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| A scale satisfies the '''generator-offset property''' (also '''GO''', '''alternating generator''' or '''AG''') if it satisfies the following properties: | | A scale satisfies the '''generator-offset property''' if it satisfies the following properties: |
| # The scale is generated by two chains of stacked copies of a ''generator''. | | # The scale is generated by two chains of stacked copies of an interval called the ''generator''. |
| # The two chains are separated by an ''offset'' (the difference between the first note of the second chain minus the first note of the first chain). | | # The two chains are separated by a different interval called the ''offset'' (the difference between the first note of the second chain and the first note of the first chain). |
| # The lengths of the chains differ by at most one. (1-3 can be restated as: The scale can be built by stacking two alternating generators (called ''alternants''), which do not necessarily take up the same number of steps.) | | # The lengths of the chains differ by at most one. (1-3 can be restated as: The scale can be built by stacking two alternating generators (called ''alternants'') a<sub>1</sub> and a<sub>2</sub>. Note that a<sub>1</sub> does not need to [[subtend]], i.e. occur as, the same number of steps as a<sub>2</sub>.) |
| # The generator always occurs as the same number of steps. For example, the generator is never both a 2-step and a 3-step. | | # The generator always occurs as the same number of steps. For example, the generator is never both a 2-step and a 3-step. |
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| The [[Zarlino]] (3L 2M 2S) JI scale is an example of a GO scale, because it is built by stacking alternating 5/4 and 6/5 generators. 7-limit [[diasem]] (5L 2M 2S) is another example, with generators 7/6 and 8/7.
| | [[File:MV3-Labeled.png|thumb|240px|Plot of at-most-decatonic MV3 generator-offset scale regions in the plane.]] |
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| More formally, a cyclic word ''S'' (representing the steps of a [[periodic scale]]) of size ''n'' is '''GO''' if it satisfies the following properties:
| | The [[Zarlino]] (3L 2M 2S) JI scale is an example of a generator-offset scale, because it is built by stacking alternating 5/4 and 6/5 generators. The 7-limit version of [[diasem]] (5L 2M 2S) is another example, with generators 7/6 and 8/7. |
| # ''S'' is generated by two chains of stacked generators ''g'' separated by a fixed offset δ; either both chains are of size ''n''/2, or one chain has size (''n'' + 1)/2 and the second has size (''n'' − 1)/2. Equivalently, ''S'' can be built by stacking a single chain of alternants ''g''<sub>1</sub> and ''g''<sub>2</sub>, resulting in a circle of the form either ''g''<sub>1</sub> ''g''<sub>2</sub> ... ''g''<sub>1</sub> ''g''<sub>2</sub> ''g''<sub>1</sub> ''g''<sub>3</sub> or ''g''<sub>1</sub> ''g''<sub>2</sub> ... ''g''<sub>1</sub> ''g''<sub>2</sub> ''g''<sub>3</sub>.
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| # The scale is ''well-formed'' with respect to ''g'', i.e. all occurrences of the generator ''g'' are ''k''-steps for a fixed ''k''.
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| This doesn't imply that ''g''<sub>1</sub> and ''g''<sub>2</sub> are the same number of scale steps. For example, 5-limit [[blackdye]] has ''g''<sub>1</sub> = 9/5 (a 9-step) and ''g''<sub>2</sub> = 5/3 (a 7-step).
| | Generator-offset scales generalize the notion of [[dipentatonic scale|dipentatonic]] and [[diheptatonic scale|diheptatonic]] scales where the pentatonic and heptatonic are [[MOS scales]]. A related but distinct notion is [[alternating generator sequence]]. While scales produced using the generator-offset procedure can be seen as a result of an alternating generator sequence of 2 alternants, the generator-offset perspective views the sum of the two alternants as the "canonical" generator, and the alternants as rather being possible choices of the offset which are effectively equivalent up to chirality. While a well-formed AGS scale requires each alternant in the AGS to subtend the same number of steps, the generator-offset property only requires each (aggregate) generator to subtend the same number of steps. |
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| More generally, we say that a scale is ''m'''''-GO''' if both of the following hold: | | Note: In Inthar's contribution to [[aberrismic theory]], this term has been superseded by [[guide frame]]s. |
| # the scale consists of ''m'' + 1 chains of stacked generator ''g'' (implying ''m'' offsets δ<sub>1</sub>, ...,δ<sub>''m''</sub> from a fixed chain), each chain having either ''j'' or ''j'' + 1 notes. | | == Mathematical definition == |
| # The scale is well-formed with respect to ''g''. | | More formally, a cyclic word ''S'' (representing the steps of a [[periodic scale]]) of size ''n'' is '''generator-offset''' if it satisfies the following properties: |
| | # ''S'' is generated by two chains of stacked generators g separated by a fixed offset δ; either both chains are of size ''n''/2 (implying ''n'' is even), or one chain has size (''n'' + 1)/2 and the second has size (''n'' − 1)/2 (implying ''n'' is odd). |
| | # The scale is ''well-formed'' with respect to g, i.e. all occurrences of the generator g are ''k''-steps for a fixed ''k''. |
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| (Thus 1-GO is the same thing as GO.)
