Expression control in automated musical instruments

an ongoing survey report

Godfried-Willem Raes

postdoctoral researcher
Ghent University College & Logos Foundation
1987-2010

 

Before the advent of electronic circuitry, musical automates, orchestrions, barrelorgans have been built using mechanical or pneumatic principles. Up to the 19th century, the pinned barrel was the device of choice to program the music into the automate. With the 19th century came the advent of pneumatic principles. All these instruments (the antique Limonaire organs, the Pianolas, the Mortier organs, the Decaps and many more) use paper rolls or cardboard books for programming and are pneumatic. In nature they are, just like their pure mechanical ancestor, binary machines: a punched hole in the roll is a note-on, no hole is a note off. Musical expression -apart from precise placement of tones in time, or global control of the wind pressure- is left out altogether in these designs which is what explains the very mechanical character of the music produced. Although is is not impossible to implement gradual and nuance control using pneumatic technology (and many attempts to do so have been performed with sometimes a reasonable effect), it is only with the advent of electromechanical or electropneumatic devices and particularly microcontrollers that is has become common practice amongst modern automated instrument builders. Instrument automation has been keeping us busy since the early seventies of the 20th century and in this survey we try to give a broad overview of technologies and approaches to the realization of musical robots with expressive possibilities way beyond the simple, if not trivial, note-on note off that has plagued automatons for way too long a time. All the mentioned technologies described here have been put into practice in one or another of our by now 43 musical robots making up the <M&M> robot orchestra. Since we put the designs into the public domain, their principles have been copied by many other builders as well. Note that here we only consider automates with acoustic sound production, excluding electronic generators (synthesizers, samplers) and loudspeakers as sound reproduction devices.

Electromechanics

First we present an overview of electromechanical devices required for the implementation of expressive possibilities in automates. We also pay attention to the circuitry involved. In a second chapter (to be even further developed later) we will delve deeper in the low level software/firmware.

1.- Automates where the sound originates from striking

examples: player pianos (strike and hold), percussion robots (strike and bounce back).

Technical solution: precise control of the striking force by modulation of the width of the excitation pulse.

Electromechanical parts: moving anchor solenoids, tubular solenoids, rotary solenoids.

Different types can be used in function of the requirements: tubular solenoids (push type and pull types do exist) being our favorites in all cases where the force has to be exerted in a vertical plane.

The picture on the left shows a large Black Knight tubular push type solenoid, used for the concussion of a couple of heavy 'bass' castanets as used in our <Simba> robot. On the right picture we see a Black Knight tubular pull solenoid used for lifting the pallets in an automated bass accordion. For piano Vorsetzers, push types become the obvious choice. On the picture we see Lucas Ledex tubular solenoids with rubber pushers designed by us. If the striking force is in a horizontal plane, it is generally better to use rotating anchor solenoids as used in organ building.The reason being that tubular solenoids operating in a horizontal plane, suffer a lot from friction and need springs to return them after striking to the original position. Gravity cannot be used in this case. The picture shows a 10 Newton double coil register magnet as produced by August Laukhuff. This type in extremely silent in operation. Brand names for the tubular solenoid types useful in musical robots are Lucas Ledex -now Saaia Burgess-, Kuhnke, Black Knight. For rotating anchor types August Laukhuff is a good source. Below some more applications of different types of rotating anchor solenoids used in automated percussion instruments:

If pulse only operation is required -as in automated struck percussion, drums, bells etc...- the drive circuit becomes extremely simple: For ease of interfacing to standard TTL logic levels, we invariably prefer to use logic level mosfets turning on with 5V such as the IRL640. The pulses, the duration of which determines the striking force, generally come from an output of a small microcontroller, although programmable 16 bit hardware timers (such as Intel 8254) can be used as well. These hardware timers have the advantage that it becomes easy to implement timing resolutions in the order of 0.1 microseconds, using a 10 MHz crystal. With microcontrollers such as the popular Microchip PIC series, we cannot do much better than achieve a resolution of ca. 18 microseconds. The resolution is a function of how many timed pulses you want to get from a single controller. For a 16 output design, the resolution will rather be in the order of 27 microseconds.

The circuit above is about the easiest one could imagine to implement note-on with velocity, including a hold as required for instruments such as pianos and touch sensitive organs. The circuit goes away with a single positive supply voltage. The disadvantage is that a lot of power resistors -one for each note- are required, leading to larger current consumption than strictly needed. From an engineers point of view it might appear silly to use such a circuit, as you might think it to be easy enough to control the power mosfet with PWM. The trouble with PWM however is that it causes audible artifacts from the solenoids. If you try to overcome these by choosing the fundamental frequency way above audio at the other hand, you will run in trouble with the dissipation and with electromagnetic radiation (EMC).

Example projects:

The same technology can be applied to the damping of the sound in some instruments, such as the vibraphone. The circuitry for a variable pulse combined with a constant hold voltage is shown below in a circuit as we used it in our <Harma> robot. This circuit is a further development of a similar circuit using darlington transistors as designed by my colleague Trimpin for his player piano.

The circuit can also be realized with a small p-channel FET instead of the PNP transistor. The schematic below shows the circuit as we applied it in <Qt> but also -with different power supply voltages and solenoids- in the latest models of our player piano.

By applying PWM (preferably using the lowest possible frequencies for reasons mentioned above) to the hold input, aftertouch can be implemented as well. The effectivity of such an approach depends highly on the mechanical design of the solenoids or valves. Conical valves are the optimum choice if aftertouch is to be implemented.

The positive hold voltage should be taken as maximum the allowable 100% duty cycle voltage for the given coil. The negative pulse voltage should be 4 to 10 times the nominal voltage for the coil. Do not go beyond the maximum rated voltage however, because the insulation of the winding may not survive it. Practical pulse duration's vary between fractions of a millisecond to ca. 50 ms. The higher the voltage, the shorter the pulses can be, and the faster the maximum possible repeat frequency. Magnetization time, frictional losses and hysteresis are limiting factors.

An alternative circuit makes use of a fast optocoupler (6N137) to drive the negative voltage mosfet:

A note on springs:

If push-type tubular solenoids are used to exert a vertical force downwards, as in the case of the piano-vorsetzer, it is generally required to fit a helical spring inside the shaft of the solenoid. Although on pianos, generally the return force of the key tends to be large enough to bring the solenoid anchors back, this is a bad practice as it slows down the reaction speed obtainable from the robot. Also the nuance possibilities will be greatly reduced. The springs should be calculated and fabricated to have just enough force to lift the anchors up at rest. Their length should correspond with the required traject of movement. It should also be noted that these springs need replacement after about 3 or 4 years of daily operation, since they loose force due to material fatigue.

