Notations for Performative Electronics: the case of the CMOS Varactor

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.

Keywords:MOSFET Varactor, Lunetta Synthesizer, CMOS, Circuit Bending, VCO, Notation, Circuit Schematics

Introduction

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 [1]:

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.

[5] Waisvisz, M. The Crackle Box, 2004.