Xenharmonic Wiki

EDT

Revision as of 06:25, 1 March 2019 by Xenllium (talk | contribs) (→EDT-EDO correspondences)


EDT: Equal Division of the Tritave (3/1, perfect twelfth). Also sometimes written as ed3.

Introduction

Western music generally revolves around the principle of octave equivalence: notes an octave apart are often perceived in western music as being the same chroma but differing in pitch height. As the octave corresponds to a 2/1 frequency ratio, it has been proposed that the next-simplest after the octave, the 3/1, can also be used to evoke a sense of chroma equivalence. This interval corresponds to a perfect twelfth in the diatonic scale, but when used to refer to an equivalence interval it is often called the "tritave".

It has been argued that pitches a tritave apart can never truly be heard as equivalent in all of the ways that octaves are, with some claiming that the tonotopic representation of the mammalian auditory system is inherently biased towards octave-equivalence. With proper context, experience, and training, however, at least some people find that they can experience some degree of tritave equivalence especially using timbres restricted to odd harmonics. It is not known whether odd harmonics actually facilitate the ability to hear in tritave-equivalence. Either way, it is certain that musically valuable organizations of pitch can arise through the equal division of non-octave intervals, regardless of whether the period is perceived as being truly chroma-equivalent, and as such the multitude of equal divisions of the tritave are rich and ripe for exploration.

The Bohlen-Pierce (BP) scale, most commonly consisting of 13 equal divisions of the tritave (although a justly-intoned version exists as well), seems to have been the first such arrangement to be seriously studied and made into music. The BP scale was independently discovered by Heinz Bohlen, John Pierce and Kees Van Prooijen. Bohlen found it while looking for triads with equal-difference tones, Prooijen uncovered it while searching for equally-tempered scales with accurate higher harmonics, and Pierce stumbled upon it trying to find consonant chords other than 4:5:6. Though they all started with different goals in mind, each of them amazingly ended up at the same destination.

Rank two temperaments

If factors of two are eliminated, the simplest possible triad is (1):(3):5:7:(9), with 1, 3 and 9 in parentheses as they're all tritave-equivalent to 1. Hence, 3:5:7 can be viewed as the fundamental consonant triad of no-twos music. The linear temperament that best approximates these chords in the moderate complexity range is the Bohlen-Pierce linear temperament eliminating 245/243, which has a no-twos mapping of [<1 1 2|, <0 2 -1|] and a pure-tritaves TE generator which is a sharp 9/7 of 440.488 cents. It possesses MOS of the forms 4L1s (pentatonic) and 4L5s (nonatonic), and larger MOS of size 13, 17, 30, 43, 56, 69 and 82. This temperament serves a function analogous to meantone in the 5-limit.

At higher complexities, the rank two 3.5.7 temperament tempering out 16875/16807 called Canopus begins to predominate. This has a mapping [<1 3 3|, <0 -5 -4|] and a pure-tritaves TE generator a slightly flat 7/5 at 581.512 cents. This has MOS of size 3, 4, 7, 10, 13, 23, 36, etc, with the 36 note MOS being particularly even.

The other no twos rank two temperament which 13edt "supports" is Arcturus, which takes an ~5:3 as a generator. I speak advisedly of 13edt supporting this temperament because the lowest-error proper MOS of it is a 2L 11s triskaidecatonic scale. However, if you do not mind having a smeary 5, you will need only a 2L 7s (nonatonic) scale to make an understandable rendition of it.

The named but not necessarily no twos rank two temperament which 13edt "supports" is Sirius, which takes a generator between ~7:6 and ~6:5. Like Arcturus, I speak advisedly of 13edt supporting it because the most proper small MOS of it is triskaidecatonic. Unlike Arcturus, there is a smaller MOS of it than this which is technically proper. However, this MOS is the Grumpy heptatonic scale the use of which is made problematic by the uniqueness of the step of the second size. It is problematic to have the step of the second size be unique in a subscale of an edx because it creates a strong sense of a second equal division of a y strictly less than x, and this sense of two different equal divisions trying to happen in the same scale causes ordinary concepts of equivalence to break down in spectacular ways. If this "problem" has not been named yet, "cross-equivalence artifacting" would be a perfect name for it.

