## Surface topology in Bach canons, II: The Torus.

Part I.

This column is based on work with Eric Altschuler, published in Musical Times, Winter 2015.

### A musical score as a topological space.

To repeat some material from Part I: topologically, a one-voice musical score is a 2-dimensional strip. The horizontal (time) coordinate runs from start to finish; the vertical coordinate runs from lower pitches to higher pitches.

A one-voice musical score as a rectangular strip.

If the score repeats exactly, that time symmetry corresponds to identifying the left (Start) and right (End) borders of the strip, forming a cylinder.

A repeating score is topologically a cylinder.

Generally, the topology of musical scores is constrained to the cylinder (and the Möbius strip in the case of some canons using inversion, as discussed in Part I) because of the impossibility of looping back in the vertical (pitch) direction. There is, however, one example in which Johann Sebastian Bach plays on our perception of pitch and tonality to give a repeatable cycle of "upward" pitch changes. This is his Canon 5 from the Musical Offering (BWV 1079).

### The Musical Offering

The Musical Offering is a work prepared by Bach after his visit to the court of King Frederick II of Prussia (Frederick the Great) in Potsdam on May 7, 1747. The King, who was himself a musician, proposed a theme on which Bach, on the spot, improvised a 3-part fugue.

The "Royal theme" proposed to Bach by Frederick the Great. Note that it starts and ands on the same note.

Frederick also challenged Bach to improvise a 6-part fugue on the theme, but, we are told, the composer declined this much more difficult task. When Bach returned home to Leipzig, he wrote out the fugue he had improvised, along with the 6-part fugue Frederick had requested, a trio sonata, and a set of ten canons, all based on the "Royal Theme." He had them printed up for the King: this was Bach's Musical Offering.

### Canon 5

Canon 5 of the Musical Offering is scored for three voices. The highest voice repeats, over and over, a version of the "Royal Theme." But Bach has modified the melodic line of the theme so that, instead of ending on the same note it started, it ends, and the next iteration starts, one full tone higher. So the first iteration starts on the note middle C and ends on D.

The ground for Canon 5, first iteration, and a graphic representation comparing it with the "Royal theme" (hollow boxes). The horizontal lines are non-standard: they are one whole tone (2 half-steps) apart, uniformly. Fs represents F-sharp, etc., Bf is B-flat. Note that the "Royal theme" starts and ends on C, but the melodic line of the ground ends one full tone higher than it starts.

The second iteration of the ground starts on D and ends on E.

The first two iterations of the ground for Canon 5, with the same graphic conventions as above. Bf' represents the B-flat above middle C.

The third starts on E and ands on F-sharp, and so forth.
The other two voices form a canon: one (the middle voice) imitates the other (the bottom voice) exactly, except one measure later and a fifth (7 half-steps) higher. The bottom voice starts on the C below middle C; the middle voice starts on the G below middle C, one measure later.
When the top voice (the "ground") starts its next iteration, the two other voices also start repeating themselves, also one full tone higher: all three voices have shifted one full tone up.
An octave is 12 half-steps; so after six iterations the ground should end on the C above middle C (C, D, E, F-sharp, G-sharp, A-sharp --or equivalently B-flat--, C). The music seems to continue spiralling upwards, but this is an illusion: the ground drops back to middle C, and the whole process is back where it began.

All six iterations of the ground for Canon5, with graphic conventions as above.

The music must be experienced. I recommend the excellent performance by Michael Monroe available here on YouTube. You can follow the score note by note on the video.

A sonogram of the Michael Monroe performance of Canon 5 shows that, despite the illusion that the pitch has continued to rise, the seventh iteration is on the same pitch as the first.

### The illusion: pitch vs. tonality

Bach is exploiting an aspect of human perception of sound: along with pitch (correlated with frequency, and therefore taking values along an ascending continuum) we are sensitive to tonality: in some essential way, middle C and the Cs above and below sound to us like the "same" note. So instead of a linear continuum, tonalities form a circle. This leads to a phenomenon called the "Shepard scale" where a musical sound seems to be rising in pitch, but yet ultimately goes nowhere. The effect can be mimicked on a piano:

These seven chords on the piano show how a rising sequence of tonalities gives the illusion of a rising sequence of pitches. In fact, the last chord is the same as the first.

The discrete "Sherpard scale" illusion is akin to the impossible waterfall depicted by M. C. Escher. Escher's images are copyrighted, but Andrew Kepert has put in the public domain this rendering of the conceptual core of Escher's woodcut.

### The Torus

If we agree to group the notes in the written-out Canon 5 ground by tonality rathen than by pitch, the image of the six iterations, above, collapses into a rectangle:

Identifying pitches an octave apart as equivalent reduces the complete cycle of the Canon 5 ground to this configuration. Since the top equivalence class (A) is adjacent in tonality-space to the bottom one (B-flat), and since the two ends are adjacent in time, the time-tonality space represented is a torus.

Note that our geometric picture of a smooth torus in 3-space has points of posiive and negative curvature. This does not make sense for a musical score. The score of Canon 5 is a flat torus, obtained by identifying opposite edges of a rectangle. It exists abstractly, but cannot be smoothly exhibited in 3-dimensional space. On this ideal torus the melodic lines of Canon 5 wind around six times in one direction, and once in the other.

On the flat torus defined by the circle of tonalities and the (modulated) repeating of the ground, the melodic line of Canon 5 wraps six times in one direction, once in the other.