Anchor shapes for tubular solenoids

The drawing below shows the three basic types of shapes for the moving anchors inside tubular solenoids. Both the push and the pull version (if different) are shown.

The first type -the most common in the industry- develops the largest holding force, since the anchor is in flat contact with the endpole of the electromagnet wound on the armature (drawn in red). The big disadvantage is that this type of anchor causes a very high noise level at the moment the anchor hits the pole. This applies even more to the pull type of the same shape. The noise can be substantially damped with a felt washer but obviously, this leads to a reduction of the holding end force. The second type shows a tapered end. This type has a much more gradual force against applied voltage characteristic. Therefore we found this type for musical robots in many cases the optimum choice. The noise is damped here as soon as we insert a spring over the tapered end inside the coil former. The smoothest operation but also the lowest holding force is obtained with the third type, where the anchor can move freely through the coil. In this case there is no real holding force and the anchor behaves somewhat like a spring on varying loads. This type can be used both for pushing and pulling. The disadvantage is, next to the low efficiency, that fitting return springs as well as end stops is mechanically rather difficult.

A note on human fingers...

When human fingers activate keys as on piano's and organs there is never a problem with noises at key release. Potential noises are damped by the design of the mechanics of the instrument. However, with instruments such as the accordion, replacing human fingers with solenoids does cause noise problems. In these instruments, when played by human fingers, the keys are released with a damping caused by stiffness and mass of the human fingers. When we replace these fingers with (tubular) solenoids, the speed wherewith the keys are released becomes much higher, resulting in lots of noise caused by the sudden (spring loaded) closing of the valves. This problem particularly plagued the design of our automated accordion <Ake>. It is also relevant for the valve action of valve-operated brass instruments. We propose three different approaches to solving this problem: a first one involves applying PWM on note-off commands such that the solenoids loose magnetization only slowly. The load on the firmware, particularly for highly polyphonic instrument becomes quickly prohibitive. Furthermore the remarks with regard to PWM mentioned before do apply also here. An alternative and second solution makes use of analog circuitry in hardware: Here we place a large capacitor parallel over the solenoid. The capacitor is charged on turn on of the mosfet via Rr. This resistor should be dimensioned at about 5 times the value of the DC resistance of the solenoid used. When the mosfet is turned off, the capacitor discharges via the series diode into the solenoid. The RC time corresponding to the product of the solenoids DC resistance and the value of Cr. Practical values for Cr are in the range of 1mF to 10mF. Since capacitors with these values invariably have to be electrolytes, they tend to be rather large. The RC time should be below the inverse of the maximum repetition rate for notes (in Hz), one wants to achieve on the instrument. A third solution, also involving analog hardware, operates similarly but this time on the gate of the mosfet. Although the circuit is very simple and does not make use of large electrolytic capacitors, it suffers from the large spread spread in the analog gate drive characteristics of the power mosfets we favor to use. The circuit also affects turn-on time. But the main problem here, using the mosfets as slow switching devices, is that it will increase their dissipation quite a bit. Thus the space (and expense...) you win by avoiding the large capacitors in the second solution is lost on the space (and cost) taken up by the increased cooling requirements on the power mosfets. All of the proposed methods have been but into practice by us. Our favorite being the second one, despite the large space penalty involved. However, the problem for all solutions presented here is that they invariably introduce a limitation on note repetition speed. Any solution we can think of for this problem requires looking ahead in software: if we know what the next note will be, we could adapt the release time accordingly. Obviously, this is not possible for a robot that is supposed to operate in real time and without any latency.

A note on the law of the hammer

Instruments wherein the sound is produced by striking an object with a beater follow the same physical laws that govern those of the hammer. The energy of the collision equals the mass of the hammer multiplied by the square of the speed on the moment of the collision divided by two:

Therefore it seems more profitable to increase and control the speed of the hammer rather than its mass. Increasing the speed was traditionally (in pneumatically driven automates) done mostly by using a longer handle on the hammer. This approach however goes severely at the detriment of repetition speed, since the movement traject becomes much larger as well. With solenoid driven beaters, the mass of the anchor has to be taken into account when it is rigidly coupled to the beater. Magnetization time limits will put limits on the maximum obtainable speed. The smaller the mass of the anchor, the faster the speed can be, but of course the impact will also be lower. As a general rule, one should take the mass of the hammer to be somewhere between one tenth and one twentieth of the mass of the object to be struck. From there one can start calculation of the required traject of movement in order to get the desired maximum amplitude. This will lead to quite good specifying possibilities for the solenoids to be used. Experimentation will be mandatory in almost all cases. It might be good to review the elementary mechanics describing collision in general:

Note that for an object at standstill, the second term on the left will always be zero. The value of v4 will be proportional to the amplitude obtained. It is a function of the elasticity of both beater and object.

Applying textbook physics formulas it is pretty easy to properly rate and design solenoid driven hammers. Let s be the traject of the hammer (we suppose the beater is rigidly connected to the anchor of the solenoid such that we can consider the moving assembly to have total mass m), than, given the response time of the solenoid (this data can be read from the datasheets provided by the supplier for a wide variety of operating conditions), we can calculate the force involved using Newton's second law:


 

2.- Automates where the sound originates from a wind flow

examples: pipe organs, accordions, reed organs, wind instruments (flutes, brass and reed-woodwind)

2.1: - Global wind pressure control:

This can be easily achieved through frequency control of the compressor motor. The speed of the possible modulations is limited by the large inertia of the motor and compressorblade combination. The modulation affects the entire instrument. The motors should be 3-phase AC induction motor types. Collector motors (universal AC/DC motors) cannot be used for they are too noisy in operation.

Example projects:

The easiest practical solution invariably consists in the use of a programmable industrial motor controller module as made by Siemens (Sinamics series), Lust Gmbh, Control Techniques, Hitachi.... These controllers all feature a 0-10V dc control input for speed control of the 3-phase AC motor. Details on programming these controllers can be found in the relevant sections describing the projects pointed to by the hyperlinks above. We found it -after doing it- not worth the trouble to design these things ourselves, since the cost would come out to be higher than the readily available solution.

The steering DC voltage nowadays is most easily derived from a PWM output on a small microcontroller. The PWM is simply filtered with an RC combination and rescaled to the required 0-10V range. Often this rescaling step can even be left behind, since most motorcontrollers can be programmed for the optimum range. For faster braking, it is advisable to program the motor controller such as to use DC injection in the windings. Braking resistors may be used as well.