The final interval which 13edt can reasonably use to generate a rank two temperament is its false 3/2 of 5 degrees. By a weird coincidence, it will generate the 5L 3s unfair father octatonic scale just as if it were an interval of an edo, except that the scale will not always contain a false 4/3 as it must in an edo. This means, most importantly, that 16/15 cannot be assumed to be a "comma" tempered out by this false Father temperament when it is taken as a temperament of full just intonation. By a second, and totally separate, weird coincidence, the well-known Bohlen-Pierce temperament is its index-2 subtemperament.

Due to the fact of its 9/7 generator, the temperament which is to BP what neutral temperaments are to syntonic temperaments does not become intelligibly a division of the tritave until extended to 17 tones whereas EDOs supporting various neutral temperaments have an "ordinary" heptatonic scale which is intelligibly a division of the octave. Additionally, 7 and 9 being consecutive odd numbers means that trying to force this temperament into a no-twos subgroup induces very poor "approximations" of less intelligible higher harmonics. To avoid this, this temperament should be assumed to be a temperment of the 3.5.7.8 subgroup tempering out 245/243 and 64/63, the familiar comma from EDOs supporting the Superpythagorean or Parapythagorean diatonic scale.

Among the EDTs tempering out 245/243, 13EDT stands out. An apt analogy can be drawn with EDOs supporting meantone: 4EDT and 9EDT are to BP what 5EDO and 7EDO are to meantone. However, in contrast to meantone, the simplest EDT supporting the BP nonatonic scale - 13EDT, the traditional tempered BP scale - is the most accurate and lowest in tuning error until 56EDT. However, there are many EDTs supporting BP temperament which also support extensions to higher-limit temperaments; in particular 2, 3, and 4 times 13 in the form of 26EDT, 39EDT and 52EDT as well as 56EDT. For tempering out 16875/16807, 13EDT again stands out, though much better accuracy can be found in more complex divisions such as 114EDT or 127EDT. All of this explains the focus on 13EDT to the exclusion of other EDTs among practitioners of the art of nonoctave composition, but it must be noted that the analysis is only valid if consideration is confined to the 7-limit, which is exactly analogous to confining it to the 5-limit with EDOs. There's a whole other world out there which has not been much explored.

For example, 15EDT very well approximates the 5th and 13th harmonics, and 12EDT, the 13th and 17th. 39EDT makes for a fine 3.5.7.11.13 system, tempering out 245/243, 275/273, 847/845 and 1331/1343, and so supporting among other things the [<13 19 23 0 2|, <0 0 0 1 1|] temperament supported by the whole suite of 13nEDTs: 13, 26, 39, 52, 65, 78 etc.

One should bear in mind that, assuming tritave equivalence, when determining which harmonics are represented, the ratios of 3 in the denominator are fungible instead of those of 2. For example making the fifth harmonic 5:3 a "major sixth" by conventional (and arbitrarily silly for the purposes of xenharmony, even with octaves) pitch class terminology.

There are other uses, or conceptualizations, of tritave-based tunings. Purely intuitive use of these myriad, assuredly xenharmonic structures comes to mind (see "EDO" versus "equal temperament"). Another intent might be to find or define temperaments (such as Magic, Hanson, etc.), or to provide exact formulae for stretching/compressing what would musically be used as an "ordinary" octave of ~2:1. (And given the stable nature of octave-based systems, some aesthetic overlap even in the most tritave-equivalent of music, would be forseeable.) For instance, the Bernhard-Stopper (19edt) temperament, might for instance be found useful in tuning pianoforti, being equivalent to 12edo, except for a 1.2 cents sharp octave which is relevant to inharmonicity.

