Using an Orchestra as a Computer: Orchestral Processing Unit

Alexis Kirke
13 min readJun 13, 2023

Can a live chamber orchestra perform computations?

Orchestras and Non-musical Processes

This article presents a composition in which an orchestra carries out a non-musical process, specifically a form of computation. Most orchestral projects involving non-musical processes are musification (i.e. using more traditional elements of sound such as melody, harmony, and rhythm). The composition Orchestral Processing Unit discussed here is neither. Orchestral Processing Unit is an instantiation of a computation.

The process used to generate the acoustic orchestral performance in this paper is computation. What is unusual about this is that the computation itself in a musical substrate. The motivation and implementation of this substrate is now explained.

Musical Processes with Non-musical Function

Virtual Machines have existed for decades, such as the Java Virtual Machine, and those which allow users to run a non-native OS. Server virtualization is common. It sacrifices processing power in the virtualization process, but this is outweighed by the advantages.

The virtual machine referred to in this article does not reduce computation time. It is more in the philosophy of the graphical user interface (GUI) interaction paradigm, also known as Window, Icon, Mouse and Pointer (WIMP) interfaces. WIMP did not increase processing power when added. In fact they reduced it because of the requirements for bitmapped screens and windows. However many tasks now would be unfeasibly slow for us without WIMP. Changing the mode of human-computer interaction opens up opportunities for increasing the usefulness of computers, even though it uses up processing power.

Given the growth in virtual computing, unconventional computing has the opportunity to greatly expand its possible modes, limiting computation only by imagination; hence the field of Unconventional Virtual Computation (UVC) (Kirke et al. 2015). Previous work using simulations to run unconventional computation was to simulate a hardware or wetware system, not to the make fundamental simulation the substrate.

One mode of UVC that could provide benefits is Human-Computer Interaction by Replacement (HCI By replacement, or HBR). HBR is an approach to unconventional virtual computing that combines computation with HCI, a complementary approach in which computational efficiency and power are more balanced with understandability to humans. Rather than ones and zeros in a simulated circuit, have the user-interface object itself; e.g. if you want data to be audible, replace the computation substrate by melodies. Some forms of HBR may not be implementable in hardware in the foreseeable future, but current hardware speeds could be matched by future virtual HBR machines.

The focus here will be on forms of HBR in affective computation or in computation that has an affective interpretation. It has been shown that affective states (emotions) play a vital role in human cognitive processing and expression. As a result, affective state processing has been incorporated into robotics and multi-agent systems. Representing and computing with affective states is an active area of research.

The dimensional approach to specifying emotional state is one common approach. In many emotional music systems, two dimensions are used: valence and arousal. In that model, emotions are plotted on a graph with the first dimension being how positive or negative the emotion is (Valence), and the second dimension being how intense the physical arousal of the emotion is (Arousal). For example “Happy” is high valence, high arousal affective state, and “Stressed” is low valence, high arousal state.

To a degree these can be represented musically. There have been a number of questionnaire studies done which support the argument that music communicates emotions and previous research has suggested that a main indicator of valence is musical key mode. A major key mode implies higher valence, minor key mode implies lower valence. For example the overture to Mozart’s The Marriage of Figaro is in a major key whereas the melancholic first movement of Beethoven’s Piano Sonata №14 (Moonlight) is in a minor key. That research also highlights that tempo is a prime indicator of arousal, with high tempo indicating higher arousal, and low tempo — low arousal. For example: compare Mozart’s fast overture above with Debussy’s major key but low tempo opening to “Girl with the Flaxen Hair”. The Debussy piano-piece opening has a relaxed feel, that is a low arousal despite a high valence. This is a rather coarse analysis, and further details can be found in, for example [Kirke thesis paper]

PMAP Representation

In the HBR protocol Pulsed Melodic Affective Processing (PMAP) (Kirke and Miranda 2015) the data stream representing affective state is a stream of pulses transmitted at a variable rate (c.f. the variable rate of pulses in biological neural networks in the brain). The pulse rates encode information (neuroscientists often use audio probes to listen to neural spiking). In PMAP this pulse rate specifically encodes a representation of the arousal of an affective state. A higher pulse rate is essentially a series of events at a high tempo (hence high arousal); whereas a lower pulse rate is a series of events at a low tempo (hence low arousal).

Additionally, the PMAP pulses can have variable heights with 10 possible levels. For example 10 different voltage levels for a low level stream, or 10 different integer values for a stream embedded in some sort of data structure. The purpose of pulse height is to represent the valence of an affective state, using pitch and keymode. There is not space to explain this here and it is not used in the example covered in this article; for more details and examples that use keymode and tempo, see (Kirke and Miranda 2015).

PMAP provides a method for the processing of artificial emotions. PMAP data tempo can be generated directly from rhythmic data and turn directly into rhythmic data or sound. Thus rhythms such as heart rates, key-press speeds, or time-sliced photon-arrival counts can be directly turned into PMAP data; and PMAP data can be directly turned into music with minimal transformation. PMAP has been applied and tested in a number of simulations. More details are available in previous UVC and PMAP publications mentioned:

a. A security team multi-robot system

b. A musical neural network to detect textual emotion

c. A stock market algorithmic trading and analysis approach (Kirke and Miranda 2012)

d. A system that keeps a photonic quantum computer in a state of maximum entanglement.