Note that wide control of operating pressure on reedpipe based instruments can be very problematic, since reed pipes do not keep tuning very well when exposed to varying pressure. This problem is non existing with flue pipes. However these pipes also maintain pitch only over a small range of pressure variation, but at least, they always return to the original pitch as the wind pressure returns to the nominal tuning value.

2.2:- Wind flow control: through valves.

These can be operated pretty fast, driven by either stepping motors or servos. Valves can be used to implement a tremulant in some cases.

Example projects:

In the accordion robot <Ake> we constructed a large 4-way valve capable of smooth switching between suction and pressure wind with all gradations in between. Our first idea to operate this valve with a bi-directional solenoid didn't work very well. The later use of a stepping motor in combination with a Melexis position sensor works nicely. In <Qt> we used a similar design for the wind flow control.

Note that commercially available solenoid valves can almost never be used in this area of applications. They are not available with large enough orifices, they generally can only operate on pretty high pressures (1 - 20 Bar) and last but not least, they make a lot of noise.

2.3:- Wind modulation and control through bellows.

The bellows can be operated either with a motor and a crank, or with a motor coupled to a trapezoidal threaded rod, or else, through a (very expensive) linear motor. Good and responsive control is possible.

Example projects:

If a trapezoidal threaded rod is used, it is best to drive it with a brushed DC motor and an appropriate controller. The starting torque should be very high to overcome friction. Sensors are required to limit the traject of the bellows. For precise control of the wind pressure, the low pressure sensors offered by Freescale may form the base of a good PID controlled loop. (Cf.. Bako).

2.4.: Individual control of notes:

Here the use of conical valves operated under PWM becomes mandatory. The picture shows the mechanism. The cone is covered with fine leather or a synthetic material such as polypel. Conical valves can also be operated with tubular solenoids. As an alternative, moving coil valves, which can be made from re-engineered loudspeakers, can be used as well. In the latter case they can be driven with bipolar analog DC current (double H-bridge). Using this technology not only the individual note attack can be controlled, but also note-aftertouch. Also it is possible to drive each note with an individual pump, driven with a solenoid, as we did in <Puff>. The picture shows the mechanism involved: underneath is a tubular solenoid (Lucas Ledex type) pushing the anchor on the carbon-compound plunger inside the glass cylinder (Airpot). In this case we used a single pulse driving circuit as described before for use in percussion instruments. However, if you go that far, it becomes difficult to obtain sustained notes unless at least two pumps are used for each note. With a single coil/pump combination you can get at the most a steady flatterzunge.

If the requirements as to the control range of attack and/or aftertouch are not too critical, flat solenoid driven pallets can be used. The types shown on the pictures are made by August Laukhuff, the left one has a 35 mm pallet, the right one 40 mm. These types can easily be converted to operate conical valves by exchanging the flat pallets with conical ones, as shown in the first picture under this heading. For good velocity control, the original springs must be replaced with a stronger type. Details can be found in our pages on the development of our 6-octave quartertone organ <Qt>.

The laws governing airflow control through round flat valves are:

The fundamental problem with gradual control of valves with solenoids is that the traject for the opening versus applied voltage is normally very steep and further that the working traject is different for opening and closing. The graphs below give typical curves:

The last curve depicted represents the best possible compromise, obtained by using conical valves in combination with a much increased spring force.

Examples projects:

  • Qt (flat valves with individual note aftertouch)
  • Puff (individual solenoid driven pumps for each note)
  • Thunderwood (bird mechanism)
  • Bomi (conical valves with individual note aftertouch)

In some of our early automates (<Piperola> and <Vox Humanola>) we have used direct acting solenoid valves to steer the windflow to the pipes. Off the shelve, such valves cannot be used unless you are ready to live with the loud clicking noises these valves produce at switching. To overcome this, we shortened the ferromagnetic anchors inside these valves with some 3 to 5mm on the lathe, replaced the back end with a circular piece of felt, and reduced the force of the return springs. Although it is possible to use these valves for velocity control of the note attacks by steering them with PWM or variable DC, the results are quite disappointing because the valve response is quite unpredictable. At the end, the valves work nicely as switches, but when you make the final bill, it comes out to be about twice as expensive as using regular valves as described before. The only area of musical automates where these solenoid valves become the device of choice, are automated tuned membrane driven car horns or ship horns driven by compressed air (1 to 6 Bar pressure). <Toetkuip> and <Klankboot> are two open air projects that illustrate this. Solenoid valves can be operated either on AC or DC, but for automated instrument use, only DC should be considered, since when driven with AC you will get a 50Hz buzz enriched with overtones from each of them...

A note on conical valves:

As noted above, the use of conical valves becomes mandatory if one wants to implement fine individual note aftertouch in windblown instruments. Since the solenoids to be used have a limited traject of movement (Tr) and proper design entails that the surface of the inlet orifice should equal the surface of the valve outlet (the surface of a cone segment or the side surface of a frustum) when fully opened, it follows that the angle of the cone becomes an essential design parameter. To facilitate calculations, we give the essential design equations below: The technical problem here is in the construction of the valve seating, rather than the valve cones themselves. The latter can be fabricated easily on the lathe or purchased from sources such as A.Laukhuff. The smaller the diameter the smaller the angle, they can be ordered in 7 different diameters. But in order to make the conical holes in the windchest one will face the problem of milling holes to these exact angles, not conforming standard available conical mills. Most of the time it can not be done on the lathe for the shapes of regular windchests (solid plates of wood or a synthetic material) make it impossible. If you do not have a CNC milling machine the only solution we found was to call on custom made mills that can be used in a regular drill. This tends to be very expensive. So far we have gone this way only for our <Bomi> robot, wherefore we used five custom made mills. Of course, once you have a set of suitable mills made, the tools can be used for many more robots and the price will come proportionally down.

Here is, as an example, the result of the calculations as performed for the construction of the conical valves in our <Bomi> robot, using A.Laukhuff cones. The last two colums give the result of the calculations in case flat valves would have been used -for the same orifices-; the regulation superiority of the cones will be obvious.