Below is a large list of EDTs; additionally, some equal divisions of the tritave are known by alternate names or have special interest:


Individual pages for EDTs

0...99

(some pages do not exist yet)

0 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29
30 31 32 33 34 35 36 37 38 39
40 41 42 43 44 45 46 47 48 49
50 51 52 53 54 55 56 57 58 59
60 61 62 63 64 65 66 67 68 69
70 71 72 73 74 75 76 77 78 79
80 81 82 83 84 85 86 87 88 89
90 91 92 93 94 95 96 97 98 99

100...199

(many pages do not exist yet)

100 101 102 103 104 105 106 107 108 109
110 111 112 113 114 115 116 117 118 119
120 121 122 123 124 125 126 127 128 129
130 131 132 133 134 135 136 137 138 139
140 141 142 143 144 145 146 147 148 149
150 151 152 153 154 155 156 157 158 159
160 161 162 163 164 165 166 167 168 169
170 171 172 173 174 175 176 177 178 179
180 181 182 183 184 185 186 187 188 189
190 191 192 193 194 195 196 197 198 199

200...299

(many pages do not exist yet)

200 201 202 203 204 205 206 207 208 209
210 211 212 213 214 215 216 217 218 219
220 221 222 223 224 225 226 227 228 229
230 231 232 233 234 235 236 237 238 239
240 241 242 243 244 245 246 247 248 249
250 251 252 253 254 255 256 257 258 259
260 261 262 263 264 265 266 267 268 269
270 271 272 273 274 275 276 277 278 279
280 281 282 283 284 285 286 287 288 289
290 291 292 293 294 295 296 297 298 299

A list of tritave reduced harmonics for easy comparison of JI and temperaments in tritave based systems.

