Mutable Instruments Shruthi User Manual
Mutable Instruments | Shruthi - User Manual
This manual documents the version 1.00 of the firmware - which has been introduced with the Shruthi XT
but can also be installed on the classic version of the Shruthi.
The Shruthi comes either in the classic version with only 4 knobs below the screen; or the XT version
which provides hands-on controls on all synthesis parameters. The panel of the XT version is divided into
3 sections: oscillators/mixer/filter ; main control ; and modulations (LFOs/envelopes). The main control
section is the same on the XT and on the classic versions.
The main control section consist of the following elements:
The LCD or OLED display shows, most of the time, the name and value of the 4 parameters that can be
directly edited with the potentiometers under the screen. When a parameter is modified, the display
temporarily shows the full name of the parameter and its value. In some other circumstances, the display
might show a confirmation messages, the content of a sequence, etc.
The clickable encoder can be used to navigate between parameters/pages, or to step through the values of a
parameter.
The four potentiometers below the display modify the synthesis parameters currently displayed on the active
page.
The 8 LEDs provide complementary information about the active page.
The 6 switches facilitate navigation between pages. The switches S1 to S4 are used to cycle between groups
of related pages ; S5 is used to switch between the synthesis pages and the sequencer/system pages ; and
S6 is used to bring the load/save page to load and recall presets.
The following connectors are available on the rear panel:
1: 2.1mm DC power jack. The Shruthi can be powered with a 7.5V-9V unregulated supply, or with a 9V
battery adapter. Make sure that the connector has a center pin (or tip) positive / outer collar (or sleeve)
negative pin polarity. Most universal power supplies allows the polarity to be selected – the polarity being
often represented by a diagram like this: - (o +, or by the indication tip: +. The power supply has a reverse
polarity protection.
2: MIDI in. This input should be connected to the MIDI out port of a sound card, master keyboard,
sequencer…
3: MIDI out. This output can be used as a MIDI thru, but also to transmit the notes generated by the Shruthi’s
internal sequencer, backup data, or for polychaining units.
4: Line-level, mono audio output.
5: Line-level, mono audio input. Note that the external audio signal flows through the VCF and the VCA - you
won’t hear anything until a note or sequence is played by the Shruthi to “open” the VCA.
In addition to the extra knobs for editing synthesis parameters, the XT additionally features:
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An on/off switch on the back panel.
A global volume knob.
A Jam button which holds a note and starts the sequencer/arpeggiator.
The Shruthi parameters are edited by blocks of four (one per potentiometer). A set of four related
parameters are grouped together on a page. For example, the Filter page will assign the four following
functions to the four potentiometers: filter cutoff, filter resonance, envelope to filter modulation, LFO to
filter modulation. The LEDs L1 to L6 display the currently active page. Moreover, the display is constantly
showing which knob does what in the current page. For example, if the filter page is active, the display will
show:
Pages are bundled together in groups, and repeatedly pressing one of the group switches cycles between
the different pages in this group. For example, pressing S1 cycles between the oscillator 1 page, the
oscillator 2 page, and the mixer page.
Pages are also grouped into two large categories: one category contains all the pages related to sound
synthesis – the ones you’ll navigate in when creating a sound – and another category contains all the
pages related to sequencing and to system settings. The switch S5 is used to toggle between the two
categories. When the synthesis category is active, the LED on top of S5 is lit. When the sequencer
category is active, this LED is not lit. The switches S1, S2, S3 and S4 cycle through different groups of
pages as shown on this diagram:
The upper list shows the pages in the synthesis category accessible from each switch, the lower list
shows the pages in the sequencer category. L7 indicates which page category is active, and the leds L1-
L6 show which page is active.
Here’s an example. The Shruthi displays:
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L7 is lit (synthesis category) and L3 is lit: the filter page is active.
Press S1 to jump into the oscillators group. L1 is lit, and the screen is now displaying:
Press S1 again to move to the next page in the oscillators group. L2 is now lit, the screen is displaying:
Now press S5 to switch to the sequencer category. L7 is now off, L1 is lit. The active page is the clock
page:
Press S5 again to get back to the synthesis category. L7 is on, and you are back to the oscillator 2
page.
When a page is active, a summary of the four parameters modified by each potentiometer is displayed on
screen. While tweaking a potentiometer, the full parameter name, its value, and the page name, are
temporarily displayed on the screen:
After a short delay, the four parameters page summary is shown again.
On the Shruthi XT, modifying any parameter through the switches or buttons on each side of the central
section also brings this temporary display.
