Dan Gibson – The Modified Cello

In September 2017 I was invited to STEIM to perform at their Connector III event. A recording of which can be found below:

This gave me the opportunity to test the new software I had been developing for ‘The Modified Cello‘ and explore the new range of possibilities it created. The new software, written in SuperCollider, is focused around improving the integration of acoustic and electronic interfaces using audio analysis and responsive mappings. The incoming audio from the cello is processed using filters, ring modulation and a delay. Their parameters are manipulated using a combination of audio analysis, expression pedals and sliders.  The diagram below shows the mappings between audio input and its processing.

fxmoddiagram1

The frequency (or time for the delay) for each effect can be mapped to the outputs of the analysis section of the software. When pitch tracking is enabled, the pedals are used to scale the output of the pitch tracker, This allows the parameters to be scaled relative to the pitches of the incoming sounds, providing intuitive control over the intervals between them.

The audio input for each effect is also filtered and used as a modulation source, resulting in a rich, complex and interrelated behaviours in the processing of the sound. The cutoff frequency for a low pass filter, acts as the frequency control for this modulation, moving from subtle and slow fluctuations to arhythmic crackling and noisy textures. This becomes especially interesting when the master output of the software is fed into the analysis rather than the dry cello signal, creating a feedback loop which produces very dynamic results.

A SenseStage MiniBee was attached to the bow and a series of buttons added for selecting which parameters are controlled by the expression pedals. Discrete movements from the accelerometer data are also recorded and used to create a wavetable oscillator. The data from each axis is used to influence each of the effects, creating modulations and fluctuations in the processing of the sound.

bowminibee2

As the day of the concert approached, I spent more time exploring the new software and adapting to its behaviours. I refined the mappings, finding and expanding sweet spots in their curves, adding custom transfer functions to allow more detailed control over their response. During this time, I also started to develop sections of material that would form the basis of the performance. These consisted of specific sounds, objects, gestures that would be used along with their combinations and transitions.

After the performance, I extended my residency to explore various ideas that occurred to me during my preparation for the performance. This mainly involved a deeper exploration of the range of sounds and responses that can be achieved with the software. This resulted in the addition of extra strings and objects being mounted on the body of the cello.

During the beginning of the performance I had used a thin metal bar placed between the strings, altering the timbre. When the end of the metal bar is plucked, it bounces up and down against the tension of the strings, creating a decaying vibrato in the sound of the strings when they are played at the same time. This inspired me to attach a piezo sensor to the end a flexible metal bar and mount it to the body of the cello. In this way, the signal captured from the piezo acts as an intuitively playable modulation source, producing bouncing, pendulum like movements which are used to influence the parameters in the software.

An extra contact microphone was also mounted on the head of the cello, allowing me to process and analyse the audio separately. This is particularly useful when placing objects underneath the strings to create a third bridge, dividing the strings into two sections. By processing each microphone signal separately, two timbrally diverse layers of sound can be controlled simultaneously, expanding the capabilities of the instrument and achieving a denser and fuller sound. Initial prototype circuits were also built using a breadboard to provide pre amplification, distortion and filtering for these microphones.

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