  • cone diameter top angle traject diameter of equivalent orifice flat pallet traject
    35mm / 15mm 110° 5.2mm 10 mm >=15mm 2.5mm
    25mm / 12mm 100° 5.0mm 7 mm >=10.5mm 1.75mm
    20mm / 11mm 85° 6.0mm 5 mm >=7.5mm 1.25mm
    16.5mm /10.2mm 81° 6.0mm 4.3 mm >=6.7mm 1.1mm
    13mm/ 8.7mm 72° 6.0mm 3 mm >=4.5mm 0.75mm

The diameter of equivalent round orifice should be taken such as to correspond with the diameter on the inlet of the organ pipes used. By increasing the traject a bit, adjustments to the exact sizings of the pipe feet are possible. For Laukhuff pallet valves, the maximum possible traject is 10mm. If you take the traject too small, resolution of the regulation possibilities will suffer. In our designs using these solenoids we limit the traject to about 5mm, a compromise between smooth regulation and speed of response. When performing the calculations, one should make sure to choose the equivalent orifices such that they are about 10% larger than the diameters of the wind inlets of the pipes, this to compensate for losses due to the curvatures, turbulencies and roughness of the valve surfaces in the windway.

In general, the sharper the top angle of the cone, the smoother the regulation will be, but also, the larger the required traject. Thus one should always try to use the sharpest possible cone for the maximum possible traject.

To facilitate these calculations we wrote a small computerprogram to generate usefull lookup tables. The program can also be used for flat valves, if you specify the top angle as 180°. From comparison of the generated data, it will immediately become clear why flat valves make poor regulators but great switches. It can be downloaded freely. (4) Here is a table with some calculated results. The numbers colored red reflect the values for standard conical valves available from A.Laukhuff. The numbers in orange are the results for flat valves operating on the same orifice as the coresponding conical valve.

angle traject orifice cone diam >= flat diam >= flat traject
170 3 68 68.6 102 17
170 4 90.7 91.4 136 22.7
160 3 33 34 49.5 8.25
160 4 44 45.4 66 11
160 5 55 56.7 82.5 13.8
160 6 66 68 99 16.5
130 3 10.6 12.9 15.8 2.64
130 4 14.1 17.2 21.1 3.52
130 5 17.6 21.4 26.4 4.4
130 6 21.1 25.7 31.7 5.28
120 3 7.79 10.4 11.7 1.95
120 4 10.4 13.8 15.6 2.6
120 5 13 17.3 19.5 3.25
120 6 15.6 20.8 23.4 3.9
110 3 5.75 8.57 8.62 1.44
110 4 7.67 11.4 11.5 1.92
110 5 9.58 14.3 14.4 2.4
110 6 11.5 17.1 17.2 2.87
100 3 4.2 7.15 6.29 1.05
100 4 5.59 9.53 8.39 1.4
100 5 6.99 11.9 10.5 1.75
100 6 8.39 14.3 12.6 2.1
90 3 3 6 4.5 .75
90 4 4 8 6 1
90 5 5 10 7.5 1.25
90 6 6 12 9 1.5
85 3 2.51 5.5 3.76 .627
85 4 3.34 7.33 5.02 .836
85 5 4.18 9.16 6.27 1.04
85 6 5.02 11 7.53 1.25
81 3 2.16 5.12 3.24 .54
81 4 2.88 6.83 4.32 .72
81 5 3.6 8.54 5.4 .9
81 6 4.32 10.2 6.48 1.08
80 3 2.08 5.03 3.12 .52
80 4 2.77 6.71 4.16 .693
80 5 3.47 8.39 5.2 .867
80 6 4.16 10.1 6.24 1.04
72 3 1.51 4.36 2.26 .376
72 4 2.01 5.81 3.01 .502
72 5 2.51 7.26 3.76 .628
72 6 3.01 8.72 4.52 .753
70 4 1.84 5.6 2.76 .461
70 5 2.3 7 3.46 .576
70 6 2.76 8.4 4.15 .691
60 4 1.15 4.62 1.73 .289
60 5 1.44 5.77 2.16 .361
60 6 1.73 6.93 2.6 .433

 

Although conical valves allow for a much better flow control than flat valves, both types show a linear characteristic within their traject of movement. The difference being that the steepness of the curve is much lower in the case of conical valves. Other than linear trajects are conceivable and possible: one could use ball valves, parabolic or hyperbolic, thus realising all sorts of trajects that can be described in a second degree equation. We have never gone that far in practice, because we do not have access to the required machinery to make the valves and their seats. Only spherical mills as well as a wide variety of balls are readily available. Herewith the required math relating to the calculation of ball valves:

It will be clear that as one takes the diameter of the ball larger, we come closer to the behaviour of a flat valve. Thus, for optimum regulation the ball diameter should be taken as small as practical but larger of course than the orifice to be regulated. The mechanism used for the sound generation in our <So> robot , an automated sousaphone, makes use of a spherical valve to control the mouthpiece.

2.5.: Very fast air pressure modulation:

The best (and cheapest) technique to achieve this in instruments operating under an air pressure not exceeding 200mm H20 (20mBar), is through large bass loudspeakers placed inside the windchest. These make very good tremulants as well.

We have been using loudspeakers as valves, air pressure modulators and even compressors since the early seventies. Since loudspeakers are moving coil devices by design, the low moving mass is responsible for their excellent responsiveness. Note that the loudspeakers are driven with sub audio frequencies (and even pure DC if used in a valve) in these applications. In any case, one should stay way below the resonant frequency of the loudspeaker.

You can drive this design even a step further by using the speaker as a vibrating membrane coupled to a resonator, thus coming close to the diaphane register as found in some 19th century pipe organs. It's a good way to achieve strong sounding basses in relatively small volumes. However, one could question here in how far one should consider such an instrument still as 'acoustical' and not as loudspeaker-sound driven... In any case, this does not seem to be a either/or question, since when properly analyzed, a continuum shows up between purely electronically generated sound and acoustically generated sound. In our automated sousaphone <So> as well as in the first versions of <Bono> for instance, a moving coil mechanism is used to make the silicone lips vibrate against the mouthpiece. This modulates the air flow coming from a small compressor and caused resonating sound from the connected instrument. These instruments at times do sound 'faulty' notes and occasional multiphonics. But, if we drive the instrument directly with a moving coil compressor driver, as we did in in the first version of our experimental cornet <Korn>, the sound is to a much larger extend determined by the electric signal applied to the driver as the acoustic coupling to the instrument is a whole lot lower than in the first case. Here 'faulty' notes can simply not occur. This last concept is therefore a border case as one could consider it to be simply a non linear loudspeaker.

