When is a Guitar Not a Guitar?  by Tom Fryer

(this article originally appeared in CHROMA Number 25 July 1999)

The title stems from a comment made by Warren Burt after a recent performance of mine. He said something along the lines of, “ Well you’ve extended the instrument so far that the guitar has all but disappeared!” This got me thinking. At the time I remember agreeing with him but  actually this is not the case. I don’t agree!! I see the guitar as a filter which creates unique results in electronic music due to its’ particular physicality. The fact that it has 6 strings allows for 6 independent MIDI channels to be easily accessed. Each string has a different sustain envelope. So in a sense you have 6 variations of a string as a controller.  Polyphony is limited to 6 independent voices (in reality this more like 4 voices or less due to physical limitations of the instrument. Strings can be caused to vibrate in many ways e.g. plucked, bowed, struck, blown upon, scraped ... all of these methods create different results and these translate expressively into the MIDI domain.

After using various commercially available MIDI guitar controllers for a number of years I became dissatisfied with the level of  control available through the use of expression pedals. (At the time I  performed  with an array  of six pedals in front of me!!). The fact that I only have 2 feet proved to be a limiting factor in manipulating more than 2 pedals at a time! Additionally  expression pedals don’t allow for a lot of nuance or fine control, especially not with any speed or precision. The search for
increased expressivity led me to investigate the use of sensors as control interfaces and in 1996 while living in Amsterdam I spent 2 weeks  at the STEIM institute using some of their hardware and software. Of particular interest to me at the time was their device the SensorLab. To quote their definition, ”The STEIM SensorLab is a small, general purpose, analogue to MIDI interface for the prototyping of musical instruments and interactive control systems.” With the SensorLab in mind  I designed  an extended MIDI guitar controller using a variety of sensors. With grateful assistance from the Australia Council for the Arts I have been able to realise some aspects of my vision.

I do not intend to describe how the sensors function in any  detail nor describe the code written for the SensorLab. My aim is purely to provide an overview of a functioning extended performance instrument that is in regular use.


The flow chart below outlines my  current performance set-up.


Fretless Guitar (Images)

A Strat style guitar on which I removed the frets. Being fretless it makes a fantastic MIDI controller as you get so much variation in pitch. With clever programming this can be used to great effect. I often assign pitchbend as a modulator of pan position allowing me to slide sounds  around in the stereo field. The pickups are a standard Strat system with 1 humbucker and 2 single coils with a 5 position
switch. The middle pickup has a phase reversal switch too. The output from the guitar goes to the
Audio in on the sampler. This allows access to the samplers FX busses. There is also a Hexaphonic
pickup for the Pitch to MIDI converter. A Hexaphonic pickup sends pitchbend and note data for each
string on a separate MIDI channel. On the fretless guitar this means that I can glissando in opposite
directions on different strings.
In performance I often prepare the guitar in various ways with alligator clips or attack the instrument
using an E Bow (electronic bowing device), slides, thimbles, a vibrator, a screwdriver or play on the
fingerboard with both hands like piano. These manipulations produce a whole world of colours
which do translate into MIDI data as well as producing interesting guitar textures.


Yamaha G50 Guitar to MIDI converter

One of many similar commercially available devices. This unit has some interesting and useful real-
time control features. The distance from the bridge that the string is picked can be translated into any of the standard continuous controllers. There is also an envelope follower which tracks the decay of
the strings. These can be assigned to performance presets so are easily accessible. The note and
controller data from the guitar is sent to the sampler via MAX but no processing occurs. This means
that there is no audio output unless I am still playing. My last vestige of traditionalism!!

SensorLab

This is where the analogue signals from the sensors and the switch closures are converted into MIDI messages. See Sensorlab for more information.

MCS2

An old Yamaha device which has inputs for two CC pedals and two switches as well as Modulation
and Pitch Bend wheels. It can also send Program Changes and has three assignable switches. I
replaced the Modulation wheel with a socket giving another analogue input. Two of the switches are
assigned to increment and decrement the Modulation value. (This is done in MAX).

Macintosh Powerbook running MAX

MAX is set up to give me visual feedback of the status of various controllers, FX assignments and 
some of the ASR10’s internal parameters. Additionally there is mapping of all incoming notes on
channel 16. (see  Hexpad Keyboard below)

Ensoniq ASR10 Sampler

Another stock standard device with a good and useable FX processor. Sounds and FX are stored on a removable hard drive. I connect a master sustain pedal here and a patch select pedal. In the ASR10,  four patches are available for any instrument (a patch consists of 1 to 8 layers of samples). This is very  convenient as you can make copies of samples in different layers that use little memory as they point  to the original data, but can have entirely different parameter assignments. So with 1 foot movement  you can move between different but related sound palettes.