When the Shruthi is displaying a page summary, the rotary encoder can be used to scroll through the
parameters. The name of the active parameter is capitalized. For example, resonance is here the active
parameter:
Rotate the encoder clockwise to make env the active parameter, rotate the encoder counter-clockwise to
make cut the active parameter. If you continue rotating the encoder clockwise for several steps, the next
page, env 1, will be shown, and its first parameter, atk, will become selected. Once a parameter is
selected, click the encoder to edit it. The encoder is now used to increment/decrement the parameter
value. Once the value has been set, click on the encoder again to move back to the 4 parameter
summary.
Note that potentiometer and encoder editing can be combined. For example, use a potentiometer to
quickly adjust a parameter, and then, while the parameter name is still displayed on the screen, use the
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encoder to fine-tune it.
A last trick: if you hold S6 while rotating the encoder, it will increment/decrement by 10 instead of 1. This is
particularly useful for browsing long lists of patches…
That’s all you need to know about the pages/navigation system!
The Shruthi sometimes flashes a symbol on the first line of the display to signal incoming MIDI messages
or the completion of specific commands.
Symbol Status
(note icon) A Note On MIDI message has been received
(pitch bend lever
icon)
~ A control change message has been received
! The Shruthi is busy accessing memory - this can interrupt sound generation
x The patch has been reset to initial settings
? The patch has been randomized
> Patch or sequence SysEx data is being sent
+ Patch or sequence SysEx data has been successfully received
#
A pitch bend message has been received
The Shruthi has received invalid SysEx data (for example a SysEx message intended for
another device)
The section describes one by one all the parameters accessible on the synthesis and sequencer pages.
Before digging into the pages details, here is a diagram of the Shruthi signal flow.
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The audio signal flow is represented in red (digital path) and then in orange (analog path):
The oscillators 1 and 2 generate digital waveforms, which are combined together by the modulator. The
default modulator is a balance control over the level of the two oscillators, but more esoteric modulation
methods are available.
The sub-oscillator (which is synchronized in pitch with the oscillator 1, but 1 octave lower) also generates a
basic waveform.
The click generator generates a short transient/click at the beginning of the note. Note that the sub-oscillator
and the click generator cannot be used at the same time. It’s bassy-beefy or clicky, not both.
The output of the modulator, the sub-oscillator/click-generator and some additional noise are mixed together.
You can adjust the balance of each ingredient.
The result is converted into a pseudo analog (10 MHz / 1bit) audio signal, and mixed in the analog domain
with an external signal coming from the Audio in jack.
The result is then sent to an analog VCF and VCA, to produce the final audio signal. Some filter boards might
offer other signal transformations such as additional filters, effects, distortions…
Each of these modules have parameters (represented by the blue arrows) which can be controlled by any
of the modulation sources listed below. However, some connections are already “hardwired” (or rather
“softwired” in the firmware):
The oscillators’ pitch always track the note played on the keyboard.
The filter cutoff frequency always tracks the note. Again, this can be disabled or attenuated by applying a
negative modulation from note pitch to cutoff frequency. The rationale behind this choice is that most of the
time, you want 1:1 tracking, so this frees up a slot in the modulation matrix for something more interesting!
Lfo 2 and Envelope 1 are always connected to the filter, their modulation amount being controlled by
dedicated parameters on the filter page.
The following routing programmed in the patch:
Source Destination Amount
Lfo 1 Oscillator 1 coarse pitch 0
Lfo 1 Oscillator 2 coarse pitch 0
Lfo 2 Oscillator 1 PWM 0
Lfo 2 Oscillator 2 PWM 0
Lfo 2 Oscillators balance 0
Step sequencer Oscillators balance 0
Velocity Oscillator 1 PWM 0
Velocity Oscillator 2 PWM 0
Envelope 2 VCA gain 100%
Velocity VCA gain 25%
Bender Oscillator 1+2 fine pitch 2 semitones
LFO Oscillator 1+2 fine pitch (vibrato) 2 semitones
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sha (shape): Oscillator 1 waveform shape.
prm (parameter): Oscillator 1 parameter. This changes the timbre of the generated waveform, in a waveform-
specific way (see below).
rng (range): Oscillator 1 pitch, from -24 semitones to 24 semitones (relative the pitch of the MIDI note).
sub (subosc): Sub-oscillator shape (see below).
The following is a list of all the available waveforms, with some applications and a description of what
adjusting the parameter setting actually does.
This simply switches off the oscillator. This might be useful for filtering/gating external signals, or to
temporarily mute an oscillator when editing a patch.
