Ricky Graham and John King > signalsundertests

Polyphonic Audio and MIDI box for Polyphonic Electric Guitar

The premise of this project was to build a hardware unit which provides polyphonic audio (separate audio signal for each string) as well as MIDI data (pitch and velocity) for each note executed which could be utilised for mapping purposes. Such a hardware unit would allow the user to maximize CPU power for signal processing. Following on from a previous “break-out box” project, I purchased the Axon AX-50 board from Terratec (Germany) to allow for MIDI data for each note. The tracking is excellent in combination with Graphtech’s “Hexpander” MIDI interface.

Polyphonic audio/MIDI box for electric guitar

Initially, I wanted to hack the board so it supported polyphonic audio and MIDI concurrently, however, the Axon board is inherently noisy and a series of the DIN pins carry a rather unfortunate mains noise rendering the board unusable for audio. So, I devised a separate power supply (9v battery) acting as a piezo amplifier, powering the Axon unit separately with an adapter. I devised a DIN splitter cable so that the Axon board is able to receive the audio derived from the guitar, whilst maintaining separate power sources, ergo eliminating the ground noise issue. My thanks to Jun Kwon for his assistance with installing a series of isolator transformer to improve the quality of Audio/MIDI data translation.

In separating the ground of the piezo amplifier from the Axon AX-50 board, one is presented with a different noise problem, whereby noise generated by MIDI data is present in the audio signals. A separate MIDI interface was tested, which reduced the noise but it did not eliminate the noise altogether. To eliminate the ground noise completely, a USB isolator may be required. This project is on-going.

Mapping Strategies

The premise of this project was to reflect pitch structure in my chosen DSP during real-time performance. I built the following patch in PD (Pure Data), in an attempt to reflect pitch structure in pre-programmed signal processing during real-time performance, which can be dictated by the chosen pitches executed by the performer while maintaining an element of improvisation. The electric guitar is inherently a multi-parameter mapped instrument. For the most part, it maintains a many to one relationship, concerning many attributes of a physical instrumental technique utilised by the performer to produce a desired sonority. It is important to maintain an approach which is conceptually clear, one that maintains musical meaning versus technical mapping.

The patch collects pitches played by the performer, then generates random index via a counter. The random index is then re-called from the [coll]. In this example MIDI note 60 (C3) is mapped to an automation clip in Ableton Live. The automation clip triggers a series of signal processes specified by the user. This example is using the built in mic on my laptop.

PD to Live – Phase 2 from Ricky Graham on Vimeo.

“Killing your Darling”

After some discussion, there is further potential to develop the patch into a more modular, versatile, and interactive performance system, which may allow the patch to operate with a greater degree of independence, encouraging the performer to engage with the system via a new performance approach, versus a learned or repetitive performance practice. I intend to develop the patch to further accommodate statistical analysis of pitch class development and additional string information, such as velocity, and note onset and offset. Thus, temporal phrasing will become an interactive part of the mapping and performance process. Other approaches include the utilisation of pick position data to trigger appropriate timbral processing, resizing the [coll] size in real-time to vary the rate at which a signal process is triggered, and the real-time drawing of automation values.

My thanks to Daniel Schorno for his feedback on this project.

Performance in Antwerp

This was a great opportunity to improvise with themes I have been developing for the forthcoming album by signalsundertests. I scored some motives to work with during the performance, but I am still developing an appropriate method of scoring pitch, timbral, and spatial attributes of each theme. I would like to work with a stave per string in future, and it is my hope that the score will also be applicable to acoustic instrumentation. All in all, an excellent experience.

signalsundertests - Antwerp

signalsundertests performing at the Live Looping festival at Arenberg, Antwerp.

Thank you to those who provided photography on the night, including Ingrid, “Sjugge”, and Benjamin. My thanks to Sjaak and Ingrid for their hospitality, and to the amazing musicians which were a part of this wonderful event. And of course, thank you to all the staff at STEIM for their support. It was a very productive and enjoyable residency.

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