Rev. 5 – Jun 2020 Page 74 of 91
an almost-pure triangle wave (noon), and a logarithmic
waveform. (Both the sine and the log are generated by
shaping the triangle wave, hence its name.)
It is possible to control this parameter, even at audio
rate, by patching an external signal to the CV input (C.2).
An Attenuator (C.3) allows for precise scaling of the incom-
ing signal. By default, the modulation input is semi-nor-
malled to the Modulation Bus output (F.2).
The signal coming from this circuit is then routed to the
Source crossfader (E.1) and then possibly to the Wavefolder
(E.4), which can be heard through the final output (more
on this below, §3.3).
PULSE SHAPER
The other waveshaping circuit consists of two sections
strictly connected: first, a comparator generating a pulse
wave and capable of pulse-width modulation (PWM),
and then another waveshaper to which the PWM output
is routed.
3.2.1 Pulse-Width Modulation (PWM)
The PWM circuit generates a pulse-wave through a
comparator. This technique requires a waveform and a
voltage value used as a reference. The comparator, as the
name suggests, compares the waveform to the reference
voltage: every time the waveform is equal to or higher
than the reference voltage, the output voltage will be
high; conversely, every time the waveform is lower than
the reference voltage, the output will be low. The alter-
nation of high and low voltages produces a pulse wave-
form, whose width depends, on the one hand, on the ini-
tial waveform fed into the comparator, and on the other,
on the value of the reference voltage.
A classic example of pulse-width modulation is
achieved by using a fixed triangle waveform and varying
the comparator threshold: this operation changes the ref-
erence voltage and thus the point where the output volt-
ages are high and low, but without affecting the duty cy-
cle, i.e. the frequency.
It has been said that both the elements of the compara-
tor (the waveform and the reference voltage) play a fun-
damental role in a pulse-wave generation: the two main
controls of BRENSO’s PWM circuit affect exactly these
two parameters.
The two-way Source switch at the bottom (D.2) selects
the source to be fed into the comparator: when it is set on
the left, the source is yellow triangle wave, straight from
the oscillator’s core, and the result is a classic pulse wave;
when it is set on the right, the source is the wave gener-
ated by the Triangle Shaper according to its knob position
(it can be a sine, a logarithmic wave or everything in be-
tween).
The Pulse Width knob (D.1) sets the comparator thresh-
old and thus varies the pulse wave, or the ratio between
the positive and negative side of the waveform (also called
‘symmetry’). When the source is the triangle wave, and
the knob is centered, the ratio will be of ~50%, which
translates into an almost perfect square wave. Moving the
knob on the left or on the right will change this ratio, thus
generating a positive or negative asymmetry).
The CV Input at the bottom (D.3) accepts any signal to
be used to vary the wave symmetry, with a dedicated At-
tenuverter (D.4) to scale or invert it: when the attenuverter
is set at noon, no modulation is applied; rotate the knob
on the left or right to apply a negative or positive modu-
lation, respectively. Patch any LFO-like signal to the
modulation input and adjust the attenuverter to achieve
the classic PWM sound.
When the Triangle Shaper is used as a source for the com-
parator, it can produce more complex results: for in-
stance, the pulse wave symmetry can be varied by modu-
lating the wave shape, without changing the comparator
threshold. Modulate both the waveshaper and the sym-
metry control to generate articulated modulations, which
are harder to obtain with more conventional PWM cir-
cuits.
3.2.2 Waveshaper
The Square Shaper circuit is BRENSO’s second
waveshaper. It further shapes the harmonic content of
the pulse wave generated by the PWM circuit (§ 3.2.1).
When its main knob (D.5) is at the leftmost position, it
emphasizes the lower frequencies; the higher overtones
will become increasingly higher until the noon position is
reached: at this point, the waveform generated by the
PWM circuit is almost purely reproduced. When the
knob is rotated past noon, the higher frequencies will be
progressively emphasized, until roughly two o’clock (75%
of the knob stroke), where they will have the highest am-
plitude. From this point onward, the lower frequencies
will be emphasized again, but with an inverted phase, un-
til the rightmost position is reached, where the signal is
the same as at the leftmost position, but with inverted
phase.
It is possible to control this parameter, even at audio
rate, by patching an external signal to the CV input (D.6).
An Attenuator (D.7) allows for precise scaling of the incom-
ing signal. By default, the modulation input is semi-nor-
malled to the Modulation Bus output (F.2).
The output of the Pulse Shaper is routed to two different
points of the circuit. First, to the yellow square wave out-
put (A.5), where it can be selected with a dedicated switch
(A.6), instead of the regular, core-derived square wave;
then, it is sent to the final wavefolding stage.
WAVEFOLDER
So far, we have described two signal paths: the one of
the Triangle Shaper, and the one of the PWM through the
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