STARI: A Self Tuning Auto-monochord Robotic Instrument Shawn Trail Leonardo Jenkins Duncan MacConnell Computer Science Electrical Engineering Computer Science University of Victoria University of Victoria University of Victoria [email protected] [email protected] [email protected] George Tzanetakis Mantis Cheng Peter Driessen Computer Science Computer Science Electrical Engineering University of Victoria University of Victoria University of Victoria [email protected] [email protected] [email protected] instruments. This instrument employs electro-magnetic string actuation making the strings resonate, creating a droning sound effect. Solenoid beaters either hit the strings di- rectly or various preparations attached to the strings cre- ating percussive effects. The author has worked with Eric Singers Guitarbot[8] and Ragtime Wests pneumatic gui- tars as Robotic Technician for Pat Metheny’s Orchestrion tour1. The author also composed and performed with EMMI’s2 robotic string instrument AMI- Automatic Monochord In- strument. Figure 1: STARI The Guitarbot is manually tuned and has no self-sensing technology. It typically goes out of tune during a concert, is cumbersome to transport and difficult to maintenance. ABSTRACT Trimpins pianos were a site specific installation and not This paper outlines the motivation, design and development practical instruments, being large and immobile. EMMIs of a self-tuning, robotic monochord. This work presents AMI inspired the design goals of STARI- being a singular, a portable, autonomous musical robotic string instrument streamlined unit with a self-contained pick-up/amplification intended for creative and pedagogical use. Detailed tests system. Its actuators feature a picking mechanism similar performed to optimize technical aspects of STARI are de- to the Guitarbot, electro-magnetic actuation and a solenoid scribed to highlight usability and performance specifica- beater hitting the strings similar to Trimpins pianos, a tions for artists and educators. STARI is intended to be string dampening mechanism, and a fretting system that open-source so that the results are reproducible and ex- affords the production different frequencies. pandable using common components with minimal finan- The motivation for this work is to explore the possibilities cial constraints. Because the field of musical robotics is so of musical-proprioception in autonomous, self-actuated mu- new, standardized systems need to be designed from exist- sical instruments. It’s intended uses are in computer music ing paradigms. Such paradigms are typically singular in pedagogy: physical music computing- sensor technologies, nature, solely reflecting the idiosyncrasies of the artist and micro-controllers; actuation- motors, solenoids, electro-magnets; often difficult to reproduce. STARI is an attempt to stan- embedded DSP- creative audio filter design, MIR, and syn- dardize certain existing actuated string techniques in order thesis; frequency analysis pertaining to music theory and to establish a formal system for experimentation and peda- audio perception; rapid prototyping- 3D printing, hardware gogy. design/fabrication; and composition and performance. This paper details the instrument ability to actuate itself for Keywords analysis and self-tune. This paper describes previous work, motivations, implementation, experiments, and future work. musical robotics, embedded DSP, MIR, physical computing, 3D printing, rapid prototyping, open-source 2. DESIGN CONSIDERATIONS 1. INTRODUCTION The monochord (Figure 2) demonstrates the mathemat- ics and resultant frequencies of musical pitch. The tonics This paper draws on previous work with proprioception in robotic musical instruments[6]. The authors have worked (open string) next higher octave is half the string slength (2:1). A third of the string’s length is three times (3:1) intimately with musical robotics, most notably: Trimpin’s: (CanonX+4:33=100) robotically actuated prepared piano the string root frequency. This relationship will continue exponentially. Our design required the instrument to self-tune to a par- Permission to make digital or hard copies of all or part of this work for ticular frequency using a motor to adjust the string tension personal or classroom use is granted without fee provided that copies are up or down and one to actuate the string. STARI is meant not made or distributed for profit or commercial advantage and that copies to be portable and compact, yet robust and flexible as a bear this notice and the full citation on the first page. To copy otherwise, to platform for testing various string actuation methods and republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. 