Mutable Instruments Streams User Manual

Mutable Instruments | Streams

Streams provides two channels of extremely flexible signal dynamics manipulation.

When we perceive a sound as “loud”, or conversely “soft”, amplitude is not the only factor at play - the
spectral content itself should be taken into account. A “dull” timbre, as obtained with a low-pass filter
removing high harmonics, will sound softer than a “bright” timbre. This intuition inspired Don Buchla to
design the so-called “Low-pass gate” - a circuit combining the effects of an amplifier and low-pass filter ­as an electronic mean of reproducing the concept of dynamics in music.

This very same motivation explains why the core circuit in Streams is an analogue, voltage-controlled filter
and amplifier (VCFA). More accurately: two of them since Streams is a dual module! What is very specific
to Streams is the way the cutoff frequency and gain of the VCFAs are controlled: a multi-purpose control
signal input, called EXCITE, is analyzed by a microcontroller to generate and emulate various gain and
frequency control schemes:

Envelope generation: the gain and frequency are controlled by an internal envelope generator tuned to
produce the natural attack/decay curve of acoustic instruments. The envelope is triggered by a pulse on the

EXCITE input.

Vactrol emulation: the EXCITE input is fed into a digital model of a resistive opto-isolator (vactrol). This
generates a slow, moody, non-linear response.

Envelope and filter follower: the signal on the EXCITE input is analyzed, and its envelope and cutoff
frequency are extracted and applied to the signal on the audio input.

Dynamics compression: the signal on the EXCITE input is used as a side-chain to limit the gain of the input
signal, just as a sidechain compressor would do. In the absence of a signal on the EXCITE input, the module
operates like a compressor/limiter.

It is of course possible to bypass the digital control section and directly drive the amplitude through a
dedicated analog CV input - but by doing so you’ll be missing some of the fun!

Streams is designed for Eurorack synthesizer systems and occupies 12 HP of space. It requires a -12V /
+12V supply (2x5 connector), consuming 30mA from the -12V rail and 100mA from the +12V rail. The red
stripe of the ribbon cable must be oriented on the same side as the “Red stripe” marking on the printed
circuit board.

This device complies with part 15 of the FCC Rules. Operation is subject to the following
two conditions: (1) This device may not cause harmful interference, and (2) this device must
accept any interference received, including interference that may cause undesired
operation.

1 of 8

Mutable Instruments | Streams

This device meets the requirements of the following standards: EN55032, EN55103-2,
EN61000-3-2, EN61000-3-3, EN62311.

A. Channel envelope shape. This knob controls the overall attack and decay characteristics of the

selected function - for example envelope times, and vactrol or envelope followers reaction time.

B. Channel modulation amount. This knob adjusts the amount of brightness (cutoff frequency)

modulation applied to the signal, in addition to amplitude modulation.

C. Channel amplitude modulation CV. This is an attenuator for the LEVEL CV input, which is directly

routed to the amplitude control port of the VCFA.

D. Channel function selector. Press the button to display the current function of a channel, and press

again to cycle through the four functions.

E. Channel amplitude response curve. This adjusts the response of the VCA, from exponential to linear.

F. Metering button. Press the button to show the bargraphs. Press again to select which signal is to be

monitored.

1. EXCITE input. Audio or CV are accepted. This is a multi-purpose control port, its function depends on

the selected mode!

2. Signal input. Audio and CV are accepted. When no jack is inserted in this input, a +3.3V signal is

internally routed to the VCFA. This allows the module to be used for CV generation - for example to
generate an AD envelope or for envelope following.

3. Level input. This input is directly wired to the amplitude control section of the VCFA, allowing the

channel to be used as a VCA in the most straightforward way. This signal is attenuated by the LEVEL

MOD knob. When no jack is inserted in this input, a +8V signal is internally generated. The LEVEL MOD

knob will thus work like an offset control.

4. Signal output.

2 of 8

Mutable Instruments | Streams

The 2-channel LED bargraph can be used to monitor the level of the inputs and outputs.

If the display is not showing the bargraph - for example if it is showing the current function of one of the
two channels - press the METER button to bring it back.

When the bargraph is active, press the METER button to cycle between the 4 visualization modes:

Note that the arrangement of the LEDs mirrors the arrangement of the physical inputs/outputs of the
channel 1 of the module.

The module detects whether an input signal is a CV or an audio signal. When the signal is a CV, the
length of the bar is proportional to the CV level. The 4 bars are lit for a CV of 8V. The LEDs are green or
red depending on the polarity of the signal. When the signal is an audio signal the maximum level
corresponds to 16V pp, and then each LED represents a level change of 6dB, with the lower two LEDs lit
in green and the upper two lit in orange and red.

When the compressor mode is active, when monitoring the output level, the bargraph will display the
amount of gain reduction rather than the final output level.

Press channel 1 or 2’s function selection button to disable the bargraph display, and then to cycle through
the four functions.

The next section explain in details how each function operates.

In this mode, amplitude and brightness are controlled by an internal envelope generator. The envelope is
triggered by sending a trigger pulse to the EXCITE input. Properties of the trigger pulse - such as its
amplitude, duration, and slew rate are taken into account to bring variations to the envelope shape.

The SHAPE knob controls the envelope shape. Over the first half of the course of the knob, envelopes
with immediate attacks and increasingly long decay times are available. Over the second half, the attack

3 of 8