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Mutable Instruments Shruthi

Hybrid digital -analog monosynth (2010)

mutable instruments

"The Shruthi-1 is a hybrid digital/analog monosynth in DIY "


KEYBOARD DIY box hardware design with few knobs (bigger version is SHRUTHI XT)  and a backlit 2x16 LCD display backlit, described originally as  "sometimes grungily digital like a PPG-Wave, fat and funky like a SH-101, videogame-y like a Commodore 64, weird and warm like an ESQ-1 ; but more often than not, truly original".   Polychaining feature, or get the 6-voices Shruthi , the AMBIKA


Shruthi rear


SOUND An 8-bit microcontroller plays the role of the two main oscillators + sub-oscillator. This allows the generation of (aliasing-free) classic analog waveforms, but also wavetable/transwave synthesis and a trove of weird digital algorithms (sinewave bitcrushing, formant synthesis, phase distortion, bit toggling, XOR modulation).  

MIDI complete MIDI implementation (large CC coverage, NRPNs, SysEx data dump including wavetable transfer)

MODULATION: a 12-slots modulation matrix

SEQUENCER: a 16-steps notes/CC sequencer and advanced arpeggiator with some unusual time warping modes, and even a

FILTER Most of the Shruthi-1s are built with the SMR-4 filter, a warm 4-poles “modern analogue” low-pass (no vintage IC).
But the filter section of the Shruthi is on its own board and can be  replaced with alternative filters boards such as dual multimode filter with serial/parallel routing or even a clone of the aggressive Polivoks filter.
Mutable instruments shruthi

Shruthi-1 specifications

Digital/analog hybrid monosynth.

2 digital oscillators

  • band-limited sawtooth with adjustable phase
  • band-limited square wave with adjustable pulse-width
  • band-limited triangle wave with adjustable waveshapping
  • phase-distortion emulation of a low-pass filtered sawtooth (casio CZ-101)
  • phase-distortion emulation of a low-pass filtered sawtooth with high resonance (casio CZ-101)
  • phase-distortion emulation of a low-pass filtered sawtooth
  • mysterious phase-distortion thing (could have been in the CZ-101 if the engineers at casio dared…)
  • phase-distortion emulation of a Jew’s harp
  • stack of 4 detuned sawtooth waves (with aliasing)
  • simplistic FM (1>2 configuration), with adjustable modulation index and frequency ratio
  • wavetables, wavetables, wavetables!
    • sweepable table of basic waveforms
    • tampura transwave
    • PPG harsh creature
    • single cycle waveforms taken from D50 patches
    • cello transwave
    • slap bass transwave
    • organ/drawbars single cycle waveforms
    • male voice
    • user wavetable, can be edited by SysEx messages
    • 9 wavetables inspired by the original PPG wave
  • harsh, aliasing-full waveforms obtained by bit-toggling
  • sine wave with variable amount of bitcrushing
  • poor man’s square with pulse-width modulation (plenty of aliasing but fat for basses)
  • colored noise (sweep from low-pass filtered white noise -> white noise -> high-pass filtered white noise)
  • lo-fi vowel synthesis (think “speak and spell”) with vowel morphing


  • adjustable oscillator balance
  • mixing algorithms: simple sum, simple sum with synchronization, ring modulator, xor modulator, sum with post-mixing tanh fuzz, mixing with sample rate reduction, bit-depth reduction, fold distortion
  • extra sources: noise, sub-oscillator (triangle, square, pulse, -1 or -2 octaves) or percussive attack generator (click, glitch, blow, metallic, pop)
  • rhythmic control of the mixer, allowing the oscillators / noise source / sub-oscillator to play in turn at each new note, or according to drum-like patterns.
  • duophonic 1-oscillator mode (when two keys are held simultaneously, oscillator 1 plays the first note ; oscillator 2 the second)

Audio quality/resolution

  • Internal digital audio signal processing chain: 39216 Hz, 8 bits.
  • Internal digital control signals processing chain: 980 Hz, 8 bits.
  • Digital outputs: 10 MHz, 1 bit.


