Until now handicapped people not having the use of one of their arms have been very limited in their choice of musical instruments...

In this article a specially modified electric guitar which attempts to widen the scope for disabled musicians is described. It was designed as a student project in the Final Year of a B.Sc. Honours Degree Course in Electronic Engineering at the City University, London and was supervised by Mrs M. Watkins, a lecturer at the University. A prototype has been built for ex-Kenny guitarist Yan Style who lost the use of his right arm in a motorcycle accident 4 years ago, and who feels confident enough to start making music again with the one handed guitar (the band by the way, will be a five piece called the Nine Arms!).

The basic arrangement is shown in the picture. The player leans on a stool and controls a set of pedals and switches which activate a plucking device near the bridge of the guitar.

First, some points about guitar playing. There are basically three ways or modes of playing a guitar. You can play a single line melody (or lead line), strum through the strings or play a number of strings simultaneously. An effective system must enable the one-handed player to use these modes of playing.

How it works

The guitarist selects the mode of playing by use of 3 mode switches labelled Lead, Strum and Simu (simultaneous). He taps on either of the side pedals whenever plucking of the strings is desired. A finger position detector on the neck of the guitar gives information on the strings the player is touching at any given moment. This detector is quite simply a circuit formed with the strings, neck of the guitar and hand as depicted in Figure 1.

The neck of the guitar is coated with a conductive paint so that when the hand is touching both a string and the neck, the inverting gate gives a high output. This is an example of CMOS at its best since the hand resistance can vary from about 5 MΩ down to 200 kΩ. The extremely high input impedance of CMOS gates allows a simple design for detection. The only catch is that on most guitars the strings form a short circuit at the bridge and string holder, but this is easily overcome by replacing these parts with a hard insulating material.

Now back to the playing. If the guitarist is in the Lead mode the system scans the strings starting from the low E going up the strings towards the high E. When one of the Rhythm pedals is tapped, only the lowest string being touched is played and any high strings being touched are ignored. This has to be so since, when playing lead, it is almost impossible not to accidentally touch strings which are not to be played. Luckily, these strings are, most of the time, of higher pitch than the one which is to be plucked.

In the Strum mode, tapping one of the rhythm pedals causes the plectrum to strum through whatever strings are being touched. If the right hand side pedal is struck, the strum is from top to bottom and vice versa for the left hand side pedal being tapped. The player chooses the speed of strumming via the central preset pedal (Strum speed control) and a row of LEDs, gives an indication of the set strum speed. At slower strum speeds a finger picking effect is obtained.

In the Simultaneous mode, a pedal being pressed causes whatever strings are being touched to be plucked at the same time.

The system also provides a dynamic mode when in Lead or Strum. This comes into operation when both pedals are pressed at the same time and causes the system to go momentarily into the Simultaneous mode, causing the strings being pressed at that given moment to be plucked as one chord. This mode of operation increases the versatility in playing dramatically as, for example, mixtures of lead and chords are easily achieved with a little practice.

The Pluckers

As can be seen in the picture, the plucking mechanism consists of motors perched over the strings with plectrums attached to their shafts. The motors are made to step backwards and forwards, simulating the movement of a hand plucking a string with a plectrum. The plucking has to take place quite near the bridge due to angle considerations and so the sound obtained has quite a high treble content. However, with good Bass/Treble controls an acceptable sound is produced. Another limitation of the plucking devices is that they always pluck at the same strength and hence a dynamically flat output is obtained from the pick-ups. This problem is solved by having an automatic gain control system.

Automatic Gain Control

The rhythm pedals incorporate strain gauges, so that a voltage is derived which is proportional to the maximum force applied to the pedal at any given time. This voltage is used to control the gain of a preamplifier stage for the guitar output. In this way the effect of different plucking strengths is simulated and the player has complete control over the loudness of the notes being played.

Although the system works satisfactorily there is plenty of room for improvement, the most pressing one being that of designing a special plucking device since higher torque is desirable for the faster plucking speeds and for a better sound. Hopefully, this will be followed by students at the City University in the near future. For now, keep your ears open for the Nine Arms!