Rev. 5 – Jun 2020 Page 72 of 91
interval (for example one octave), according to the mod-
ulation amount.
The main difference between these two techniques is
that Linear FM generates sidebands which are equally
spaced above and below the carrier frequency, while ex-
ponential FM does not. This happens because the expo-
nential modulation is asymmetrical: if an A=440Hz
waveform is modulated exponentially, and the modula-
tion amount is +/-1 octave, the carrier frequency will os-
cillate between 220Hz and 880Hz, which is 220Hz below
and 440Hz above the original frequency. Such modula-
tion also causes a shift of the central frequency: in this
case, it will be 550Hz, which is exactly 330Hz above
220Hz and below 880Hz. This, in turn, will generate a
perceived detune of the original pitch, which will be dif-
ferent every time the carrier frequency is changed.
Sidebands are the sum and difference of the carrier and
integer multiples of the modulator. There can be some
cases, however, in which the difference between the car-
rier and the modulator would provide a negative number.
Since negative frequencies are not physically possible,
these sidebands are usually inaudible. For example, if the
carrier frequency is 150Hz and the modulator frequency
is 200Hz, the first couple of sidebands would be at 350Hz
and −50Hz. However, a conventional analog oscillator
stops oscillating whenever it reaches 0Hz, thus removing
part of the spectra.
For this purpose, an approach called Thru-Zero FM
has been implemented: with this technique, the negative
sidebands (the ones located “below zero”) are generated,
but with an inverted phase. The result is a richer and
more natural musical timbre with even less pitch drifting
than may occur with analog FM.
2.2.1 FM Routing
Both oscillators of the BRENSO can work as carrier
and modulator at the same time: this means that the
green oscillator can modulate the yellow one, which in
turn modulates back the green. This technique allows
creating extremely complex sounds with just two oscilla-
tors, whose final spectral content may even reach the
realm of noise.
To achieve this result, BRENSO is equipped with two
FM buses, one for the yellow and one for the green oscil-
lator (B.7 to B.12). The scope of each bus is to offer an
explicit modulation routing with advanced capabilities.
Each FM bus has three main controls: the big one is the
FM Deviation knob (B.6) connected to its CV Input (B.7) and
Attenuverter (B.8), while the other two smaller knobs are the
Linear TZFM Attenuator, and the Exponential FM at-
tenuator. The Deviation knob sets the overall modulation
that it is applied to the oscillator, while the two attenuators
(B.10, B.12) determine the specific amount of Linear
Through-Zero (TZ) and Exponential FM. All the knobs in-
crease the value through a clockwise rotation, starting
from the leftmost position, where no modulating signal is
routed.
By default, the linear and exponential modulation
source of each oscillator is the other one: the green oscil-
lator is thus semi-normalled to the yellow Linear TZFM
and Exponential FM inputs (B.9 and B.11), and vice versa.
It is also possible to break the normalization and use an-
other signal by patching it to the desired input. In this
case, the respective attenuator knob will attenuate the ex-
ternal signal amplitude. It is thus possible to use up to four
different modulation sources at the same time.
In order to produce audible effects, one must set both
the Deviation knob and either the Linear TZ or Exponential
attenuator to a value higher than zero (the leftmost posi-
tion).
This bus design offers two great advantages: on the one
hand, it allows combining linear and exponential FM in-
dependently per each oscillator. On the other, by having
independent CV inputs on the two buses, it is possible to
control the modulation amount over each oscillator with
different sources, thus creating more articulated timbres.
SYNC
Syncing refers to a variety of techniques originally devel-
oped to improve and stabilize the relative intonation of
two or more analog oscillators.
The common ground is that one oscillator should be
used as a reference to which other oscillators must be
compared and, if different, corrected: different correction
techniques translate into different syncing circuits.
It soon became clear, however, that in certain sync cir-
cuits, an exaggerated modulation of a “slave” oscillator
introduced pleasant overtones to the final sound, and
these techniques became widely employed in sound syn-
thesis to generate more complex timbres. This is the case
with, for example, Hard Sync, which is often imple-
mented in sawtooth-core oscillators. This circuit takes
two oscillators, called ‘master’ and ‘slave’, and forces the
slave’s waveform to reset back to 0 at each master’s duty
cycle. Modulating the slave oscillator’s frequency will re-
sult in a rich sound which changes timbre without chang-
ing pitch since the waveform still resets at the master’s
rate. The downside of this process is that every time the
slave wave resets and thus drops back to the beginning of
the duty cycle, it may produce a sort of unpleasant
“spike,” which becomes more and more noticeable as the
modulation gets deeper.
BRENSO is equipped with two different circuits, which
provide different results and are designed to accomplish
different tasks: they are called Lock and Flip Sync. They
can be selected through a three-way switch for the green
oscillator (B.16), and via a jumper on the back of the PCB
for the yellow oscillator.
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