2.6.: Acoustic impedance convertors for pressure driven monophonic wind instrument

When thinking through the acoustical function of wind instruments, be they lip-driven or reed-driven, we always have to do with an air column with an adjustable resonant frequency coupled to a driver. In order to obtain proper resonance, the driver shouldn't be too stiff nor frequency selective. It should be very low impedance since the resonator will convert high pressure and small amplitude at the mouthpiece side into low pressure and high amplitude at the point of contact with the surrounding air. Thus a wind flow does not appear to be essential for the acoustic functioning of the pressure driven instrument. Human players however, can only set their lips into vibration (this also applies to reeds of course) by directing a windflow and using the elasticity of either lips or reed. If our muscles were only fast enough, we could play the instrument without using our lungs. This analysis led us to the development of sound compression motors coupled to properly designed acoustic impedance converters as replacement for lips and reeds in wind instruments. Note that this does not apply to air flow driven instruments such as flutes, which we have treated above. Pressure driven instruments acoustically behave as resonators closed at the driven end. Instruments developed according to this line are:

If the impedance converter is well designed ( the orifice ought to be as small as reasonable, although this goes at the detriment of sound pressure), the waveshape of the driving signal, provided it has enough partials, becomes pretty unimportant and the instrument will produce a sound pretty close to the sound obtained by players. However, attack and envelope will have to be controlled by the electronic driver using amplitude modulation. If this is left out, the produced sound will invariably sound synthetic, particularly on sustained notes. In all our robots making use of this technology, we implemented a wide range of expression controllers to this purpose. Particularly for the higher pitched instruments, this approach was very fruitful. The reason why it is so difficult to make a full-mechanical sound source for these instruments is that the required speed of movement and the mass to be moved are too high for electromagnetic devices. From this constatation, it becomes logical to examine the possibilities of realizing the mechanical sound sources on a sub-miniature scale and picking up the vibrations with transducers. These signals, after amplification, can then be used to drive the acoustic impedance convertors. The resulting sound is a lot more natural than what can be obtained with synthesized waveforms. However, such an approach cannot be used if one wants to automate existing instruments. An alternative way to improve sound results with digital oscillator driven impedance convertors consists of using audio feedback from the instrument and using this signal to modulate the driving signal in the software. This however, requires very fast processors. If the frequency range is limited, the acoustic impedance converter can also be equipped with a mirliton like resonator in the embouchure part. Good results require many days, if not weeks, of experimenting with different materials and geometry's.

Compression drivers are produced by different manufacturers either for use in public address sound reinforcement systems where they are coupled to exponential horns (megaphones), or as tweeter drivers for speaker systems. In fact they are like loudspeakers -moving coil devices- but lack a sound projecting membrane. The specifications vary with powers ranging from 5 W up to 150 W and impedances like 4 Ohm, 8 Ohm, 16 Ohm, 800 Ohm etc. If you are designing automated instruments using acoustic impedance converters as described above, you should be aware of the fact that the load on the driver represented by the converter changes the impedance of the driver considerably. Also, the impedance is a function of the driving frequency. So for example, one of the drivers we have used (a made in china driver rated 100W at 16 Ohm) has an impedance of 15 Ohm at 1 kHz with no acoustic load. However, when loaded with an impedance converter with a long capillary, this impedance, measured at the same frequency of 1 kHz, rises to 32.8 Ohm. At 100Hz (measured impedance 11.4 Ohm) nor at 10 kHz (measured impedance 26,7 Ohm), the loading effect on impedance is substantial. These facts dictate the need for linearising or equalizing lookups on the generator firmware level or in the amplifier stages.

Also it is important to understand the way compression drivers work: if they have a membrane with surface Sm and they are loaded with a horn with an orifice Sh equal to Sm, than then the compression ratio Sm/Sh is unity. In audio applications, the compression ratios are in the order of 2 to 4, meaning that the surface of the orifice of the load, Sh is only half to one fourth of Sm. In the interest of a natural sound, for automated instruments the compression ratio should be taken as high as practical. The upper limit is where the air compression starts hindering the movement of the membrane too much. Taking into account these considerations, our choice for a tweeter driver in our oboe robot <Ob>, becomes logical. The compression ratio in this robot is about 1:25.

The electric signal for the compression driver can be obtained in two different ways: either one can make use of a suitably designed audio-type amplifier, or one can generate the power signal directly using two phase shifted PWM outputs on a dsPIC type microcontroller. In the last case a custom designed power output transformer (push-pull) may be required to match the impedance of the compression driver.

The circuit as we used it for our automated oboe <Ob> as well as for <Korn> looks like:

The circuit using an audio amplifier, as used for our automated valve trombone <Bono>, as well as for <Autosax> and <Heli> looks like:

Note that also in this circuit a transformer is used. But, since here we only have to cope with small signal levels, ordinary good quality audio line level transformers readily available on the market, can be used.

 

3.-Instruments where the sound originates from or is influenced by rotation, rotating or linear friction such as in bowed instruments, sirens, the rotating valves in vibraphones, the tremulant in reed organs...

Technology to be used: Frequency control of AC motors, PWM control of DC motors, linear motors, servos and/or stepping motors.

Example projects:

A particularly hard problem is encountered whenever one attempts to automate bowed instruments. The pressure of the bow against the string as well as the bowing speed have to be controlled in great detail. To control the pushing pressure of the bow against the string, softshift magnets driven with variable DC or PWM can be used. The picture shows the mechanism used to achieve this in our automated hurdy gurdy where we use a rotating round belt as a bow.

Note that these soft-shift magnets, although extremely expensive, are relatively slow responding devices. The forces involved here exclude the use of moving coil mechanisms. Pneumatic cylinders would be ideal here, if they didn't suffer so much from exhaust noises...

The rotators in vibraphones can be driven with 7.5 degree per step stepper motors easily. To avoid noises its best to drive the rotating shaft with a rubber or nylon belt. The same principle can be applied to the typical Doppler based vibrato mechanism found on many larger reed organs. On the original instruments, this rotator -functionally very similar to the Leslie effect- is driven by a simple pneumatic motor. The disadvantage is that the vibrato speed becomes an intrinsic function of the windpressure and thus of sound volume. By replacing the pneumatic motor with a silent DC motor (we have used tape recorder motors for this with great success) we can control the vibrato speed independently. Also we found that replacing the blades -normally made from cardboard- with more reflective material such as polished steel (thin Hasberg measurement blades) makes the entire mechanism a lot more effective.

The picture shows the tremulant mechanism as we made it for our <harmO> robot.

 

4.- Instruments where the sound originates from shaking.

Maracas, Angklungs, bells, shakers, thundersheets...

Bipolar electromagnets or solenoids can be used, with single pulse-time control in both directions. Useful solenoids can be found in the catalogues of Kuhnke, Emessem as well as August Laukhuff, where they are presented as register traction magnets. Shaking frequency is limited to the low frequency ranges up to about 30 Hz. For medium shaking frequencies, motor driven vibrators can be used.

