Polymetric structures

Polymetric expres­sions are the basic rep­re­sen­ta­tion mod­el of musi­cal data in Bol Processor. The word is a mix of polypho­ny and polyrhythm, the for­mer evok­ing super­posed streams of musi­cal events, and the lat­ter a met­ric adjust­ment of their durations.

This page illus­trates the syn­tax of sim­ple expres­sions and their inter­pre­ta­tion by the poly­met­ric expan­sion algo­rithm exposed in paper Two algo­rithms for the instan­ci­a­tion of struc­tures of musi­cal objects (Bel 1992). This process may get extreme­ly com­plex since a whole musi­cal work — e.g. Beethoven’s Fugue in B flat major — is han­dled by Bol Processor as a sin­gle poly­met­ric structure.

In this tuto­r­i­al, sim­ple notes (“C4″, “D4″ etc.) are used fol­low­ing the “English” con­ven­tion. All time-setting process­es could be illus­trat­ed with sound-objects or sim­ple notes in alter­nate con­ven­tions: “Italian/Spanish/French” or “Indian”.

Symbolic versus physical duration

Music nota­tion sys­tems (for humans) make use of sym­bol­ic rather than phys­i­cal dura­tions. Their units are beats rather than (milli)seconds.

In west­ern con­ven­tion­al music nota­tion, notes and rests are rep­re­sent­ed with par­tic­u­lar signs indi­cat­ing their rel­a­tive durations.

For instance, if the time sig­na­ture is 3/4, we will have 3 quarter-notes (crotch­ets) in a bar (see pic­ture). A half-note (min­im) lasts twice longer than a quarter-note in the same con­text. Other rel­a­tive dura­tions are expressed in the same manner.

To get the phys­i­cal dura­tion of a note we need an addi­tion­al piece of infor­ma­tion: the metronome val­ue, for instance “mm = 100″ mean­ing 100 beats (quarter-notes) per minute.

A metronome val­ue (by default 60 bpm) is declared in the set­tings file of a Grammar or Data page. With this set­ting, note “E4″ on a Bol Processor score rep­re­sents a “E” of the 4^th octave played in 1 beat with phys­i­cal dura­tion 1 second.

This con­ven­tion extends to sound-objects labelled with arbi­trary names whose default dura­tions are set by the streams of MIDI events or sequences of Csound instruc­tions they are made of. Mapping sym­bol­ic to phys­i­cal time for the per­for­mance of struc­tures of sound-objects (with their met­ric and topo­log­ic prop­er­ties) is a sophis­ti­cat­ed process accom­plished by a time-setting algo­rithm. A prac­ti­cal exam­ple is dis­cussed on page Interactive impro­vi­sa­tion with sound-objects.

Polymetric expression

Typical forms of poly­met­ric expres­sions are:

• field 1, field2 or {field 1, field2} indi­cat­ing that field1 and field2 should be super­posed and the total sym­bol­ic dura­tion should be adjust­ed to that of field1;
• field1 • field2 or {field1 • field2} indi­cat­ing that field1 and field2 should be in sequence and the sym­bol­ic dura­tion of each field should be adjust­ed to that of field1;
• {expres­sion} is equiv­a­lent to expres­sion.

Curled brack­ets ‘{‘ and ’}’ are required to pro­duce mul­ti­level expressions.

➡ Periods notat­ed as bul­lets ‘•’ on Data and Grammar win­dows are con­vert­ed to plain peri­ods before send­ing to the con­sole, due to its rejec­tion of some Unicode signs.

For instance, {C4 D4, E4 F4 G4, E5} yields the fol­low­ing time struc­ture with a metronome set to 60 beats per minute:

The use of the first field to set the total dura­tion is high­light­ed by the fol­low­ing exam­ples in which fields appear in a reverse order:

Rests (silences) may be notat­ed “-” for single-unit rests, or with inte­ger num­bers and ratios. The fol­low­ing shows a single-unit rest and a more com­plex one last­ing 2.5 beats:

Polymetric struc­tures can be mul­ti­level, for instance:

The same rules of time-setting apply to sequences in which com­mas are replaced with peri­ods. For instance:

Superpositions and sequences can be com­bined (even in mul­ti­level expres­sions) such as:

Undetermined rests

Undetermined rests are a pow­er­ful fea­ture of poly­met­ric expres­sions used to avoid uneasy cal­cu­la­tions. The poly­met­ric expan­sion algo­rithm cal­cu­lates (sym­bol­ic) dura­tions pro­duc­ing the least com­plex expression.

They may be notat­ed “…” or “_rest” in Data or Grammars.

➡ Since the con­sole does not rec­og­nize this Unicode sym­bol, it is rewrit­ten as “_rest” by the PHP interface.

