Using modularSID

The modularSID is divided into four sections: one global section for parameters that affect the entire chip, and three individual sections corresponding to the SID’s three voices.

The SID is a hybrid digital–analog chip with many software-controlled parameters. modularSID makes almost all of these parameters accessible for manual adjustment or voltage control. Most controls follow one of a few recurring input types:

Unipolar Analog Inputs

These inputs accept positive control voltages only and consist of a jack and a knob.

When no cable is connected, the knob provides manual access to the parameter’s full range. When a cable is connected, the input expects a positive voltage between 0 V and 5 V. The knob then acts as an attenuator, scaling the incoming signal from 0% to 100%.

Digital Inputs

These inputs consist of a jack, a latching or momentary button, and an LED. The LED always indicates the actual state being sent to the SID.

When no cable is connected, the button directly controls the parameter. When a cable is connected, the jack expects a typical digital Eurorack signal. Voltages of 1.7 V or lower are interpreted as low/false, while voltages above 2 V are interpreted as high/true. Pressing the button overrides the incoming signal and forces the parameter into its high state.

Bipolar Analog Inputs

The Audio In is the only input intended for bipolar analog signals.

Global Section

  • Audio Out – The output of the entire chip as it is provided by the SID, slightly amplified for Eurorack levels.
  • Digital Volume – The SID has an internal volume register with just 16 raw steps. This parameter shoule be left in maximum in most cases. However, it was used by C64 coders to create certain effects. So you can access this parameter either manually or using a unipolar CV voltage.
  • Audio In – The SID signal offers to run an external audio signal through its filter section. This is the jack plus an attenuator knob to do this. The peak LED lights up when the signal comes close to the maximum leve (clipping). However, if this LED flashes from time to time it does not necessarily mean that clipping occurs.
  • The right side of the global section is dealing with all kinds if filter aspects. Each voice has a flag (a digital parameter) deciding whether it should run through this filter section or bypass it on its way to the Audio Out. The following parameters are available to manipulate the filter. Some combinations likely result in effectively mute the signal, but this is entirely logic and expected behaviour.
    • High Pass / Band Pass / Low Pass are common digital switches deciding whether the high, band and/or low pass is active.
    • Filter Res and Filter Cutoff are analog inputs making these classic filter values available.
    • Filter Ext Audio is the switch to decide whether the Audio In signal is being sent thrrough the filter section or not.
  • The digital Mute parameter removes Voice 3 from the audio mix in order to use it as a modulation source make itself inaudible.
  • Soft Reset signals the modularSID to reset the SID chip.

Voice Sections [1,2,3]

  • VCO Frequency The VCO Freq input and the Coarse, Fine, and Offset controls work together to set the oscillator frequency of the voice. Coarse and Fine adjust the overall response to the incoming CV (Scaling / Tracking). Offset shifts the resulting pitch range. Without an external CV, the oscillator will still be tuned across a limited range; an external pitch CV is required to access the full frequency range.
  • Quantization snaps the oscillator frequency to discrete pitch values corresponding to musical notes. This makes it easier to obtain stable melodic intervals from a continuously varying CV.
  • Ring Modulation alters the Triangle waveform of the current voice using the oscillator of another voice. The modulation routing forms a fixed cycle:
    • Voice 1 is modulated by Voice 3
    • Voice 2 by Voice 1, and
    • Voice 3 by Voice 2.
      Changing the frequency relationship between the two oscillators produces additional harmonics and metallic, bell-like, or otherwise complex sounds. Ring Modulation has no effect unless the Triangle waveform of the modulated voice is active.
  • Sync hard-synchronizes this oscillator to another SID oscillator by periodically restarting its waveform cycle. This produces more complex, harmonically rich timbres, especially when the two oscillators run at different frequencies.
  • Filter routes this voice through the SID’s shared global filter section. When disabled, the voice bypasses the filter and is sent directly to the output stage.
  • PWM Duty Cycle sets the pulse width of the Square waveform. Changing or modulating the duty cycle alters its harmonic content, ranging from a broad square-wave sound to increasingly narrow and thinner pulse sounds. It has no effect unless the Square waveform is active. Waveforms: Square, Triangle, Sawtooth, and Noise
    • Square produces a pulse waveform whose shape is controlled by PWM Duty Cycle.
    • Triangle has a comparatively soft and rounded sound and is also used by the Ring Modulation function.
    • Sawtooth produces a bright waveform rich in harmonics.
    • Noise generates an irregular noise signal suitable for percussion, effects, and sound textures. Multiple waveforms may be selected simultaneously, although the resulting combined waveforms can vary between original SID versions and replacement chips.
      Note: On the 8580, selecting Noise together with another waveform can cause the oscillator to lock up and fall silent, sometimes requiring a reset or power cycle to recover.
    • Gate and Trigger
      • Gate directly controls the SID envelope: while the gate is active, the envelope progresses through Attack and Decay and then remains at the Sustain level. When the gate is released, the Release stage begins.
      • Trigger converts a short incoming pulse into a gate event and restarts the envelope, making it suitable for sequencers, drum triggers, and other brief control signals.
    • ADSR Envelope
      • Attack determines how long the sound takes to rise from silence to its maximum level after the gate is activated.
      • Decay sets the time required to fall from the maximum level to the Sustain level.
      • Sustain defines the level held for as long as the gate remains active.
      • Release determines how long the sound takes to fade out after the gate is released. Each parameter can be set with its knob and controlled through its corresponding input.

Tuning

Recommended Oscillator Tuning Procedure:

  1. Connect a device that provides a stable, accurately calibrated 1 V/octave signal to the VCO Freq input. (At any:frequency we use an Arturia KeyStep 37 as our reference source.)
  2. Be sure to enable Quantization.
  3. Set the Coarse, Fine, and Offset controls to approximately the 10–11 o’clock position.
    • Scaling/Tracking: Coarse and Fine control the “attenuation”, meaning they scale the incoming CS voltage.
    • Tuning: Offset on the other hand, shifts the signal relative to a constant bias, which causes the effective CV to increase at high frequencies and decrease at low frequencies.
  4. Set Attack to minimum and Decay to approximately the 3 o’clock position. Set Sustain and Release to minimum as well.
  5. Connect a tuner to the modularSID output, or use an acoustic tuner to monitor the oscillator frequency. Support for tuning with a connected GUI2C display may be added in the future.
  6. Play a note and adjust the Offset control until the oscillator reaches the intended pitch. We recommend starting with D♯1.
  7. Play notes across several octaves and alternate between the Offset and Coarse controls until the tuning is accurate across as wide a range as possible.
  8. The Fine control is usually not required, but it can be used for small final corrections.

A correctly adjusted oscillator should typically track approximately from C1 to B6. The exact settings and achievable range may vary slightly depending on the power supply, temperature, SID chip, and connected CV source.

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