values when playing C2 and C7. Use the measured values to calculate an appropriate scaling factor.

There is also a trim pot on the underside of the board for hardware-based scaling adjustment.

The FM code is quite computationally expensive, but well handled by the Teensy 4.1 at its default clockspeed of 600MHz. If you wish to overclock, be aware of the extra current required, and the power (I²R) going through the 10Ω resistor R9.

Control

• Design
• Bill of materials
• Construction
• Firmware
• Top

Nearly all control of the module is straightforward and immediate, available by directly manipulating the knobs, sliders and switchs on the panel.

However, there are some aspects that are less obvious and somewhat hidden, and I will describe those here.

Envelopes

I use 4 pots for the envelopes as follows: First I use a slide switch to select the envelope "mode" - one of AD, ASR, ADSR or Independent. Then, in each mode, the 4 pots are used in different ways.

• AD mode
  • pot 1 is A for all carriers
  • pot 2 is D for all carriers
  • pot 3 is A for all modulators
  • pot 4 is D for all modulators
  • S is 0, and R is irrelevant.
• ASR mode
  • pot 1 is A for all carriers
  • pot 2 is R for all carriers
  • pot 3 is A for all modulators
  • pot 4 is R for all modulators
  • S is maximum, and D is irrelevant.
• ADSR mode
  • One pot for each of A, D, S and R, and the same envelope applied to each operator.
• Independent mode
  • Each pot controls the envelope for its own operator, with a continuous change between some specific fixed shapes (as shown on the panel).
  • Specifically, it starts with long attack and fast decay to zero.
  • Then the attack is reduced and sustain increased as it moves to a gated envelope.
  • Then attack and release are reintroduced to give it a more typical rising then falling envelope shape.
  • Then it moves into faster attack with release.
  • At maximum clockwise rotation it switches into drone mode.
  (Obviously independent mode doesn't come close to covering all possibilities, but it does cover plenty of cases and is far better than trying to manage 4 pots per operator just for the envelope.)

Algorithms 11-20

The 10 basic algorithms are shown on the front panel, and the feedback operator switch allows you to choose whether feedback should be applied to operator 2 or operator 4.

However, a straightforward implementation of the operator 2 feedback alternatives would mean some cases are redundant. Specifically, algorithms 14, 15 and 20 would just just relabellings of algorithms 4, 5 and 10 respectively. I thought it would be better to make them provide genuinely different sonic possibilities, and so I implemented the following idea that works well, provides plenty of additional sound possibilities, and doesn't require any complicated or magic interaction with the controls.

Algorithms 14, 15 and 17 introduce operators with triangle or square wave oscillators to provide variation from their operator 4 feedback analogues, as shown by the small illustration in the top right of their box in the above figure.

More signifcantly, algorithm 20 now has 2 sine wave oscillators and 2 triangle wave oscillators, and the fine tuning controls are overridden to perform wave folding. A clockwise rotation folds the wave symmetrically, while an anti-clockwise rotation folds asymmetrically.

Secret Menu

The "secret" menu is accessible by pressing and holding the reset switch. The Slider LEDs will light in turn, and releasing will select the associated option:

• LED 1: panic — all notes off.
• LED 2: V/Oct mode — monophonic, MIDI ignored.
• LED 3: MIDI mode — 16 voices, V/Oct and gate inputs ignored.
• LED 4: Toggle aftertouch — aftertouch will raise the level of all modulators.
• Beyond: the Teensy will restart.

In MIDI mode, CC1 will also relatively raise or lower the level of all modulators. Velocity and pitch-bend are supported.

Serial connection

A small number of commands can be sent over the usb serial port.

• c: print the current configuration
• z: toggle print configuration on change
• p: panic (all notes off)
• s: toggle oscillator sync
• t: show the temperature of the Teensy
• h: show the help

Congratulations! You're done.

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