1 http://www.patmetheny.com/orchestrioninfo/ PACRIIM’13, August 27-29, 2013, University of Victoria, Victoria, BC. 2 Copyright remains with the author(s). http://www.expressivemachines.com/ ª*&&& Figure 2: Monochord interfacing techniques. Design problems that STARI ad- dresses for improved functionality and increased flexibility were as follows: AMI received control input via a PC us- ing Max/MSP. Our goals were that STARI be autonomous, Figure 3: Flowchart of STARI eschewing the PC dependancy. Max/MSP requires a PC and is relatively expensive. We chose the software platform Puredata (Pd), a similar programming environment, being free and ubiquitous within the computer music community. We designed a string tensioning system fixed to a board Pd runs on the Beaglebone3, being Linux compatible. This measuring 25” x 4” x .75”. The bridges (Figure 4) were custom designed (using the free Autodesk Inventor Fusion affords embedding the computing environment on the in- 4 strument, while taking advantage of the softwares existing program) and 3D printed and spaced 14” apart. DSP libraries for MIR analysis. Like the Guitarbot, AMI is tuned manually. Our design goals were to develop a “self- aware” instrument capable of sensing it’s own frequencies played in real-time in order to continually monitor and cor- rect itself autonomously during use, should the instrument detune. An important aspect to consider when designing the fre- quency estimation-to-string actuation process is a tuning threshold (Table 1) of acceptable estimation. A certain range of tolerance is necessary when determining if the string is“in-tune”or not. In a conventional, Western equal-tempered scale (A=440Hz), the octave is divided into 12 discrete semi- tones. Each semitone consists of 100 cents, allowing for extremely fine-tuning. Table 1 shows example for the rela- Figure 4: Bridge CAD drawing tionship between target, frequency and its high (Fhigh)and low (Flow) threshold, respectively. The string was fixed to the board through a hole- the strings stopper fixed to the body underside the same as Table 1: formulas for suitable tuning thresholds an electric guitar. The other end was wound to a standard Tuning Threshold tuning peg mounted to a custom bracket designed to with- F Hz target = 200 desired frequency stand the tension of a tuned string. Because this force was n = 30 cent threshold relatively severe, a substantial bracket was required to fully F F ∗ (n/1200) . Hz high = 1 (2 ) = 203 4 brace the tuning peg. The end of the peg meant for turning F 3 = F 1/(2(n/1200)) = 196.5Hz low was fitted with a custom bracket fixed to the stepper motor axis. The stepper motor presented substantial body vibra- 3. SYSTEM DESCRIPTION tion requiring a secure enclosure to brace it to the wood. STARI is utilizes a collection of commercially available elec- We also used large, dense rubber feet to further reduce the trical components, guitar string and pickup, a micro-controller, vibrations in efforts to reduce the resulting noise produced microcomputer, and custom built 3D printed structural com- when in use. Figure 5 shows the 3D printed mounts. ponents (see Figure 1). The instrument is implemented us- An H-bridge is mounted next in line, bolted to the wood ing a motor driven tuning mechanism. The stringss signal foundation. The servo motor required a bracket for mount- is acquired via a passive single coil electro-magnetic guitar ing. We found that it oscillated, interfering with the motor s pickup and routed to the Beaglebone for analysis and pro- performance and ability to pluck the string. We solved this cessing. The signal is output to a speaker for amplification. by bolting it to a 3D printed bracket (Figure 6). It was The string is actuated by standard guitar pick fitted to a mounted just off center from the string with just enough servo motor and controlled natively from Pd or remotely via reach for the pick to pluck the string sufficiently. A small slit external interface. The string tunes itself using the embed- was cut into the arm of the motor so a pick could be fit and ded analysis software determining the frequency. Control glued on. This was a convenient solution and prevented us signals are sent to the stepper motor to tighten/loosen the from having to alter the structure significantly. The wiring string, raising/lowering the pitch respectively, until reach- from the stepper motor was wrapped in heat shrink to keep ing the target pitch. Figure 3 shows a flowchart of STARI’s the unit tidy and secure. The battery pack was zip-tied to operation. the body of the wood through holes drilled into the body, the Beaglebone was mounted above (Figure 7). The guitar 3.1 Mechanical Design pickup is placed nearer the opposite bridge from the step- 3http://beagleboard.org/Products/BeagleBone 4http://labs.autodesk.com/technologies/fusion Figure 7: Beaglebone, battery, pickup, pick unit Figure 5: Motor enclosure, tuning mount, bridge. Figure 8: Canopy model and photo of the interface per motor to avoid magnetic noise interference and plugged into the Beaglebones“audiocape”5 via 1/8” connector. Stage and an Output Stage. The Input stage deals with the capture of the signal from the string andvsubsequent processing to determine its frequency. The Output stage deals with the control and actuation of the motor to ad- just the strings tuning.