Notations for Performative Electronics: the case of the CMOS Varactor
Mon, 03/08/2021 - 15:18 — AdrianFreed
Summary
Demonstrating an unusual application of a CMOS NOR
gate to implement a CMOS varactor controlled VCO, I explore the idea of circuit
schematics as a music notation and how scholars and practitioners might create
and analyze notations as part of the rich web of interactions that constitute
current music practice.
This demonstration is an invitation to practitioners and a provocation to scholars
to take a closer look at the notations in play with what (by analogy with live
coding [2]) is now sometimes called “live music hacking” [Georgina
Born, Personal Correspondence]. The practice of integrating the assembly and
modification of one’s instrument into a performance is certainly older than
this term would suggest, independent of a particular technology or time and
practiced in other performing arts such as puppetry.
If the
gestures assembling electronic devices that produce sounds are part of a
musical act, circuit schematics can be understood as a specialized kind of
music notation. I will point out some easy analogies between circuit schematics
and conventional music notation with an eye to revealing how contemporary
notations both enable and limit performance. I introduce an example of how new
notation can enable discovery of new musical affordances with the specific case
of MOSFET Varactors, devices that allow for voltage controlled capacitance
modulation. The new notation for MOSFET varactors shows practitioners how to
easily connect low cost CMOS digital IC’s to coax their transistors into
operating in an unusual analog capacitive modulation regime. CMOS circuits are
currently highly favored by synthesizer bricoleurs who build under the rubric
of “Lunetta synths”. I will concurrently demonstrate the notation, its
implementation in a device and how MOSFET varactors can be employed readily in musical
contexts that favor aesthetics of indeterminacy and also those that favor tight
repeatable control.
Circuit diagrams are a Music Notation
Circuit diagrams,
construction diagrams and notes constitute a music notation for circuit benders
and live hackers. These diagrams differ in important ways from traditional
circuit diagrams used as blueprints for industrial production of electronic
devices. Industrial production requires stability of notation, whereas live
music hacking requires plasticity and ways to “animate” flux in circuits as
they evolve during a performance. We don’t have to look far to find analogous
differences in conventional staff music notation and notations designed to
support improvisational games and other dynamic strategies. It is important to
acknowledge that conventional printed musical scores are annotated and “hacked”
by practitioners in their passage from bookstore to concert stage.
Analytical Models of Notational Practice
Scholars
interested in an analytic frame to study these notations might consider Max
Weber’s tripartite model of authority and particularly the concept of
rationalization. It is productive to remember that Weber didn’t claim this
process was unilinear although his studies of various institutions strongly
lean towards rationalization[4]. The many processes producing the diversity of
highly personal notations and interpretations of circuit diagrams found in the
live hacking community might be called “irrationalization” or even “perverse
engineering” [Sha Xin Wei, personal correspondence]. Such terms have the
unfortunate effect of stigmatizing vernacular engineering. The idea that
professional engineers have stabilized the “right way” to notate circuits is an
illusion maintained for rhetorical purposes such as synthesizing alterity:
professional practice in fact concurrently accommodates charismatic
contributions of individuals, corporate structures and government-sponsored
interventions. For example electronic devices and structures still carry the
names of individuals e.g. the “Norton” amplifier, aka LM3900, aka Current
Differencing Amplifier (CDA).
Bourdieu’s
field model may be more useful starting point [1]
than Weber’s for accounts of semiosis of
music notation as part of a dynamic flux of consensus and dissensus as
stakeholders enter and leave various scenes (fields), posture, perform, share
and obfuscate.
Ontologies for Notational Diversity
The following
categories may be helpful in making sense of the tremendous diversity of music
circuit notations to found in the online diy synth community, e.g.,
electro-music.com/forum/forum-160.html:
Prescription/Description
Level of Detail
Literal/Figurative
Speculative/Tested
Partiality/Completeness
Here are two contrasting
examples of notations that can readily be analyzed with the frame I propose:
Iconographic Riffs
The triangle
shape is the basis of the standard iconography for digital gates and analog
op-amps (operational amplifiers). The triangle is a metonymic borrowing of the
head of an arrow. It is used to visually reinforce a signal direction with its
concomitant design pattern: an output is connected to inputs but outputs are
not connected together. Transistors that implement gates and op-amps are there
to provide amplification. This amplification serves to isolate the influence of
outputs on inputs (impedance conversion) and inputs on adjacent inputs
(“crosstalk”, common mode rejection). This is a convenient fiction as it is
allows composable designs and “blackboxing”. However, as practitioners attest,
it can be arduous maintaining this fiction and so various additional
connections can be seen flowing from the top and bottom of these triangles to
represent connections to compensation networks, decoupling capacitors and
related mechanisms.