Let us start with a triv­ial exam­ple. In {C4 D4 E4, … F4 G4}, unde­ter­mined rest “…” will be replaced by a single-unit rest:

This solu­tion pro­duces the sim­plest poly­met­ric expres­sion. The same sim­ple case is that of {… C4 D4 E4, F4 G4}:

If a field of the poly­met­ric expres­sion con­tains sev­er­al unde­ter­mined rests, these are assigned equal dura­tions — in such a way that the com­plex­i­ty of the struc­ture remains min­i­mal. Consider for instance {… C4 D4 … E4, A4 F4 G4}:

An unde­ter­mined rest may even be assigned dura­tion 0 in case this yields a sim­pler expres­sion. For instance, in {… C4 D4 … E4, F4 G4} dura­tion 0 yields a “three in two” polyrhythm where­as dura­tion 1 would yield “five in two”. The cri­te­ri­on for eval­u­at­ing com­plex­i­ty is get­ting the low­est com­mon mul­ti­ple (LCM) of the num­bers of units in each field, in effect 6 against 10. Therefore the solu­tion is:

Every field of a poly­met­ric expres­sion may con­tain unde­ter­mined rests. Consider for instance {… C4 D4 E4, A4 B4 F4 … G4}. Here, again, assign­ing dura­tion zero to each unde­ter­mined rest yields the sim­plest struc­ture since “four in three” (LCM = 12) is a bet­ter trade-off than “five in four” (LCM = 20).

A more com­plex struc­ture is assigned to {C4 D4 E4, A4 B4 F4 … G4 A4, C5 … D5} with rests of 1 unit in the sec­ond and third fields. The LCM of 3 and 6 is 6, which is the low­est val­ue achiev­able for this structure.

Note that there is an equiv­a­lent solu­tion in terms of com­plex­i­ty: assign­ing dura­tion 0 to the rest in the third field. When sev­er­al solu­tions are valid, the algo­rithm selects in pri­or­i­ty the one con­tain­ing the fewest null-duration rests.

A sim­i­lar case is {C4 D4 E4, A4 B4 F4 … G4 A4, C5 … D5 E5}:

Here, the first rest has been assigned 1 unit and the sec­ond one 3 units. This yields the LCM of 3 and 6 = 6. Another opti­mal (equiv­a­lent) solu­tion would be to assign 0 to the sec­ond rest, but it has been dis­card­ed due to the heuris­tics of avoid­ing null-duration rests.

Replacing com­mas with peri­ods yields the same struc­ture in a sequen­tial form:

Tied notes, tied sound-objects

Sound-objects or sim­ple notes can be con­cate­nat­ed (“tied”). Consider for instance:

and its vari­a­tion with ties notat­ed “&”:

The time inter­val of a tied note/sound-object may cross the lim­its of (tree-shaped) poly­met­ric struc­tures. For instance:

The chal­lenge of han­dling tied events is dis­cussed on page Tied notes.

Real music is “polymetric”

Rules and heuris­tics asso­ci­at­ed with poly­met­ric expres­sions make sense when deal­ing with real musi­cal items. Notably, they made it pos­si­ble to import MusicXML scores and inter­pret them as Bol Processor data (read page).

Check for instance Mozart’s musi­cal dice game, this “Charles Ames” exam­ple and Harm Visser’s demos.

Further reading

Bel, Bernard. Rationalizing musi­cal time: syn­tac­tic and symbolic-numeric approach­es. In Barlow, Clarence (ed.) The Ratio Book. Den Haag: Royal Conservatory - Institute of Sonology. 2001: 86-101.

Bol Processor shares a few fea­tures, with respect to pat­tern rep­re­sen­ta­tion, with the TIDAL Pattern Language for the Live Encoding of Music (Alex McLean & Geraint Wiggins, 2010).

An Indian con­cep­tion of time can be seen most clear­ly in the ven­er­a­ble Bol Processor sys­tem for algo­rith­mic music, cre­at­ed by com­put­er sci­en­tist Bernard Bel from work on notat­ing tabla rhythms and devel­oped over forty years. Drawing from Indian clas­si­cal music, it includes an expres­sive approach to time set­ting that seems unique to the algo­rith­mic music field, in which sound events are orga­nized in terms of inter­re­la­tion­ships before being mapped to phys­i­cal time. Although not a live cod­ing sys­tem itself, it has been heav­i­ly influ­en­tial on the design of the TidalCycles sys­tem, par­tic­u­lar­ly its embed­ded “minino­ta­tion” lan­guage for describ­ing rhythm in the Bol Processor and more gen­er­al­ly its rep­re­sen­ta­tion of music based not on the dura­tion of events (as in staff nota­tion) but on the dura­tion of cycles.

Alan Blackwell, Emma Cocker, Geoff Cox, Thor Magnusson, Alex McLean, Live Coding: A User’s Manual, MIT 2022, page 195.