4.a: Bipolar electromagnets

Example projects:

For small objects (bells and rattles) bistable electromagnets as used for registration knobs in organs can be used: The type shown uses two separate coils. By steering them with two independent PWM signals, you can get intermediate positions easily. For good control, a position sensor and a PID regulating system is required. For larger loads and forces, the solenoid shown in the picture below is suitable. Note that solenoids with higher forces -and thus more moving iron mass- show inherently also a much slower response. We used this type in <Klung>, our automated angklung. A type made by Emessem in the UK (since 2007 named Magnet-Schultz Ltd.) looks like:

If the shaking frequency ought to be very high or very randomized, here again cheap loudspeakers can be used, as we did in the rain-mechanism in our <Thunderwood> robot. When solenoids are used, they should have two different windings. The choice of commercially available bi-directional solenoids is extremely small. For some applications it is possible to combine two solenoids to implement bi-directional movement without using return springs. This is what we ended up with in the design for the rotary valve mechanism in our automated trombone: <Bono>. It is not too difficult to make bi-directional solenoids yourself provided you have a lathe and some winding experience.

4.b: Motor driven vibrators.

These devices have applications in a wide range of industrial production: sieving, mixing and separation of granular components... They consist of a motor (generally a 1 or 3 phase AC induction motor) with a protruding axis on both ends on which eccentric weights are mounted. By adjusting the position of the weights, the amplitude of the vibrations can be regulated. In applications for musical automates wherein fast shaking is a requirement, good control of rotational speed as well as of amplitude becomes a requirement. For ac-motors, standard 3-phase motor controllers can be used. If control of acceleration and amplitude is required, the same technology can be applied but one should rather go for hybrid stepping motors. Steering the magnitude of the motor current will yield a good control over vibrational amplitude whereas programming of the stepping patterns allows control over the vibrational wave form. In our experiments shaking frequencies up to 400 Hz have proven to be possible. An intrinsic problem is presented by the own-resonances of the vibrator in combination with the load. Under resonance conditions, self-destruction is easily reached.

Commercially available AC-motor vibrators are available from Italvibras (Italy). Type Vibtec M3/4-S02 is a monophase device with a centrifugal force rating of 2 to 6 kg, powerful enough to vibrate even the largest thundersheets. Models with much higher forces are available from the same source.

The model shown on the picture weights 850 g and has a power rating of 20 W. It is very silent in operation. For variable frequency use, we advise to use them with an amplified sine wave. Make sure the voltage is reduced when the frequency goes down.

5.- Instruments usually bowed or struck, with ferromagnetic strings or blades.

On such instruments electromagnetic devices can be used to steer very precisely the excitation of the strings or steel blades. Precise tuning of the strings or objects is mandatory for good resonant operation. Also, the driving circuitry should have extremely stable as well as precise frequency synthesizing. For this purpose we now use Microchip 30F3010 microcontrollers. (ds-PIC's). Although it is very well possible to design instruments in this category such as to be self-tuning, (automated guitar tuning devices are a commercially available example), we have never considered to implement it, for the weight of the motors involved becomes quickly prohibitive.

Example projects:

  • Hurdy (e-drive mechanism)
  • Aeio (two phase e-drive mechanism)

The inherent problems you encounter here have to do with the low coupling factor between coil and object. The higher you want the excitation amplitude to be, the lower the coupling factor becomes because you will have to increase the distance between string or object and the electromagnet. The electromagnetic force is inverse proportional to the square of the distance... As yet we do not have an adequate solution for this problem and thus all the designs making use of this technology suffer from a very low efficiency, say very high current consumption versus sound output.

In the 12-stringed <Aeio> robot we used an electromagnetic string driver operating in two phases. Electromagnets are mounted to both sides of the string and by controlling the duty cycle of the driving signals, string motion can be controlled to a quite large extend. The result comes pretty close to a bowed string sound.

The string driver assembly as used in the <Aeio> robot (opened up) looks like: When mounted, the electromagnets face each other: Note that the mechanical assembly should be made of a non ferromagnetic material. We use stainless steel (AISI304). It will be clear that the excitation force can be controlled by changing the amplitude of the excitation pulses. It is quite seducing to implement this by PWM-ing the pulses on the microcode sound generation level However, there are some caveats here, in that easily audible artifacts are produced caused by the too low carrier frequency of the PWM signal. A cleaner approach would be using voltage controlled current sources (using LM317 variable voltage regulators for instance) for each of the coils. However, apart from the far greater complexity of the circuitry, keep in mind that the dissipation tends to be quite high.

In the string excitation drawing above, we have shown an almost sinusoidal movement. Such movement in practice will only be met when the excitation force from the magnets is very low as compared to the stiffness of the string. When we take the spring-behaviour of the string into account, the movement shape of the string under excitation will rather look like:

Notice that the zero-cross happens already in the B phase! Also note that the curvature in the A phase depends on the distance between string and magnets, on the excitation force as well as on the string elasticity. In any case, the waveshape thus obtained will come closer to that of a real bowed string, wherefore a sawtooth shape is generally assumed.

Experiments are being conducted in using 3 separate coils driven in 3 phases, thus the rotation of the string can be better controlled and the coupling factor should become a lot higher. Also the string driver can be made movable along the length of the string. We will report on the results of these experiments in due time.

Obviously the problem with the low efficiency of e-drives is not encountered when we deal with electronically amplified instruments such as electric guitars. But in this article we very much on purpose leave out the possibilities of using electronic amplification. Here we want to deal with pure acoustical sound and how to get it under close control, exclusively.

Another use of electromagnetic drive can be found in the control of reeds in single reed instruments (saxophones, clarinets, bagpipes). Here we do not bring the blade or reed into resonance but contrariwise impose our vibrational mode onto the reed. In order for this to work, the free resonant frequency of the reed must be a lot higher than the highest pitch you want to generate. This dictates the use of pretty thick spring metal reeds and as a consequence, pretty high magnetizing forces. Dual coil systems operating in two phases have proven to be the most workable and reliable. The sound color can be greatly influenced and controlled by controlling the phase angle between the currents in both coils.

Example projects:

Our attempts to realize oboe and bassoon reeds this way, so far, were not very successful, but research is continued. We can only hope flexible piezoelectric material (yes, we know of Kynar, but this stuff does not work here...) becomes available one day. Acoustic impedance converters (as mentioned before) gave by far the best results so far when it comes to reed-driven instruments.

