Demonstration 4
Markov Chaining

Introduction

Demonstration 4 provided the practical component for Chapter 6: “Conditional Selection I — Markov Chains” of my unpublished textbook on composing programs. It illustrates how Markov Chains can be employed to compose a piece of music. Markov chains are consistent with the selection pattern of G.M. Koenig's Project Two, to the extent that there is a sequence of events such as phrases or notes, and that even attributes such as duration and pitch are being drawn from a supply. In Markov's scenario, supply elements are called “states”, and there is an additional aspect of conditionality: The likelihood of selecting state i for event j depends on the state previously selected for event j-1.

The piece may be regarded as a chain of rhythmic units in the sense of “unit” used to explain Demonstration 2 and Demonstration 3.

Features of the Markov Matrix

A Markov transition matrix is a two-dimensional array of probabilities:

p1,1   p2,1   …   pi,1   …   pN,1
p1,2   p2,2   …   pi,2   …   pN,2
  ⋮   ⋮   ⋮
p1,j   p2,j   …   pi,j   …   pN,j
  ⋮   ⋮   ⋮
p1,N   p2,N   …   pi,N   …   pN,N

Where N is the number of states, where each pi,j ≥ 0, and where for each row of the matrix (that is, for each j),

p1,j + p2,j + … +  pi,j + … + pN,j  =  1

The fact that each row sums to unity is why I use the term probability in favor of the less restrictive term weight.

Expanding from a one-dimensional distribution to a two-dimensional transition matrix adds a full order of complexity into the designer's task. Exercising control over the succession of choices means that among the multiplicity of available sequences, the matrix designer can use high probabilities to emphasize some sequences, use low probabilities to deemphasize other sequences, or use null probabilities to exclude still other sequences entirely.

Another feature is the ability to control the propensity for the chain to linger on one specific state. Such control is exercised through waiting probabilities, which are positioned along the diagonal of the matrix. Thus the waiting probability pi,i controls how often state i transitions to itself. Each waiting probability has an associated waiting count Ci,i. This waiting count gives the average number of times the chain will locally repeat state i before moving away. The two values pi,i and Ci,i have the following relationship:

Ci,i  =   1
1-pi,i
 
pi,i  =   1
1-Ci,i

Compositional Directives

The structure of Demonstration 4 is simpler than in previous demonstrations because there is no level of organization intervening between the individual notes and the piece as a whole. Each of the four attributes characterizing a unit — duration, articulation, register, and degree — proceeds independently of the others.

Articulation

Articulation is controlled through a percentage indicating the probability that an iteration of the composing loop will produce a note instead of a rest. There are four values: 10%, 17%, 29%, 50%. The waiting counts in each case will be 91%, which means the chain will linger upon one of these values for an average of

1
1-p
  =   1
1-0.91
  =  11.11

notes and rests before transitioning to a different state.

Transitions to new states emphasize the sequence 10% → 50% → 17% → 29% → 10%. On-track transitions have 5% probability, while off-track transitions have 2% probability. Since

91% + 5% + 2% + 2% = 100%,

the requirement of summing to unity is confirmed.

Average Note Duration

There are four average durations available to any note. Expressed in sixteenths, the averages for notes are D1=2, D2=3, D3=5, and D4=9; the averages for rests are half as much: 1, 1.5, 2.5, and 4.5.

Thus we can make each average duration persist over a whole-note duration by selecting Ci,i so that Ci,iDi=16, or

pi,i  =  1 -  Di
16
p1,1  =  1 -  2
16
  =  0.875
 
p2,2  =  1 -  3
16
  =  0.8813
 
p3,3  =  1 -  5
16
  = 1 0.688
 
p4,4  =  1 -  9
16
  =  0.438

The waiting probabilities for all four values have been calculated so that the chain lingers on each value for approximately two measures before moving on to a new value. Table 1 illustrates how the average length over which a value holds sway may be computed as the product of the average duration itself and of the waiting count, which is the reciprocal of the waiting probability. We should recognize that control over how long an average duration holds sway is very loose, since both the waiting count and the individual durations are random. The number of random mechanisms affecting a process is often known as the degrees of freedom. The more degrees of freedom, the more the individual states of the process deviate from the norm.


Figure 1: Profile of Demonstration 4.

Figure 1 graphs the evolutions of average duration, articulation, and register in Demonstration 4. The rates of these evolutions directly result from the waiting probabilities controlling how individual states transition to themselves. Since waiting probabilities for articulations are high in Figure 1 (a), articulations evolve very slowly; by contrast, the waiting probabilities associated with the unison in Figure 1 (d)) are consistently zero, so no two consecutive notes repeat the same degree.

Also pertinent to the medium-term behavior are the stationary probabilities shown in Figures 2 (a-c). Stationary probabilities1 appear for average duration in Figure 2 (a); for articulation in Figure 2 (b); and for register in Figure 2 (c).

   
Figure 2 (a): Stationary probabilities for average durations.   Figure 2 (b): Stationary probabilities for articulations.   Figure 2 (c): Stationary probabilities for registers.

Register

Note Rhythms

The Markov chains controlling average note durations, articulations, and registers produced graphs which lingered for some numbers loop iterations before transitioning to the next value. This happened because the waiting probabilities — those located along the diagonal of the matrix — were nonzero. By comparison, the process used to decide whether a loop iteration should generate a note or a rest prescribes total certainty that a rest will never transition away to another rest but only a level of probability that a note will transition to another note. The process is in fact a two-state Markov chain, where the probability of transitioning from a note to another note is determined by the articulation percentage described above.

