The Lattice Harp: a New Hybrid Instrument and Controller
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THE LATTICE HARP: A NEW HYBRID INSTRUMENT AND CONTROLLER Colin Raffel, Nick Kruge, Diane Douglas, Edgar Berdahl, Wendy Ju Stanford University CCRMA, Stanford, CA, USA {craffel,nkruge,eberdahl,wendyju,ddouglas}@ccrma.stanford.edu ABSTRACT world. While many single-plane instruments exist, such as the This paper describes the Lattice Harp, a new musical instru- hammered dulcimer, harp, and zither, the concept of a two- ment which functions simultaneously as an acoustic instru- dimensional string instrument has not been fully explored. ment and a hardware controller. The Lattice Harp consists of By adding a second layer of strings and allowing them to two overlaid, perpendicular planes of strings which are elec- be tuned freely, different string crossings can represent dif- trically connected to a microcontroller. The microcontroller ferent music intervals between notes. For example, if both detects when two strings are touching and treats this data planes are tuned to the same major scale, each crossing along as a button being pressed. Furthermore, each string has an a single string corresponds to a distinct interval in the scale. individual piezoelectric pickup, which allows for the strings Furthermore, the strings can act as visual cues for distances to be separately amplified and processed while providing a along each string because of the regular placement of the method for continuous control. The pairing of normally sep- bridges. This is particularly useful when finding harmonic arate performance paradigms affords new, interesting, and node locations; for example, the midpoint of a string is de- unexpected performance techniques. lineated by the fourth and fifth strings on the opposite axis. 1. INTRODUCTION Most modern music controllers use knobs, buttons, and sim- ilar components which are mapped to sound generation pa- rameters. However, an instrumental music performer has historically been considered to be someone who plays a phys- ical instrument, not someone who manipulates control de- vices, as expressed in [9]. Most products fall either into the category of a musical instrument or a controller, and most frequently those that qualify as both owe much more to one type of device than the other. For example, augmented gui- tars have been designed which can be used for musical soft- ware control, but they maintain the form factor of a guitar, which was not intended for such use [2]. The main motivation behind the Lattice Harp is to ex- plore the possibilities of a device which is designed for both Figure 1. Prototype of the Lattice Harp instrumentation and control purposes. The end result is a useful tool for either application, but is more importantly an exchange between these two paradigms. With a typical 2. DESIGN AND CONSTRUCTION hardware controller, few unexpected “features” arise. After all, such devices are typically engineered so that they work A prototype Lattice Harp can be seen in Figure 1. The exactly as intended, with very little room for error or side ef- Lattice Harp consists of a wooden frame on which sixteen fects. However, the process of discovering unexpected uses strings are mounted, organized in two sets of eight traveling of an instrument can promote creativity and musicality. For perpendicularly. For the prototype’s frame, we chose a solid example, John Cage’s prepared piano is an entirely unin- block of bamboo. Bamboo has a high strength-to-weight tended performance method for the piano but has proven to ratio, which ensures that the frame does not buckle under be a popular modern use of the instrument [10]. The Lattice tension and makes the instrument more portable. To allow Harp brings the artistic process of discovery to the hardware the user to play the strings from above and below the frame, Figure 2. Different ways of creating button presses. a) Cross section of the strings with no pressing. b) Plucking the strings with the fingernail to create a momentary button press. c) Pressing the strings together to connect them electronically without sound. d) Pressing multiple strings at once. we hollowed out the center section of the wooden block. volves one plane of strings tuned to a major scale, with the Each side of the frame terminates eight of the strings. On other eight strings tuned to accidentals or major mode exten- two sides, the strings are wrapped around tuning pins and sions. In any tuning, the different string crossing locations travel over individual, two-layer bridges in which piezoelec- represent different intervals, allowing for a graphical way of tric transducers are sandwiched. On the other two sides, the playing music. This concept has been explored in commeri- strings share a large bridge and terminate at fixed end pins. cial hardware controllers such as the AXiS 64 and Omni- The horizontal distance between strings generously approx- chord. As an example tuning, a major scale with extensions imates a normal picking finger size. The string planes are in the key of