Switches and Sensors

2 Momentary switches on headstock - these swap SensorLab modes. The SensorLab can have 8
operational modes which can completely re-assign it’s functionality. I currently use 2 modes which
change some of the sensor assignments. These 2 switches select 1 of 2 modes. This could be achieved using 1 switch as a toggle but this would require visual feedback to be certain of the mode in use, with 2 discrete switches there is no doubt.

Hexpad Keyboard
This was originally a cheap Tandy keyboard which I cannibalised. I removed the circuit board and substituted 2 Hexadecimal data entry pads for the keys. These are mounted on a small Jiffy box containing the circuit board. This creates a compact MIDI keyboard. The MIDI note numbers are mapped to the appropriate SysEx string or Program change in MAX. This is the case for channel 16 only, notes on  other channels are sent out to the sampler immediately.

Hexpads
2  Hexadecimal data entry keypads mounted on the guitar body. They have identical function to the floor unit switches (see below), with the addition of an extra row of 8 switches that  select 1 of 8 tables to which the accelerometer is mapped. The duplication of function between the footswitches and the Hexpads gives flexibility during performance.

Custom Footswitch Unit
This is an array of 3 rows of 8 switches. (fig 2) The bottom row sends out SysEx strings which select, de-select or stack any of the 8 instruments on the ASR10. Next the middle row, (reading from right to left) the first 6 switches toggle sustain (controller 64) on MIDI channels 1 to 6. This corresponds to strings 1 to 6 on the guitar. The 4 switches in the top right-hand corner each send a SysEx string to select FX variations in the ASR10. The ASR10 allows 4 variations on the currently loaded effect algorithm to be selected.  The first 4 switches in the top row control selecting and loading FX from the Hard disk via “Virtual Button Pushes” (a SysEx string replicating a button on the front panel of the ASR10). Switch 1 selects the FX directory, switch 2 is the LOAD button and switches 3 and 4 scroll up and down the FX directory.The remaining switches, 5 and 6  in the top row, increment and decrement CC#4. I use this to modulate sample loop points in the ASR10.

 


Joystick 
The x and y axes transmit modulation and CC#4 respectively (modes 1 and 2). All guitars
should have a joystick, they are a fantastic and highly intuitive control interface.

Pressure strips (top of neck and back of neck )
Aftertouch strips from Yamaha keyboards. They consist of 2 conductive strips which change resistance as they are pressed together. (currently out of service due to lack of conductive epoxy)

Linear motion potentiometers x2
These are potentiometers that operate in a linear direction as
opposed to a rotary direction. They are spring loaded so they return to 0 after displacement. They are located on either side of the headstock. Thin plastic cable is attached to them at one end and to the guitar body at the other. Any displacement of the cable produces a corresponding change in
resistance. The varying voltage is converted to pitchbend and aftertouch in Mode 1 and modulation
and CC#4 in Mode 2.

LDR (light dependent resistor)
A light sensitive resistor. This is another wonderful control interface. Currently out of service. The problem is with ambient light levels at venues. In a controlled situation (where it works!!) you feel  as if you are sculpting the air and sound simultaneously.

Accelerometer
A motion sensitive device. This is often attached to a plastic Trout. (No joke!!) The signal produced is mapped to 1 of 8 note-tables. This is currently done in MAX  but will  be implemented in  the SensorLab in the near future. The tables are naturally set up to play fish scales ;-) usually using 10 tone equal temperament. The Pitchtables are set-up  in the ASR10.

All the guitar mounted sensors and switches are routed using ribbon cable to an RS 232 socket
mounted on the back of the guitar. This socket allows connection to the SensorLab.

So there you have it.  This is obviously a highly personal system tuned to my idiosyncrasies but the
principles could be applied to extend the capabilities of any instrument or non-instrument. I still see
it as playing the guitar but it is a little removed from the cheap Korean acoustic I got for Xmas as a
fourteen year old. If you are interested in hearing the results drop me an email and I’ll inform you of
any upcoming gigs. Hey you could even buy a CD!!!

I would like to thank Rainer Linz for his invaluable advice and expertise.