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Jerboa ATtiny85 Modular Synthesizer

JERBOA ATtiny85 Modular Synthesizer

The idea is to create an Analog Modular Synthesizer using a single ATtiny85 microcontroller for each module.

So although those chips are digital microcontrollers, the signals going in and out of them are analog voltages, like in a real analog synthesizer.

What's a *Modular* synthesizer? It means that it's composed of lots of individual modules, each of which usually does a well-defined, often simple operation. Examples are

  • VCO: Voltage Controlled Oscillator: the output frequency depends on the input voltage. (There's often a "one volt per octave" convention.) If it's input is varying, you get Frequency Modulation (FM)!

  • LFO: Low Frequency Oscillator: It's like a VCO but it produces much lower frequencies.

  • Sample/Hold: On some condition, it samples its input voltage and outputs that voltage until the next sample time.

  • VCF: Voltage Controlled Filter: A High, Band, or Low-pass filter in which an input voltage determines the cutoff frequency.

  • VCA: Voltage Controlled Amplifier: An input voltage controls how much amplification or attenuation. Basically it multiplies two signals. This also gives you Amplitude Modulation (AM).

    Kinds of signals in a Modular Synthesizer

  • Audio: The voltage (in time) is the signal (in time).

  • Frequency Level: The (instantaneous) voltage is the (instantaneous) frequency. Usually "1 volt per 1 musical octave". Thus the voltage is (a constant plus) the log of the frequency.

  • Other levels: For example, the amount to amplify or attenuate in a VCA.

  • Gates: Digital signals, or analog signals interpreted as digital. Could describe the press/release of a key, or a time pulse in a synchronizer, or when to light an LED.

    "Patch Anywhere": The goal that any inputs and outputs can be connected any way you want. This lets you abuse the above signal types, wiring the to each other, to achieve weird effects.

    My design achieves "patch anywhere" as long as you only use the designated inputs & outputs of each module. (Connecting other things, like shorting +5V to Ground, obviously can cause bad problems.)

    I love the ATtiny85s!

    They're $1.20 in single quantities, $100 for a hundred.

    8 PIN DIP! Easy to handle, easy to solder, fits in breadboards.

    Robust! They're very forgiving of accidental short cirucits and don't require special anti-static handling.

    They're the kid brother of the $2 28-pin ATmega328 chip in the Arduino.

    Their I/O functions are very similar to, and usually a subset of, the Arudino's capabilities.

    The Arduino programming software (and IDE) can be used on Linux, Mac, or Window, to program ATtinies.

    The outputs can source or sink enough current to power an LED.

    SparkFun sells a $20 programmer for these chips, that uses USB to connect to Linux, Mac, or Windows.

    Nice features of the "ATtiny85"

  • Internal clocks for 16MHz, 1MHz, 125kHz, and other speeds. I'm using 16MHz since I care about sampling rates but don't care about power efficiency.

  • ADC: One Analog-To-Digital converter with 4 possible input pins. Going a little slower, you can get 10 bit accuracy, but I'm taking 38,460 samples per second with 8 bit accuracy. I'm using 3 analog inputs, two wires A and B and one knob K. I sample A every other time, so 19,320 per second, so a Nyquist frequeny about 9600Hz. In the other slot, I usually sample B, but 1/256th of the time, I sample the knob K.

  • PWM: Instead of true analog outputs, the chip supports three Pulse-Width-Modulation outputs. I only use one. This uses short pulses (at 250kHz) of varying width (duty cycle) to simulate an analog output. I run it through an 200 ohm resistor and then have a 0.1 uFd capacitor to a power rail (that's a Time Constant equivalent to 50kHz, logarithmically midway between the 250kHz pulses and our Nyquist frequency around 10kHz) to smooth out the pulses.

  • LED: Any pin on the chip can be a digital input or output. I usually put an LED on the same output pin that the programmer does. Making it blink in various ways is a useful debugging technique (there is no printf!).

    Interlude on debugging

    Besides the most basic "turn a LED on or off", debugging features of Jerboa that proved useful:

  • Moctal: = "morse" plus "octal": Call Moctal(n) to output a debugging byte, I output dits for 0s and dahs for 1, spaced like this: dah-dit di-dah-dit di-dah-dit for octal 0222. This can happen on the LED while everything else is still running.

  • Fault: Call Fault(n) to stop all interrupts and produce "flashy" LED pulses in groups of n. For fatal assertions.

    My Standard Jerboa Module

    This is my standard Jerboa module. They are all the same except for the programming on the chip.

    The chip only has 5 useful I/O pins (the other three are +5V, Ground, and Reset*). So I have to pick and choose which subset of capabilities. I chose 3 analog inputs, 1 analog (PWM) output, and one digital output for the LED.

    Here's what I name the signals:

  • A: analog at full 19,320 samples per second: If a module has one audio input, it comes in A.

  • B: another analog input, at almost 19,320 samples per second. Usually for some level input that is not audio.

  • K: An analog knob (potentiometer) sampled 75 times per second. Adjusts some parameter on each modules. A lot of the fun in modular synthesizers is frobbing the knobs!

  • F: The output, a function of A, B, and K. The external resistor and capacitor smooth this out to an analog signal.

  • L: The LED. This digital output can also be used as a degenerate analog input.

    I line the ATtinies down a breadboard in a repeated cell shape. In addition to the chip and connections to ground and +5V power rails, there's two resistors, one capacitor, and an LED in each cell.

    Current Demo

      /--> LFO --> LFO --> LFO --\--> PeriodicHold -> VCO --> ECHO --> Speaker
     |                            |
      \                          /

    The three LFOs are just VCOs at low frequencies. Making a loop with these creates chaotic, unpredictable output (thanks to Mark Lentczner for this idea).

    On a regualar period, the output of an LFO is sampled and held.

    That held sample determines the freqency of the VCO.

    The Echo lets you continue to hear the old tone for a short while after a new sample is held. (It has a 450-byte history, where the oldest is averaged with the input, and that average is output and saved at the front of the history.)

    Programming a Module

    The hardest code has gone into my libraries, mainly "jerboa.h". Writing a module is often very simple.

    Where Arduino sketches have "setup()" and "loop()", use a capital letter instead, and the Jerboa runtime will call your routines "Setup()" and "Loop()".

    Jerboa provides these inputs:

  • InA()
  • InB()
  • InK()

    And you can output with these:

  • OutF(byte x)
  • LedOn()
  • LedOff()
  • LedToggle()

    Here is the actual "Sawtooth VCO" module:

    #include "/tmp/jerboa/jerboa.h"
    #include "/tmp/jerboa/generated-vfo-table.h"
    word phase;  // 16-bit phase, 0x0000 to 0xFFFF.
    void Setup() {}
    void Loop() {
      // Sum all 3 inputs, for a 0-to-15V input:
      word offset = word(InA()) + word(InB()) + word(InK());
      // Look up how much to increment the phase.
      phase += pgm_read_word(VFO_TABLE+offset);
      OutF(phase>>8);  // Analog output of sawtooth wave.
      if (phase & 0x8000) {  // Digital output blinking LED.
      } else {

    The "phase" is a 16-bit unsigned "word". The "VFO_TABLE" was generated by a program in Go. It basically performs the AntiLog function ("2 to the x").

    It's configured to know how much to add to the phase for each voltage level, 0V to +15V, since three 5V inputs are added together, InA() InB() & InC().

    When the phase overflows, that's a full output wave cycle.

    Happy Hacking!

    Work in Progress: https://github.com/strickyak/jerboa-attiny85-mod-synth

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