Due to lack of space, only (a) will be briefly described. The security robot team simulation involved robots with two levels of intelligence: a higher level more advanced cognitive function and a lower level basic affective functionality. The lower level functionality could take over if the higher level ceased to work. A new type of logic gate was designed to use to build the lower level: musical logic gates. PMAP equivalents of AND, OR and NOT were defined, inspired by Fuzzy Logic.

The PMAP versions of these are respectively: MAND, MOR and MNOT (pronounced “emm-not”), MAND, and MOR. So for a given stream, a PMAP segment of data can be summarized as mi = [ki, ti] with key-value ki and tempo-value ti. The definitions of the musical gates are (for two streams m1 and m2):

MNOT(m) = [-k,1-t] (1)

m1 MAND m2 = [min(k1,k2), min (t1,t2)] (2)

m1 MOR m2 = [max (k1,k2), max (t1,t2)] (3)

It was shown that using a circuit of such gates, PMAP could provide basic fuzzy search and destroy functionality for an affective robot team. It was also found that the state of a three robot team was human audible by tapping in to parts of the PMAP processing stream, and listening to keymode and tempo.

PMAP Half-Adder

The main focus of this article is the application of PMAP to a dynamic re-configurable orchestral performance. “Orchestral Processing Unit” (OPU) is a chamber orchestra performance using the tempo encoding of PMAP. Because of an orchestra’s size and slow reaction, and the need for a performance to have enjoyable coherent development, it was decided that an orchestral performance using PMAP would have to focus on the PMAP process, rather than the PMAP process controlling something else. Furthermore it was desired to make the process a form of computation that was understandable to the audience in a more intuitive way. So using an orchestra as an affective computation system would be too complex for the audience to understand and therefore experience. Thus a more traditional form of computer circuit was implemented in orchestral PMAP: a two-bit addition system. Such a system, when looped through twice, allows the addition of two numbers between 0 and 3.

The orchestral PMAP adder is implemented using two passes through a half-adder. A half-adder is implemented in traditional computation as seen in Figure 1. S is the sum of binary numbers A and B. If the resulting sum is greater than 1 bit in size, it is set to 0 and the carry flag C is set to 1. A half-adder allows two numbers between 0 and 1 to be added as shown in Figure 1. This is why two passes are needed through the PMAP half-adder: to allow numbers between 0 and 2 to be added.

Fig. 1. Circuit and truth table for a half-adder (Circuit attribution: inductiveload [Public domain], from Wikimedia Commons. Truth Table © 2019 Alexis Kirke)

The most musically controllable way of making a half-adder in PMAP is to focus on two tempo values, and remove the pitch dimension. It is problematic to use the pitch dimension in a musical composition as having multiple lines playing in the orchestra in which one is major and one is minor will usually lead to unpleasant dissonance.

For OPU a logic level is represented by a monophonic musical phrase. If the phrase is played with its original note lengths it is considered to be logical 1, if played at half its original note length, it is considered as logic 0. Then the AND gate in Figure 1 can be replaced by a MAND gate. The XOR gate is replaced as followed. XOR is constructed in Boolean logic using AND, OR and NOT gates:

Output = (A AND NOT(B)) OR (NOT(A) AND B) (4)

Because the MAND is functionally a two dimensional Fuzzy AND gate, and because the tempo in OPU has only two states: a minimum and a maximum, and because in the extreme a Fuzzy AND is identical to an AND gate: the AND gate can be replaced by a MAND gate. Similarly for the OR and NOT gates. Thus we have:

Output = (A MAND MNOT(B)) MOR (MNOT(A) MAND B) (5)

Equation (2) is then implemented in the orchestra as shown in Figure 2.

Fig. 2. PMAP half-adder, looped through twice (© 2019 Alexis Kirke)

Procedure in Performance

The processing is implemented as follows. Melodies — call them MA and MB — represent inputs A and B in Figure 1. For MA a logical 1-value is represented by MA played with its defined note lengths. A 0-value is represented by MB played with its note lengths at half the note length of MA — i.e. the melody is played at twice the speed, and takes half as long to play. Figure 3 shows the two possible states for one of the clarinets during a particular part of the calculation / performance. The upper line is logic 1 and the lower line (with twice the note lengths) is logic 0 (only first half shown).

Fig. 3. Examples of logic 1 and logic 0 from part of the clarinet score (© 2019 Alexis Kirke)

Each group of instruments in Figure 2 has a paper score showing, at every point in the performance, the two possible phrases they could be playing: the maximum tempo phrase, and same phrase played at half the tempo. The conductor indicates to players when they should play and whether they should play the top line or the bottom line.

Note — due to the size of the orchestra, and the time and style constraints — the conductor performs the carry calculations and storage — these are not included in Figure 2. As will be seen later, the conductor is given a rule to say that if both the trombone and viola represent a 1-bit, then the conductor should make a note of this. The existence or non-existence of these notes will effect which musical lines they indicate to the players later in the calculations.

