Electroacoustic feedback instrument from Portuguese musician Henrik Ferrara

In this post we present Crescente by Henrik Ferrara, a performance instrument which integrates FM synthesis, delay lines, and acoustic feedback loops. Henrik has used a Bela starter kit alongside Trill Bar and Square to create a delicate and intuitive electroacoustic instrument.

Silver Apples of the Moon

Crescente sits somewhere between a digital and physical musical instrument. It presents itself as a DMI (Digital Music Instrument) with qualities taken from the electroacoustic universe. The character of its sound comes from a polyphonic FM synthesiser processed with a delay effect and controlled via the instrument’s sensors.

Cresente electroacoustic instrument.

In parallel, Crescente offers the possibility of creating physical feedback loops which resonate the material and structure of the sound box. These feedback loops are created between a piezo vibration sensor built into the front faceplate and a contact transducer which produces the sound of the instrument. The feedback signal is first processed before being sent to the transducer making the feedback a controllable element in the instrument’s design.

The insides of the instrument and the embedded piezo vibration sensor.

The instrument is powered by a common 5V powerbank and can be reprogrammed at any time using an USB connection cable to the Bela. The electrical circuit and microcomputer are easily accessible by removing the instrument’s rear covers. For this reason, in addition to being reprogrammable in the digital domain, it is also modifiable in its electronic components.

Sensors and Mappings

Touching the black square of Crescente (Trill Square) with your fingers works, at first, as a way to initiate notes from the FM synthesiser. Depending on the force applied to these touches, the notes will have a greater or lesser amplitude and greater or lesser duration, where more strength = greater amplitude and longer duration. Also, the position of the touches in the square controls the delay parameters: the position of the finger along the x-axis (horizontal), sets the feedback value, and along the y (vertical) axis sets the time value. Thus, placing a finger in the different x and y coordinates, in the square, triggers different delay effect responses.

The Trill Square sensor for controlling the FM Synthesis.

Three pressure sensors (FSRs) are placed on the side of the instrument. Counting from the FSR closest to the Crescente base, the mappings are as follows:

  • The first FSR controls the dampening factor from a Karplus Strong synthesis block, applied to the feedback loops. This equates to how much of the piezo signal is fed back into the system. The greater the applied force and, consequently, the greater the value of dampening, the greater the possible intensity of the feedback loop.
  • The second FSR signal increases the amplitude value of an oscillator which can be used to interfere with the timbral characteristics of the feedback loops or as a drone. This oscillator has a sinusoidal waveform and the frequencies it can emit correspond to the tonic notes of each of the scales implemented in the FM synthesiser. To select one of these tonics, the Trill Bar sensor must be pressed with 1 finger (first scale), 2 fingers (second scale) or 3 fingers (third scale). By default, this oscillator takes the first position (1 finger press).
  • The third FSR signal controls the value corresponding to the “length of the string” in the Karplus Strong synthesis block, making the pitch change.

Pressure sensors (FSRs) placed on the side of the instrument.

As previously mentioned, next to the Trill Square a piezo disc is embedded in the faceplate of Crescente. This contact microphone can be struck with hands, or other objects, serving as an exciter for the Karplus Strong synthesis model. The sound production resembles that of a stringed instrument when played in this way. The piezo’s main function however is to allow feedback loops to grow by capturing the vibrations of the instrument when the instrument is producing sound. When struck in conjunction with pressing the first FSR, the feedback becomes audible and can be controlled via the amount of pressure on the FSR. One of the ways to increase the intensity of the feedback loop is by pressing a large area of the palm against the piezo or the Trill Square sensor, increasing body contact with Crescente’s front face.

Increasing body contact with Crescente’s front face in order to increase the intensity of the feedback loop.

Next to the Trill Bar sensor is a fourth FSR. The mapping of this FSR is similar to the second FSR and also corresponds to the amplitude of an oscillator. This oscillator produces lower frequencies (in a logic similar to the tonics of scales, selected by the number of fingers on the Trill Bar sensor). These notes, which can also be used as a drone, are very efficient in creating beatings effects when used in conjunction with the feedback loops.

Playing drone notes from the second oscillator in order to create beating effects.

The Trill Bar sensor has musical notes mapped along its axis. These notes control the FM synthesiser. The number of fingers on the sensor chooses between scales. Using one finger, each position corresponds to a note of an Arabic scale. Two fingers selects a Hindu scale, one octave above the first scale; three fingers a Gypsy scale, two octaves above the first scale.

By pressing the sensor with greater or lesser force the detected note can be detuned slightly. This means that the spectrum of possible notes to play is not restricted to the notes of these predetermined scales. The production of these notes by the FM synthesiser must be accompanied by a trigger, whenever you intend to play a new note. This trigger comes from the Trill Square sensor.

Exciting notes from the Trill Square and controlling pitch from Trill Bar.

In addition two potentiometers are used to determine the Index and Ratio parameters associated with the implemented FM synthesizer. This means that the “tuning” of the instrument’s timbre can be altered by real-time adjustments to the position of these potentiometers.

Two potentiometers for timbre control.

Building Crescente’s body

Before making a final decision regarding the shape of the instrument as a crescent moon, two prototypes were built using K-Line, pins and paper tape. When building these two boxes it was possible to experience the sensation of holding the object, creating a notion of dimension and weight, which contributed to decisions on the placement of different sensors in the final revision. After several tests with these prototypes the final shape of the moon was chosen. From there, the construction of the prototype box and assembly of the circuit took place in parallel. This iterative design process made it easier to make crucial technical and aesthetic decisions, highlighting details such as the placement of k-line beams for the stability of the box or where openings for the passage of sensor cables should be for example.

From sketch, to prototype, to final form.

Crescente’s body was built following one of the designs that emerged from this process, manifesting as the shape of the moon. From an ergonomic point of view, this shape main advantage is the fact that it fits into the performer’s body, almost as a “hug”. Visually, this shape allows frontal and lateral communication with the audience of the musical performance, forcing the hands to be placed in a certain position to play the instrument successfully. The materials used in the construction of the prototype allow the whole instrument to vibrate thanks to the contact transducer, thus making it possible to hear the sound produced come out of the instrument itself and also to provide a sensation of haptic feedback to the instrumentalist. From the minijack output, the instrument can be connected to an amplifier or mixing console, in order to increase its overall volume, ideal for many live musical performance contexts.

About Henrik Ferrara

Henrik Ferrara.

Henrik Ferrara is a musician, photographer and multidisciplinary author living in Vila Nova de Gaia, Portugal. He recently completed a Masters Programme in Multimedia, specializing on Sound Design and Interactive Music, at Universidade do Porto, Portugal. He’s currently interested on the subject of new musical instruments and interfaces. Henrik frequently takes part on independent projects, many of which with LickSickDick’s arts collective. These usually focus on the fields of cinema, video, music, performance and photography.

Henrik has always kept a strong relationship with music and has participated on some bands and other musical projects such as Stop!Estra, ATA OWWO + GUILLIO and Geleia Mista. He’s also a founding member of the LickSickDick collective where he regularly plays alongside other artists and musicians. For more information see his instagram or bandcamp.