This waveform is perfect for basses and brass sounds. The parameter controls the waveshapping - when
its value is increased, an increasingly large section of the waveform is shifted up. Note that this is not
intended to be a perfect, drawn with a ruler, sawtooth. It contains a bit of high-pass filtering to make it
sound more Juno-y. This waveform is band-limited. Thus, only a limited amount of aliasing artifacts will be
heard when playing high-pitched notes.
The parameter controls the pulse-width. This waveform is perfect for simulating a clarinet, for basses,
“hollow” sounds or Depeche Mode-like leads. Note that this is not intended to be a perfect, drawn with a
ruler, square wave. It contains a bit of high-pass filtering to make it sound more Juno-y. This waveform is
band-limited and only a limited amount aliasing will be heard when playing high-pitched notes.
Note that there’s a slight difference in sound when moving the parameter from 0 to 1. To offer the best
sound quality, the pulse width = 50% flavor is read straight from a wavetable at full sample rate, while the
pulse width > 50% flavor is obtained from two dephased sawtooth waves, evaluated at half the sample
rate. For bass sounds, for which aliasing is not going to be a problem, it is recommended to use pwm
instead of square to get a beefier sound.
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A pure waveform, which serves as a good basis for flute or soundchip-like leads. The parameter controls
some kind of waveshapping, clipping the bottom of the waveform. This waveform is band-limited and will
still sound fine above C5.
This waveform uses phase distortion to recreate a low-pass filtered sawtooth by progressively “pinching”
the phase of a sine wave. The parameter controls the brightness of the sound: from a sine wave to a
sawtooth, then from a sawtooth to a sawtooth gone through an ugly transistor amp. Good for dirty bass
guitar sounds or clavinets.
This waveform uses phase distortion to recreate a sawtooth sent through a low-pass filter with high
resonance. The parameter controls the resonance frequency. Useful for doubly-filtered sounds, formants,
or anything where its synthetic, cheesy, feel will shine.
This thing is hard to describe and must be experienced. It sounds like a resonant filter sweep, but has a
very hollow, synthetic quality. It may or may not have been in the Casio CZ-101. This waveform is likely to
result in whacky sounds, vaguely evoking hardsync.
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Yet another waveform that makes little sense. When the parameter is set to 0, it sounds like a fat
superposition of a square wave and a sawtooth. Sweeping the parameter value progressively increases
what sounds like the resonance of an odd, goofily saturated analogue filter.
You know the trick: one oscillator playing a low-frequency square wave and another oscillator a higherfrequency sine wave, the first one resetting the phase of the second… and the two of them sent into a
ring-modulator! Now roll this into one single sound generator, the parameter of which controls the
frequency ratio of the two oscillators: we call it zsync… The resulting sound is close to a Jew’s harp or
Morsing - band-passey and formantey.
As the name implies, this waveform made of four stacked sawtooth waves is useful for pads (when a
copious amount of filtering is applied) or for harsh trance leads. The parameter controls the amount of
detuning between the four waves. Note that no bandlimiting is happening here, so this thing doesn’t sound
quite good above C5… but it’s doing a perfect job in the bass range!
The parameter controls the modulation strength. This oscillator provides the base material for metallic
sounds, bells, metallophones, or the next 386 DX hit.
When the fm oscillator is selected, the range parameter plays a slightly different role than usual: instead
of controlling the main pitch of the note, it controls the modulator frequency, and has a drastic impact on
the timbre.
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All these waveforms are wavetables, comprising 16 single-cycle waveforms. The parameter scans the
wavetable, smoothly interpolating between each waveform. Some of these tables are “transwaves”: the
single cycle waveforms were extracted from different “stages” of a sample, and you can somehow
recreate the original sample by using an envelope that will sweep the parameter from 0 to 127.
waves are 16 basic waveforms, or rather two series of 8 basic waveforms: waveforms 9-16 are one
octave higher.
tampur is a transwave extracted from a looped Tampura note.
digitl is a PPG-wave classic.
metall is made from single cycle waveforms extracted from classic D50 patches.
bowed is a transwave extracted from cello sounds.
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slap is a transwave extracted from an SQ-80 slap bass patch.
organ contains 16 different mixtures of pure tones, ready for some “Light my fire” action.
male is another PPG-wave classic, it sounds vaguely like a human male voice.
This wavetable has two peculiarities: it is smaller than the other ones (8 single cycle waveforms, each of
them being 129 samples long), and it resides in RAM instead of residing in flash ROM. By default, it is
loaded with the same content as digitl, but its content can be altered by SysEx messages. You can find
here some examples of MIDI files containing user wavetables.
A palette of 8-bits sounding waveforms obtained by applying bitwise operations to a basic sawtooth wave
(something now known as “biscuiting”).
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