  • Exchangeable analog signal processing boards:
    • SMR4mkII: 4-pole LPF and linear VCA built with the widely available LM13700 transconductance amplifier and TL074 op-amp
    • Dual SVF: Two 2-pole multimode filters with serial/parallel routing
    • Digital filter: Digital emulation of a 3-pole low-pass / hi-pass + a bunch of lo-fi digital effects (distortion, bit-crushing, delay, comb filter)
    • Polivoks: A clone of the filter of the dark and gnarly Polivoks ; with LP/BP modes ; a switchable distortion, and brutal filter FM feedback
    • 4-Pole Mission: A clean 4-pole based on the high quality SSM2164, from which are derived 15 different filter responses (LP, HP, notch, allpass…) according to a technique popularized on the Oberheim Xpander. Additional circuitry provides 4 different “flavours” of the resonance self-oscillation, from pure and liquid to chaotic and distorted


  • LFOs: 2, with square, sawtooth, triangle, sample & hold, step sequence, or wavetables shapes. Syncable to tempo. Adjustable “raise/attack” time. LFOs can be free-running, sync’ed to envelope retriggers, or retrigger envelopes.
  • envelopes 2 ADSR
  • modulation matrix: up to 12 source -> destination connections. Adjustable modulation amount in the -100% — 100% range
  • modulation sources:
    • LFO 1/2
    • step sequencer
    • first / second half of the step sequence
    • arpeggiator gate
    • mod. wheel
    • aftertouch
    • pitch bender
    • DC offset
    • CV inputs 1/2/3/4
    • MIDI CC 16 to 19
    • noise generator
    • env 1/2
    • random value resampled at each note on
    • note velocity
    • note number (eg for filter tracking)
    • keyboard gate
    • audio signal from the mixer output (decimated at 1kHz)
  • modulation destinations:
    • filter cutoff
    • VCA gain
    • pulse width/Parameter osc. 1/2
    • wide pitch osc. 1/2
    • pitch, applied to osc 1 & 2 simultaneously
    • very fine pitch, applied to osc 1 & 2 simultaneously
    • oscillator balance
    • noise level
    • subosc level
    • resonance
    • PWM CV output 1
    • PWM CV output 2
    • Envelopes attack speed
    • LFO1 FM
    • LFO2 FM
  • New modulation sources can be created by combining 2 existing modulation sources with mathematical operations


Clock/tempo generator

  • internal clock: 40-240 BPM, with some additional extra fast tempi for effects (270, 300, 360, 480, 600, 720, 960)
  • external MIDI clock with divider
  • 6 groove templates (swing, shuffle, push, lag, human, monkey) with adjustable amount


  • 1-4 octave range
  • up, down, up&down;, random
  • 15 preset rhythmic patterns + 1 programmable rhythmic pattern
  • warp: creates complex rhythms by cyclical repetitions of steps (eg 1212343456567878 instead of 12345678)
  • latch/hold


  • 16 steps, with legato/slide, velocity, and modulation
  • 4 modes corresponding to different interactions between the sequence and keyboard play: latch / transpose / loop recorder / override


  • custom page in which 4 user-selectable parameters can be assigned to the 4 editing pots
  • microtunings/key maps for arabic scales (1/4 tones) and a selection of hindustani ragas
  • portamento, legato on/off
  • master tune

System and memory

  • complete MIDI implementation, including patch editing by CC or NRPNs, patch and sequencer data transfer, full memory backup, wavetable transfer.
  • MIDI out modes: off, thru, thru+knob movements sent as CC, sequencer/arpeggiator generated events only, polychaining, split (plays notes below split point, forwards the other notes)
  • patch memory: 144 patches and 144 sequences stored in EEPROM (no backup battery required). Can be upgraded to 464 / 464.
  • firmware update by SysEx

Hardware and connectors

  • power: 2.1mm connector, requires 7.5V-9V 300mA supply (can be unregulated). Center pin positive (+), outer collar negative (-)
  • audio input/output: 6.35 Jack (Mono audio out), 6.35 Jack (External signal in)
  • cv inputs: up to 4 CV inputs for easy experimentation with controllers. Warning: only use a signal in the 0-5V range. Deeper or higher peaks in the signals might damage the Shruthi-1’s ADC.
  • cv outputs: up to 2 PWM CV outputs.
  • MIDI: in, out
  • serial: extra UART TX pin available on control panel board (might be used in future firmware updates to control extension boards)
  • ISP connector for firmware development
  • controllers:
    • 8 facade LEDs (current page / modulation matrix visualization)
    • 6 navigation switches
    • 4 pots for patch editing
    • clickable rotary encoder for fine parameter adjustment or navigation
    • backlit 2x16 LCD display

video demo

Do you use this machine ? do you like it?  Let us know in the comment field just below 


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Mutable Instruments SHRUTHI Mutable Instruments SHRUTHI
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