RestNote
Rest01
Note1-pp

Table 1: Note/Rest transition matrix.

Note durations are selected according to the negative exponential distribution, but applying John Myhill's procedure to limit the ratio of maximum to minimum durations to 8.

Note Degrees

The progression of chromatic degrees reduces from a second-order Markov chain of chromatic positions to a first-order Markov chain of chromatic intervals. Figure 3 details the relative weights associated with each consecutive pair of intervals. Though intervals range from 1 to 11 (unisons are excluded), ‘wrap-around’ arithmetic keeps the current degree always within the range from 1 to 12.


Figure 3: Stylistic matrix for Demonstration 4 — Each entry depicts a pair of two rising chromatic intervals with the middle degree fixed arbitrarily at B. The number of semitones in an interval corresponds to the extent of chromatic displacement.

The approach just described resembles the INTERVAL feature of G.M. Koenig's Project Two. Such matrices will be frequently employed in subsequent Demonstrations, where they will be designated as stylistic matrices. The current matrix acts both to enforce absolute constraints and to promote desirable tendencies, in this case to encourage a consistently dissonant style.

The language of constraints is inherently dogmatic. Practices are either correct or incorrect. Just bear in mind that the dogma applies only to this particular composing program. These Demonstrations in no way suggest that adherence to universal standards will improve the quality of your music. Artusi lost that argument to Monteverdi centuries ago. Rather, the take-away lesson from these Demonstrations is that if there certain patterns run contrary to the style your program is intended to generate, and if you are able to formulate constraints forbidding these patterns, then there are methods which you can use to enforce such constraints, that some methods are more efficient than others, and that where these methods occur in the decision-making logic can also make a difference.

Stationary probabilities calculated from Figure 3 appear in Figure 4.


Figure 4: Stationary probabilities for chromatic intervals.

A transcription of the musical product appears in Figure 5.


Figure 5: Transcription of Demonstration 3.

Implementation

The explanations to follow focus variously on four attributes of musical texture: average note duration, articulation, register, and chromatic degree. The purpose is to tease out the strands of code that affect a particular attribute, and thus to illustrate Markov chaining in play. The explanations are peppered with line numbers, but you are are by no means expected to chase down every line of code. Rather, you should follow through with line numbers only when you have a specific question that the narrative is not answering.

The source code for program DEMO4 appears as Listing 1. This program implements the one-tiered structure described above as a single composing loop (lines 56-81) which iterates for each note or rest and which continues until sixty measures of 2/4 time have been filled out (line 78). At the beginning of an iteration, the program chooses an average duration, expressed in sixteenths, and a rest/play probability. Line 59 executes a random branch to determine whether to compose a note (lines 61-69) or a rest (lines 71-75).


Listing 1: Program DEMO4.

Average Note Duration

Variables and arrays pertaining to average duration of notes (equivalently, average tempo) have names ending with AVG. The average duration of rests is half as large.

Articulation

Variables and arrays pertaining to articulation have names ending with ART. Articulations are expressed as the probability that a rhythmic unit will serve as a rest.

Register

Variables and arrays pertaining to register have names ending with REG. Registers are expressed as the lowest position in a twelve-semitone gamut.

Chroma

Variables and arrays pertaining to chromatic intervals have names ending with TVL. Chromatic intervals are expressed as integers from 1 (minor second) to 11 (major seventh).

The loop completes in line 80 with a call to subroutine WNOTE. This version of WNOTE takes as argument #5 the note/rest flag REST. If REST is true, register (argument #3) and degree (argument #4) are ignored. If REST is false, WNOTE uses the library subroutine IOCT to place IDEG in the register indicated by IREG.

Executing a Markov Transition

The library subroutine MARKOV3 implements random selection with probabilities conditioned by previous choices. Calls to MARKOV require five arguments:

  1. RESULTMARKOV selects a state (supply value) and returns the value in this location.
  2. VALUE — Supply of state values. Duplicate values are not permitted. VALUE must be an array whose dimension in the calling program is NUM (argument #5 below).
  3. TRANS — Array of transition probabilities. Trans must be a real array in the calling program whose duration is NUM by NUM. The probabilities must be non-negative, and every row of the array must sum to unity.
  4. IDX — Index to previously-selected state. IDX must be an integer in the calling program. This argument provides the element of feedback. Since FORTRAN passes subroutine arguments by address, each call to MARKOV updates variable IDX to the newly selected state.
  5. NUM — Number of states. NUM must be an integer in the calling program.

MARKOV first calculates a row offset I=NUM*(IDX-1). This means that the upcoming state selection will be governed by the probabilities TRANS(I+1), TRANS(I+1), … TRANS(I+NUM). According to the restrictions given for array TRANS above, these probabilities must sum to unity. MARKOV then uses the methods of the library subroutine SELECT to select the new value of IDX, where the ‘chooser’ value R= is unscaled. MARKOV completes by setting RESULT=VALUE(IDX).

References

  1. The procedure for calculating stationary probabilities is explained in Automated Composition, Chapter 6, pp. 6-8 to 6-9.
  2. Understand that the note/rest mechanism for Demonstration 3 essentially matches what happened both in Demonstration 2 and Demonstration 3. These previous implementations were in fact doing Markov chains for this particular mechanism.
  3. Subroutine MARKOV is presented in Automated Composition, Chapter 6, pp. 6-16 to 6-18.

© Charles Ames Original Text: 1984-11-01 Page created: 2017-03-12 Last updated: 2017-03-12