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.
Mutable Instruments | Streams
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