Also may be found convenient: http://www.nonoctave.com/tuning/twelfth.html

EDT-EDO correspondences

EDT EDO Comments
8edt 5edo 8edt is equivalent to 5edo with ~11 cent octave compression. Equivalently, 5edo is 8edt with ~18 cent stretched tritaves. Patent vals match through the 13-limit.
9edt Neither 9edt nor 10edt is equivalent to 6edo.
10edt
11edt 7edo 11edt is equivalent to 7edo with ~10 cent stretched octaves. Patent vals differ in the 7-limit, but neither can really be said to represent the 7th harmonic with a straight face.
12edt 12edt entirely misses 2/1, falling halfway between 7 and 8 edos.
13edt The equal-tempered BP scale cannot be considered equivalent to 8edo.
14edt 9edo There is a lot of mismatch between the pure-octave and pure-tritave tunings, but the patent vals match through the 13-limit. Great for stretched-octave pelog!
15edt 15edt entirely misses 2/1, falling halfway between 9 and 10 edos.
16edt 10edo Similar situation to 8edt~5edo. Patent vals match through the 17-limit.
17edt Neither 17edt nor 18edt is equivalent to 11edo.
18edt
19edt 12edo 19edt is 12edo with ~1.2 cent octave stretch, and consistent through the 10-integer-limit. Patent vals match through the 7-limit.
20edt Neither 20edt nor 21edt is equivalent to 13edo.
21edt
22edt 14edo Similar situation to 11edt~7edo, but the equivalence is rough. Patent vals match through the 11-limit, with the exception of 5 (which neither represents well).
23edt 23edt entirely misses 2/1, falling halfway between 14 and 15 edos.
24edt 15edo This is only a rough correspondence, as the (8n)edt ~ (5n)edo sequence begins to break down. Patent vals match through the 13-limit, with the exception of 7 (24edt is only consistent through the 6-integer-limit, with discrepancy for the 7th harmonic).
25edt 16edo Also only a rough correspondence; 25edt corresponds to 16edo with ~17 cent octave stretch, and patent vals match through the 5-limit. 25edt is only consistent through the 6-integer-limit, with discrepancy for the 7th harmonic.
26edt Double BP scale entirely misses 2/1, falling halfway between 16 and 17 edos.
27edt 17edo 27edt is 17edo with ~2.5 cent compressed octaves. With the exception of 5 (which neither represents well), patent vals match through the 13-limit.
28edt Neither 28edt nor 29edt is equivalent to 18edo.
29edt
30edt 19edo 30edt is 19edo with ~5 cent stretched octaves, and consistent through the 10-integer-limit. Patent vals match through the 7-limit.
31edt 31edt entirely misses 2/1, falling halfway between 19 and 20 edos.
32edt 32edt cannot be considered equivalent to 20edo.
33edt 33edt cannot be considered equivalent to 21edo.
34edt 34edt entirely misses 2/1, falling halfway between 21 and 22 edos.
35edt 22edo 35edt is 22edo with ~4 cent compressed octaves, and consistent through the 12-integer-limit. Patent vals match through the 11-limit.
36edt Neither 36edt nor 37edt is equivalent to 23edo.
37edt
38edt 24edo Same ~1.2 cent octave stretch as 19edt~12edo. 38edt is consistent through the 6-integer-limit. Patent vals match through the 19-limit.
39edt Neither 39edt (Triple BP scale) nor 40edt is equivalent to 25edo.
40edt
41edt 26edo 41edt is 26edo with ~6 cent stretched octaves, but only consistent through the 10-integer-limit, with discrepancy for the 11th harmonic. Patent vals match through the 7-limit.
42edt 42edt falls exactly halfway between 26 and 27 edos. It entirely misses 2/1, but nails the "double octave" 4/1, so it strongly resembles the scale with generator 2\53 of an octave.
43edt 27edo 43edt is 27edo with ~6 cent compressed octaves, and consistent through the 10-integer-limit. Patent vals match through the 7-limit.
44edt Neither 44edt nor 45edt is equivalent to 28edo.
45edt
46edt 29edo 46edt is 29edo with ~0.9 cent compressed octaves, and consistent through the 16-integer-limit. Patent vals match through the 89-limit.
47edt Neither 47edt nor 48edt is equivalent to 30edo.
48edt
49edt 31edo 49edt is 31edo with ~3.3 cent stretched octaves, and consistent through the 12-integer-limit. Patent vals match through the 11-limit.
54edt 34edo Same ~2.5 cent octave compression as 27edt~17edo. 54edt is consistent through the 7-integer-limit. Patent vals match through the 17-limit, with the exception of 7.
57edt 36edo Same ~1.2 cent octave stretch as 19edt~12edo. 57edt is consistent through the 9-integer-limit. Patent vals match through the 31-limit, with the exception of 11.
60edt 38edo Same ~5 cent octave stretch as 30edt~19edo. Patent vals match through the 5-limit.
62edt 39edo 62edt is 39edo with ~3.6 cent compressed octaves. Patent vals match through the 5-limit.
65edt 41edo 65edt is 41edo with ~0.3 cent compressed octaves, and consistent through the 16-integer-limit. Patent vals match through the 19-limit.
68edt 43edo 68edt is 43edo with ~2.7 cent stretched octaves, but only consistent through the 6-integer-limit, with discrepancy for the 7th harmonic. Patent vals match through the 5-limit.
73edt 46edo 73edt is 46edo with ~1.5 cent compressed octaves, and consistent through the 18-integer-limit. Patent vals match through the 17-limit.
76edt 48edo Same ~1.2 cent octave stretch as 19edt~12edo. Patent vals match through the 11-limit.
79edt 50edo 79edt is 50edo with ~3.8 cent stretched octaves, and consistent through the 10-integer-limit. Patent vals match through the 7-limit.
81edt 51edo Same ~2.5 cent octave compression as 27edt~17edo, but there is a lot of mismatch between the pure-octave and pure-tritave tunings. Patent vals differ in the 5-limit.
84edt 53edo 84edt is 53edo with ~0.04 cent stretched octaves, and consistent through the 10-integer-limit. Patent vals match through the 61-limit.
87edt 55edo 87edt is 55edo with ~2.4 cent stretched octaves, but only consistent through the 4-integer-limit, with discrepancy for the 5th harmonic. Patent vals match through the 11-limit, with the exception of 5.
92edt 58edo Same ~0.9 cent octave compression as 46edt~29edo. Patent vals match through the 17-limit.
95edt 60edo Same ~1.2 cent octave stretch as 19edt~12edo. Patent vals match through the 7-limit.

Multiples of 13EDT which approximate EDO

Also, on the topic of multiples of 13edt, 26 (double) and 39 (triple) offer very good harmonic approximations, the former of the 8th, 13th and 17th partials, and the latter of the 11th and 13th. However, quadruple and quintuple, ie. 52 and 65edt, also exist offering good approximations of the octave. 52edt is very nearly 33edo, and 65edt is practically identical to 41edo.

See also

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