These
normative mechanisms are the ones famously transgressed in the “Crackle Box” [5]
which invites the use of the human body to connect
an op-amp IC’s pins thereby deliberately inverting the aesthetic ascribed to engineers
eliminating “undesired” noise and oscillations into “desired” sounds of the music
performer. Such transgressions sit uncomfortably in the well-worn trope
“conventional trained engineer vs untrained hacker/artist maverick.”
Transgression and resistance is also part of the history of professionally
engineered components. The DTL logic series of the early 1970’s has passive
resistive pull-up resistors on gate outputs. This breaks the convention of
one-output-to-multiple-inputs by allowing multiple outputs to be connected
together in what is known as the “wire-or” design pattern. TTL and CMOS
circuits developed after DTL used a different mechanism (tri-state) to allow
outputs to be connected together in buses.
Blackboxing
(triangling, sic.) the transistors inside integrated circuits fixes functionality
thereby moving the challenge of designing system behaviors to the topology of
connections around those fixed functions. This separation of concerns for
device manufacturers and system integrators allows each to optimize designs in
their own design spaces. However there are many circuit configurations where
blackboxed devices don’t behave according to their idealized description
(sometimes referred to as the “recommended operating conditions”). In these
situations it is convenient to introduce notations that reflect salient devices
properties, a partial rupture of the opacity the term black box implies.
MOSFET Varactor
I propose the
following modification of the NOR gate symbol to represent MOSFET varactors [3]:
The term
“varactor” refers to devices that allow for voltage modulation of capacitances,
the storage elements of electrical charge. Implemented with specially
constructed diodes, varactors became the preferred solution to the FM-radio tuning
problem affording electronic presets for favorite stations. One of the common
aesthetics privileged in the CMOS (“Lunetta”) DIY synth. scene is the use of
cheap CMOS circuits for all active devices at the exclusion of op-amps and
exotic or expensive components such as diode varactors.
The
varactor depicted belowuses a voltage applied to one input of a
standard CMOS 4001 2-input NOR gate logic to modulate the capacitance (w.r.t.
ground potential ) of the other input . It is trangressive both functionally in
that it is an analog use of a “digital” circuit and structurally in that it challenges
the fiction that gate inputs have the symmetry their black-boxed Boolean logic truth
table implies. Notice that the addition of pin numbers to the MOSFET varactor
in application circuits such as the VCO (voltage controlled oscillator) in Figure
3. This is analogous to fingerings being added to conventional music notation to
manage, for example, when the timbre is different for notes sounding at the
same pitch on different strings of a chordophone.
Pin numbers
identify the inputs with the requisite properties to manifest the capacitance
change. This is a third sort of transgression because it breaks the desirable separation
(that we also see in conventional musical notations) between description (staff
notation) and prescription (tablature).
MOSFET Varactor VCO and Waveshaper
The asymmetric placement of the input lines of the
icon emphasizes the difference between the roles of the input nodes. Most logic
diagrams have the signal data flowing from left to right so the vertical
reorientation of the NOR gate above signals that something special is going
on–further reinforced by the unconnected output of the NOR gate.
The output of the NOR gate
is an interesting signal with musical uses. In a certain range of voltage
inputs (those approximately above Vcc/2) the gate no longer changes capacitance
and the output provides a periodic waveform that is waveshaped by the voltage.
This interesting and useful behavior is not advertised in the Varactor
iconography. Such plurifunctionality presents a future notational challenge.
The example of the MOSFET Varactor, shows that
notation, artifact and sound exploration are entrained in an ontogenetic
dance–a dance performed in wood sheds, bedrooms, on internet bulletin boards, and
in academic papers.
References
[1] Born, G. The social and the aesthetic: for a
post-Bourdieuian theory of cultural production. Cultural Sociology, 4
(2). 171-208, 2010.
[2] Collins,
N. Live Coding of Consequence. Leonardo, 44 (3). 207-211, 2011.
[3] Pao-Lung,
C., Ching-Che, C. and Chen-Yi, L. A portable digitally controlled oscillator
using novel varactors. Circuits and Systems II: Express Briefs, IEEE
Transactions on, 52 (5). 233-237, 2005.
[4] Sterling,
J. and Moore, W. Weber's analysis of legal rationalization: A critique and
constructive modification. Sociological Forum, 2 (1). 67-89,
1987.
Comments
Control Voltage