B is shown in Figure 3. As each vertical string vertically offset by a few millimeters so that the strings only is traversed, the corresponding interval increments or decre- touch when they are meant to, as shown in Figure 2a. ments by one, making the tuning more intuitive. Each of the end pins is connected electronically to an AVR microcontroller. The microcontroller treats each string as a row or column in a typical scanning button matrix [6]. Specifically, one dimension of strings acts as a series of nor- mally high input pins, while the other dimension cyclically sets its pins low. In this way, when two strings touch, the mi- crocontroller detects a low voltage on the corresponding in- put and subsequently sends out a serial message. To ensure reliable press data, we also used a counter-based debounc- ing method [7]. With two planes of eight strings, the Lattice Harp functions as a 64-button controller, with a virtual but- ton at each string intersection. Each of the bridge-mounted piezoelectric elements is separately connected to a DB25 connector by shielded audio cable. We created a number of breakout snake cables to connect to the DB25 jack accord- ing to various Lattice Harp setups. 3. TUNING AND ACTUATION The Lattice Harp’s design allows it to be freely tuned as a sixteen string instrument. On a basic level, if each string Figure 3. A possible tuning of the Lattice Harp, with inter- plane is tuned equivalently, the Lattice Harp begins to act vals labeled at each string crossing like a mandolin, piano, or other instrument that relies on sympathetic vibrations [8]. A more interesting tuning in- The instrument’s strings can be made to vibrate through a variety of methods. The orginal intention was to have the pick closed the circuit and the microcontroller registered a strings plucked with the players fingers or a pick, so that button press. The main drawback of this method was that when two overlaying strings were plucked together, a but- a press event would occur before the string was plucked if ton press would simultaneously occur. Similarly, the player the user rested the pick on a string intersection. We also can actuate the strings by striking them with a mallet, in obtained fairly reliable control data when relying on strings the way that a hammered dulcimer is played. The strings touching during a pluck without a metal pick. This method can be set to vibrate gradually through use of electromagnet is shown in Figure 2b. When treating two vibrating strings devices, including the commercially available eBow and the as a press location, the data was not as reliable, but neverthe- Electromagnetically Prepared Piano [1]. Sympathetic vibra- less provided interesting additional control possibilities. On tions can also occur, depending on the instrument’s tuning. the Lattice Harp, it is also possible to cause a press event by physically pushing crossing strings together, much like 4. DATA MAPPING a force-feedback version of a typical button pad. Figure 2c visualizes this type of pressing. This also gave reliable The connected microcontroller sends messages over USB data, and provided a way to control software without any which loosely conform to the Monome serial protocol [3]. resulting acoustic sound. One interesting side effect of this In this way, the Lattice Harp can make use of the growing method was that as the string was pressed down further, it number of software applications written for button matrix would begin to touch additional strings, so that multiple but- devices [4]. On the connected computer, the serial messages ton presses were registered as a result of more pressure, as can also be translated to OSC (Open Sound Control) [11] or seen in Figure 2d. Finally, when simply placing or drop- MIDI and sent to compliant software. Output levels from ping metal objects onto the strings, the microcontroller reg- the piezoelectric transducers can be treated as separate con- isters press locations where the objects rest. Because of the trol data by processing the signals in software. In this way, a physicality of this method, the player can create press event button press event can occur when two perpendicular strings patterns with a clear visual basis. are vibrating, as shown in Figure 4b. The amplitude output of the transducers can also be used as analog control, so that One anticipated problem with the Lattice Harp was ghost- as the player causes a string to vibrate more, a software pa- ing. Ghosting is a well-known effect in button matrices rameter increases. One possible mapping could measure the which appears when multiple button events occur simultane- time between any two button presses to set the tempo of an ously [6]. Because of the way the matrix is scanned, if three echo effect, use the location of a button press to determine button presses occur in locations that correspond to corners what strings the effect was applied to, and use the output of a rectangle, the button at the fourth corner of the rectangle levels of the corresponding strings to set the regeneration will also be registered as down.