A note on the phenomenon of frequency doubling and spectrum shift:

When a coil moves in the magnetic field of a permanent magnet, the coil will follow the ac input signal and thus the movement of the coil will be at the input frequency of the signal. This happens in normal loudspeakers. Likewise, if a coil is wound on a non moving permanent magnet, the force exerted on a ferromagnetic object in the neighborhood (string, membrane, reed, tongue...) will strictly follow the frequency and wave shape of the driving signal. This happens in the old style telephone receivers and ancient headphones as used for Morse telegraphy. These devices typical use a U-shaped permanent magnet with two coils connected in series, one over each leg. In front of the poles of the magnet a round thin iron membrane is placed such at it does not make contact with the poles. However, if a non-permanent core is used for the coil (or if the core loses its magnetization...) , the frequency of the force will be twice the frequency of the input signal if the ferromagnetic object on which the force is exerted it not permanently magnetized itself. Therefore a string driver as used in <Hurdy> must electrically be operated at half the frequency required, since the mechanism itself will operate as a frequency doubler. The same applies to membranes and reeds driven by weak-iron core solenoids.

This explains why most ac driven buzzers designed for the mains voltage and frequency 50 Hz or 6 0Hz, sound 100 Hz or 120 Hz. Coils with permanent magnet cores are very often used as pickup elements as in electric guitars, phonograph turntable cartridges and some types of contact microphones. If used as force output transducers one has to realize that the ac voltage applied to the windings will after enough time fully demagnetize the core. Another perspective with relevance for sound producing devices is that you can drive a weak iron core solenoid with a signal superimposed on a variable DC voltage. In many cases this gives you control over the spectral content of the so produced vibrations. This is clarified in the drawing below:

It will be clear that the spectral content, both in the case of frequency doubling as in the DC-offset case described here, will contain a very large amount of very high components. If this technique is applied, it is important to realize that the spectrum will become a function of amplitude as well. We have applied it in to advantage in our robots <So>, <Bono>, <Korn> and <Autosax>.

Often one will be compelled to drive the coils with square waves. Most of the time they will make use of PWM, but that aspect is not immediately relevant in this context. There is a pitfall in this case, which is shown in the upper drawing below:

If a bipolar square wave is used to drive a coil, the force exerted by the electromagnet thus formed will tend to be continuous! (Of course, due to the time required to build up a NS magnetic field followed by the building up of an inversely polarized SN magnetic field, there will be a ripple in the force curve proportional to RL as well as to core material constants). This way it will be impossible to excite an object with a given frequency (apart from harmonics that will be produced as a consequence of finite magnetization time - the magnetic poles have to invert at the frequency of the signal, causing slow slopes on the force square wave and thus many spectral components and artifacts enter into the game- ). The square wave bipolar ac drive will lead to a nearly constant force with ripple on the object. This will lead to a high dissipation in the core material, leading a very strong heating up of the assembly. However, if the core is a permanent magnet, this force will follow the frequency. In that case it will go up and down around the constant force of the permanent magnetic field. With a unipolar square wave drive, the force will follow the frequency of the driving voltage. If in that case (lower drawing) a permanent magnet is used as core material, the force will either vary between the constant force of the magnet and the extra force added by the drive (in case the polarity of the driving voltage corresponds the the polarity of the magnet), or else, between the constant force of the magnet and the opposing and smaller force caused by the inverse polarization of the driving voltage. It follows that in case permanent magnets are used as core material, correct poling of the excitation voltage becomes very important. In fact, electromagnets in a mechanical way behave a bit like diodes or rectifiers in pure electronics. A word or warning though: if you use PWM with a high frequency with substantial power on permanent magnet cores, demagnetization is likely to happen at a pretty fast rate. For those amongst you that remember that technology: it's like erasing heads on analog tape recorders... So if you really need it, it might be better to go for regular solenoids driven with a (variable) DC offset current. It can be done either by using coils with separate windings or else as shown below.

6.- Plucked string instruments

Here there are different possible approaches. The mechanism as found in harpsichords does not lend itself very well to expression control and hence should by bypassed in the context of this survey. To implement a plectrum with precise control of the striking force (speed), a stepping motor driving a rotating plectrum may be used. If the plucking has to be repetitive (such as in mandolins), a small DC motor can be used as well. For dynamic control it then should be mounted on a motor- or soft-shift solenoid driven slide. Rotary solenoids can also be considered here. A type produced by Magnet-Schultz Ltd. is shown on the picture. Return springs can be added if required.

Overcoming the MIDI constraints and bottlenecks

1. The 7-bit constraint

As I have mentioned in the treatment above, for most implementations of expression control, we use precise time controlled pulses. The minimum resolution of the timers to be used is 16 bits. Now, standard midi is basically a 7-bit protocol. Thus using midi it is impossible to offer the finest possible resolution to the user. We have to remap the relevant section of the useful range into the 7 bit range offered by Midi.

The procedure to do so starts with determining the minimum pulse width wherewith the valve starts opening. (tmin). For percussive instruments, you should of course take the minimum value required for the hammer to just strike the object. Next determine the shortest pulse width wherewith the valve fully opens and wherewith a further increase of pulse duration does not make a perceivable difference anymore. (tmax) Note that generally these limit values will be different from device to device, and form note to note. They may also shift a bit over time due to wear of the mechanics. The useful timing range is now tmax-tmin. Now it would seem easy enough to just remap this range onto the 1-127 range covered by midi. Generally speaking this almost never leads to good results. Neither the solenoids nor our ears have linear characteristics. The mapping should be described using at least a second degree equation. To find out what curve suits best a smooth mapping of the range, we use simple curve fitting software (Gaussfit). To bring this to a good end, one should determine some 5 intermediate points, starting with the middle of the range.

The equation found should then be implemented in the firmware of the microcontroller and here obviously the use of lookup tables will impose itself. First of all, because most microprocessors are integer math based and secondly because modern microprocessors have more than enough memory available to store the lookups.

For ease of maintenance, we invariably implement sysex commands on our robots, allowing the experienced user to upload different lookup tables. To select between these, midi program change commands are implemented.

2. The midi bottleneck

Midi as a protocol should now be considered outdated. Mostly because it is way to slow to control large instrumental setups when many expression controllers have to be send. As an alternative, preserving some compatibility, UDP/IP can be used. This topic is treated in another short article. However, MIDI is still the lingua franca for musical instrument control and therefore we have spend quite some effort in using it up to its extreme limits. In our robot orchestra we have so far some 45 machines each listening to one or two midi-channels. It will be clear that this dictates the use of a multiport midi device, our favorite (although far from perfect) being the Midiman 8x8. However, even though this gives us 128 midi channels, this means that each machine has to handle all the interrupts generated by the midiflow for many other machines. This invariably leads to glitches, lost bytes and timing problems. To overcome these it is a smart idea to build dedicated midi filters outputting only the midi data relevant for a specific machine. Here is a possible approach, using a fast dsPIC 30F3010:

The dip-switch is used to select the midi channel to be selected for pass-through. The jumper (JP1) allows you to pass also the first adjacent channel. This was done in the perspective of our quartertone automates, since these use two midi channels.

