Rev. 5 – Jun 2020 Page 15 of 91
· D.1 Blue Noise (+3dB/oct spectrum);
· D.2 White Noise (0dB/oct spectrum);
· D.3 Pink Noise (-3dB/oct spectrum);
· D.4 Red Noise, also known as brown or Brownian (-
6dB/oct spectrum).
Each noise “color” has its own distinct tone, which can
be used for sound-designing purposes.
Theoretically, White Noise has an equal distribution of
intensity across all the frequencies per bandwidth. Prac-
tically, in the analog domain, White Noise is a sound
which has a flat spectrum in the audible range, or, in sim-
pler terms, which can “hit” all the frequencies with equal
amplitude at the same time. It is perceived as a highly
inharmonic sound but, to the peculiar nature of the hu-
man ear, it appears slightly unbalanced towards higher
frequencies.
For this purpose, SAPÈL features also a Pink Noise out-
put. Pink Noise is basically a White Noise filtered through
a −3dB/Oct filter, which “smoothens” its higher fre-
quencies in order to deliver a more “balanced” sound for
the human ear. Pink Noise features an equal distribution
of intensity per octave, instead of bandwidth.
Red Noise is similarly generated, but the filter has a
slope of −6dB per octave. The result is a low, “rumbling”
tone.
Finally, Blue Noise is a kind of inharmonic sound whose
intensity increases proportionally to the frequencies. In
other words, the higher part of the spectrum will appear
to be louder, and the overall result is a high-pitched hiss-
ing sound which lacks bass frequencies.
3 VOLTAGE SAMPLING
The yellow and green generators, as said above, work
independently the one from the other. Each of them sam-
ples three random values at the same time: one which is
unquantized (i.e. with no fixed “pitch”), one quantized in
semitones and one quantized in octaves.
The sampling process happens when a trig or gate acti-
vates the S&H circuit, which “picks” the value played by
the noise source at a given time, and “holds” it until an-
other trig or gate is generated. There are four ways to
activate the S&H circuit for each of the two generators:
internal clock, external clock, manual S&H button, exter-
nal S&H gate, plus an “extra” mode which combines the
clocks (let it be internal or external) of both the green and
yellow generators.
By default, each generator is driven by an internal
clock. The clock section generates a regular clock signal
(called Main Clock) and two Random Clocks (on which see
below). Both the yellow and the green random sources
have their own independent clock. The regular train of
impulses provided by the Main Clock is used to sample the
random values and is also routed to the Main Clock output
(A.4). The Random Clock does not affect the S&H circuit,
but it is available for advanced modulation purposes
through the Random Clock output (A.5).
INTERNAL CLOCK AND CLOCK MODULATION
The built-in clock frequency is managed by the Clock
Rate knob in the center of the module (A.1). Rotate the
knob counterclockwise to decrease the sampling rate and
clockwise to increase it.
It is possible to modulate the frequency of the clock via
the Gate/CV Modulation Input (A.7).
This input can have two separate functions, selectable
via the dedicated switch (A.8): it can route the incoming
CV to modulate the clock frequency, or it can use any
voltage higher than 3V to trig the S&H cluster (see below,
§3.5).
When such a switch is set to the right, the incoming CV
will modulate the clock frequency: a positive CV will in-
crease the clock speed, and a negative one will decrease
it.
EXTERNAL CLOCK
It is possible to use any external trig to activate the S&H
by patching it to the External Clock Input (A.1).
Whenever a cable is patched to this input, the internal
clock is bypassed (i.e. it will no longer trig the S&H cir-
cuit). This input welcomes trigs and gates only: this means
that in order to sample a value, it needs an incoming sig-
nal with a really steep rising edge, such as square waves,
pulse waves or sawtooth waves with negative ramp (be-
side of course trig and gate impulses). Other kind of im-
pulses such as sine or triangle waves will be ignored.
CLOCK MIX
It has been said that the green and yellow sections of
SAPÈL have their own independent clock generation. It
is possible, however, to blend them in a more creative
way through the Single/Both switch (A.3), which feeds the
other generator’s clock into the one currently in use. To
activate the clock mix, set the switch to the position
marked by a square of the other generator’s color.
This feature works with both internal or external clocks,
and it affects the sampling section only: all the clock out-
puts will maintain their regular behavior.
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