To see how one of the two half-adder calculations are done, suppose the conductor wants to input a 1-value to the top input in the circuit in Figure 2. The piano is the input, output and storage register of the adding process (it does not perform any processing). So the conductor will indicate the piano should start playing MA. Consider the top line of processing in Figure 2, which can be written as:

Horn = Piano (6)

Trumpet = NOT(Horn) (7)

The conductor next indicates to the horn that they should play what the piano (the input register) is playing from their score. The horn plays MA as well. The next stage is the trumpet playing the MNOT of MA. The definition of MNOT is that the maximum tempo becomes the minimum tempo, and vice versa (equation 1). So the conductor will indicate to the trumpet to start to play MA at double note length (half the speed). The next musical gate along this line of Figure 2 is the MAND. Based on equation (2), the MAND will output the minimum tempo for all inputs, unless both of its inputs are maximum tempo. This MAND gate has inputs from the percussion and the trumpet. Given the trumpet is playing minimum tempo, the MAND output — the Trombone — will be told by the conductor play the phrase at a minimum tempo. The whole trombone score is shown in Figure 4.

Fig. 4. Single Instrument Score Example — Tenor Trombone (© 2019 Alexis Kirke)

The conductor was given a “verbal score” reproduced in Figure 5 to perform the calculation. The first column — the segment column — refers to markings on the conductor’s full score. This score is too large to include clearly, but an excerpt is shown in Figure 6. Segments S1 to S29 perform two XOR calculations. S31 is actually a special case for the conductor instruction set, as shown in Figure 7. This involves combining carries to create the final sum. These carries are noted down by the conductor during the playing of Segments S4 and S19. The pianos are also a special case. As the calculation / performance, continues, the pianos act as registers — storing intermediate calculation results. Thus the piano can store two bits of data: in the left and the right hand.

Looking at Figure 5, the four bits are input at segments S1, S3, S16 and S18. S1 and S3 are the least and most significant bit for the first input; S16 and S18 are the least and most significant bit for the second input. After the conductor guided through to S31, the piano plays the three output bits.

Performing a Calculation in Concert

The performance took place at the Peninsula Arts Contemporary Music Festival, performed by the Ten Tors Orchestra and conducted by Simon Ible. A recording of the PMAP movement is available online (, starting at 6m15s. The two numbers added in the performance were 3 and 3. These were sent to the conductor around 48 hours before the performance (he had had the re-configurable score for a few weeks before this). The binary values were 11 and 11, so what was actually sent to the conductor were the instructions to get the piano to play the top line of the score at each of its four input points. The result of the sum meant that two carries of 1 were stored by the conductor, and this led to him instructing the pianist to play certain chords at the end whose constituent lines represented binary 110 — which is 6 in decimal, the result of 3+3.

The orchestra succeeded in calculating accurately. The author did not attend the rehearsals, providing only scores and instructions. The accurate performance indicated the musicians’ ability to interpret a computation with the conductor. The fact the conductor asked for the calculation inputs to be provided 48 hours before the performance indicates that he wanted to limit the degree to which the musicians had to make live choices in responding to him during live performance. However he was never given the “hard” score — i.e. the actual fixed musical score for a 3+3 calculation, but only the protocol and the inputs.

Is such a performance is enjoyable to an audience, or just a curiosity? Apart from positive informal audience feedback there was a positive review by a music critic in Drowned in Sound:

“Binary code is reinterpreted as musical melodies, and the Ten Tors orchestra pulsate in a powerfully pretty and pointillistic Reichian performance piece…Orchestral Processing Unit holds its own as an intensely beautiful experience.”

This is one person’s opinion but supports the anecdotal feedback from the general audience.

Fig. 5. Conductor Verbal Score (© 2019 Alexis Kirke)
Fig. 6. Conductor Score Excerpt (© 2019 Alexis Kirke)
Fig. 7. Alternative Conductor Instructions for Final Calculation (© 2019 Alexis Kirke)


This article introduced the Unconventional Virtual Computation protocol PMAP. PMAP has been demonstrated previously to have non-musical functionalities. Orchestral Processing Unit (OPU) demonstrated OPU and examined if it could be used artistically. OPU instantiated two half-adders using tempo-based PMAP across orchestral instruments. A protocol was provided to the conductor, describing how to control the orchestra live as a PMAP circuit, and the inputs (3 and 3) were provided 48 hours before the performance. The orchestra was able to perform the calculation, whilst playing well, and the resulting performance was judged favourably by observers.

A. Kirke et al. “A Hybrid Computer Case Study for Unconventional Virtual Computing,” International Journal of Unconventional Computing 11, №3–4, 205–226 (2015).

A. Kirke and E. Miranda, “Pulsed Melodic Affective Processing,” Simulation 90, №5, 606–622 (2015).

A. Kirke and E. Miranda, “Application of Pulsed Melodic Affective Processing to Stock Market Algorithmic Trading and Analysis,” in Proceedings of 9th International Symposium CMMR 2012 (London: Springer, 2012).



Alexis Kirke

Alexis Kirke is a screenwriter and quantum/AI programmer. He has PhDs from an arts faculty and from a science faculty.