Feedback and sensing

If the hardware is well designed, precise and reliable, it is generally better to go without any kind of force-, position-, pressure... sensing devices. Automated regulation of whatever parameter always comes with a price tag, not only financially, but more important, it goes at the detriment of timing precision as well as reliability. One of the most common mistakes amongst automate builders and robot designers in the area of musical instruments, is in trying to overcome deficient or poorly build hardware by adding sensors and regulating loops in software. You invariably end up with a shaky and unreliable construction, plagued by under- and overshoot. However, there are many cases in good automated instrument design where you have almost no choice. We give a few examples, before we delve a little further into the technologies and components available.

1.- Automated rototom playing robot: <Rotomoton>

Here we used large stepping motors to rotate the central spindle of the drums in order to tune them. As we did build the robot, it came out that the useable traject shifted quite a bit with the time of use. Also the traject came out to be highly temperature sensitive, this due of course to the properties of the Mylar membranes on the drums. For these reasons we provided each motor driven drum with sensors such that the beginning and end positions can be set. The microcontroller automatically adjusts the number of steps according to the signals from the sensors. First we used microswitches, but these had too much hysteresis and so we replaced them later with non-contact proximity induction sensors by Pepperl+Fuchs.

2.- Automated bass accordion : <Bako>

In this robot, the bellows are driven by a trapezoidal thread driven by a strong DC motor. In this case we needed to provide end-sensors but also a PID regulating loop for the pressure. The rotational speed of the motor has to adapt for the air consumption, this being a function of both the number of notes played and their pitch. In this case we used a bipolar Freescale low pressure sensing device to measure the actual air under- or overpressure inside the bellows. We could have gone in this case without sensors and regulating loop, but that would entail really hugh lookup tables for all the different combinations of notes versus dynamic level. Memory constraints as well as the enormous work to fill the lookups with correct values pushed us into the direction of a PID regulating loop.

3.- Quartertone organ : <Qt>

In this case, we mounted two flap valves in the two windchest channels to modulate the air flow. The valves are driven by stepper motors. The problem was that we could not guarantee that when the motor stops, the valve position is known exactly. Thus we mounted Melexis sensors on the valve axis, such that the actual position of the valves can be read out by the microprocessor anytime. This way it became possible to use the valves as reliable expression controls on the organ.

As can be seen from the examples, the PID regulating loop with sensors was always added because of inherent problems with the electromechanical devices (slip on stepper motors, temperature changes) or limitations of the microcontrollers. This is not to say that we would tend to reject autoregulation, but only that this technology should only be called upon when all other possibilities are exhausted.

A good example candidate for autoregulation offers the latest design of our piano Vorsetzer. The existing model, baptized <pp2> works to great perfection, but... it needs specific look up tables for each grand piano on which you want to use it. Pianos do show great variation in dynamic range, in touch-mapping on loudness as well as in key stiffness and repetition speed. To automate the generation of lookup tables, we started a project whereby the vorsetzer-piano can become selfregulating. As sensors we use a normal acoustic microphone, to measure the sound output versus key-force input combined with a measurement of the counterinductive voltage generated over the solenoids when activated. Our experiments have shown that this induction spike (normally always damped with a diode) is to a certain extend a function of the mechanical resistance the anchor meets when pushing a key down. Thus it is related to the 'touch' of the piano. In this project, the automation is used prior to the actual concert, just like it is the case with a human professional pianist, who also will insist to play-in on a given piano prior to a public concert.

Under no condition would we use this technology to steer the piano in the course of a performance, because the time required for the regulating loops to adjust well enough in real time, would make the Vorsetzer sound very sluggish rather than responsive.

[To be completed...]

Notes:

(1) Pretty complete catalogue of all our automated and other instruments.

(2) Composers guide to the M&M robot orchestra

(3) More texts by the author with regard to robotics and sensors

(4) Software utility for the calculation of conical and flat valves (link to the source code). The compiled program (compiled with the PowerBasic Console compiler) is here, and requires our math library g_indep.dll

(5) This survey treats only the mechanics and control of expressive possibilities in automates, the expressive use of such automates in music is a completely different story. That story is told in part in my composition teaching, my composition software projects (GMT) and in my articles on sensor technologies used to translate expressive properties of human gesture into data that can be used to control automates and other sound generating devices.

This survey was first written and published in 1987. It undergoes continuous updates as our research and experience into this area expands and progresses. Feel free to reference it, but please always link to the original source.

Credits & Acknowledgments:

The firmware for most of the Microchip PIC based controllers in the robots and automates described here was developed in close collaboration with Ing.Johannes Taelman. The MPLAB platform, provided by Microchip, was used.

Many experiments on electromagnetic devices have been carried out with the assistance on Ph.D. student Troy Rogers. Shaking devices were researched in collaboration with Ph.D. student Laura Maes.

Part of the research results presented here where obtained thanks to the support of Hogeschool Ghent, where I am currently employed as a full time post doctoral researcher, paid 70% of a normal wage however..

Thanks to the Logos Foundation, funded by the Flemish Government, where my instrument building workshop and electronic research lab are based. They also provide me with all facilities to bring this research to artistic and presentable results.

Manufacturers of electromechanical devices treated here:

BENADE, Arthur H., "Fundamentals of Musical Acoustics", 2nd edition, ed. Dover Publications Inc, New York,1990 [ISBN 0-486-26484-x]

BOWERS, David Q., "Encyclopedia of automatic musical instruments", Vestal Press, New York, 1972 [ISBN 0-911572-08-2]

BROOKS, Rodney A., "Flesh and Machines. How Robots Will Change Us", ed. Pantheon Books, NY 2002

BUCHNER, Alexander "Mechanical Musical Instruments", ed.Greenwood Press, Westport, Connecticut, 1978. [ISBN 0-313-20440-3]

DUFFIN, William John, "Electricity and Magnetism", ed. W.J.Duffin Publishing, Cottingham East Yorkshire, 2001. [ISBN 0-9510438-1-1]

HAYT, William H. "Engineering Electromagnetics", ed. McGraw-Hill Inc, Tokyo, 1974 [ISBN 0-07-027390-1]

ORD-HUME, Arthur W.J.G, "Barrel Organ", ed. George Allen & Unwin, London,1978 [ISBN 004789005-3]

Last revision: March 9, 2010

dr.Godfried-Willem Raes