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| An ''m''-GO scale can be interpreted as a scale in a rank-(''m'' + 2) [[regular temperament]] (though the specific tuning used may be of lower rank), with basis ''p'' (period), ''g'', δ<sub>1</sub>, ...,δ<sub>''m''</sub>.
| | [[Category:Scale]] |
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| == Other definitions ==
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| * A strengthening of the generator-offset property, tentatively named the ''swung-generator-alternant property'' (SGA), states that the alternants ''g''<sub>1</sub> and ''g''<sub>2</sub> can be taken to always subtend the same number of scale steps, thus both representing "detemperings" of a generator of a single-period [[mos]] scale (otherwise known as a well-formed scale). Only odd GO scales and xyxz can satisfy this property. The Zarlino and diasem scales above are both SGA. [[Blackdye]] is GO but not SGA.
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| == Theorems ==
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| === Proposition 1 ===
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| Let ''S'' be a 3-step-size scale word in L, M, and s, and suppose ''S'' is SGA. Then:
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| # ''S'' is unconditionally MV3 (i.e. MV3 regardless of tuning).
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| # ''S'' is of the form ''ax by bz'' for some permutation (''x'', ''y'', ''z'') of (L, M, s).
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| # The cardinality (size) of ''S'' is either odd, or 4 (and ''S'' is of the form ''xyxz'').
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| # If |S| is odd, then S = aX bY bZ is obtained from the (single-period) mos aX 2bW by replacing all the W's successively with alternating Y's and Z's (or alternating Z's and Y's for the other chirality).
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| [Note: This is not true with SGA replaced with GO; [[blackdye]] is a counterexample that is MV4.]
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| ==== Proof ====
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| Assuming SGA, we have two chains of generator ''g''<sub>0</sub> (going right). The two cases are:
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| CASE 1: EVEN CARDINALITY
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| O-O-...-O (n/2 notes)
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| O-O-...-O (n/2 notes)
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| and
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| CASE 2: ODD CARDINALITY
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| O-O-O-...-O ((n+1)/2 notes)
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| O-O-...-O ((n-1)/2 notes).
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| Label the notes (1, ''j'') and (2, ''j''), 1 ≤ ''j'' ≤ (chain length), for notes in the upper and lower chain respectively.
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| In case 1, let ''g''<sub>1</sub> = (2, 1) − (1, 1) and ''g''<sub>2</sub> = (1, 2) − (2, 1). We have the chain ''g''<sub>1</sub> ''g''<sub>2</sub> ''g''<sub>1</sub> ''g''<sub>2</sub> ... ''g''<sub>1</sub> ''g''<sub>3</sub>.
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| Let ''r'' be odd and ''r'' ≥ 3. Consider the following abstract sizes for the interval class (''k''-steps) reached by stacking ''r'' generators:
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| # from ''g''<sub>1</sub> ... ''g''<sub>1</sub>, we get ''a''<sub>1</sub> = (''r'' − 1)/2*''g''<sub>0</sub> + ''g''<sub>1</sub> = (''r'' + 1)/2 ''g''<sub>1</sub> + (''r'' − 1)/2 ''g''<sub>2</sub>
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| # from ''g''<sub>2</sub> ... ''g''<sub>2</sub>, we get ''a''<sub>2</sub> = (''r'' − 1)/2*''g''<sub>0</sub> + ''g''<sub>2</sub> = (''r'' − 1)/2 ''g''<sub>1</sub> + (''r'' + 1)/2 ''g''<sub>2</sub>
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| # from ''g''<sub>2</sub> (...even # of gens...) ''g''<sub>1</sub> ''g''<sub>3</sub> ''g''<sub>1</sub> (...even # of gens...) ''g''<sub>2</sub>, we get ''a''<sub>3</sub> = (''r'' − 1)/2 ''g''<sub>1</sub> + (''r'' − 1)/2 ''g''<sub>2</sub> + ''g''<sub>3</sub>
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| # from ''g''<sub>1</sub> (...odd # of gens...) ''g''<sub>1</sub> ''g''<sub>3</sub> ''g''<sub>1</sub> (...odd # of gens...) ''g''<sub>1</sub>, we get ''a''<sub>4</sub> = (''r'' + 1)/2 ''g''<sub>1</sub> + (''r'' − 3)/2 ''g''<sub>2</sub> + ''g''<sub>3</sub>.
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| Choose a tuning where ''g''<sub>1</sub> and ''g''<sub>2</sub> are both very close to but not exactly 1/2*''g''<sub>0</sub>, resulting in a scale very close to the mos generated by 1/2 ''g''<sub>0</sub>. (i.e. ''g''<sub>1</sub> and ''g''<sub>2</sub> differ from 1/2*''g''<sub>0</sub> by ε, a quantity much smaller than the chroma of the ''n''/2-note mos generated by ''g''<sub>0</sub>, which is |''g''<sub>3</sub> − ''g''<sub>2</sub>|). Thus we have 4 distinct sizes for ''k''-steps:
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| # ''a''<sub>1</sub>, ''a''<sub>2</sub> and ''a''<sub>3</sub> are clearly distinct.
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| # ''a''<sub>4</sub> − ''a''<sub>3</sub> = ''g''<sub>1</sub> − ''g''<sub>2</sub> != 0, since the scale is a non-degenerate AG scale.
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| # ''a''<sub>4</sub> − ''a''<sub>1</sub> = ''g''<sub>3</sub> − ''g''<sub>2</sub> = (''g''<sub>3</sub> + ''g''<sub>1</sub>) − (''g''<sub>2</sub> + ''g''<sub>1</sub>) != 0. This is exactly the chroma of the mos generated by ''g''<sub>0</sub>.
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| # ''a''<sub>4</sub> − ''a''<sub>2</sub> = ''g''<sub>1</sub> − 2 ''g''<sub>2</sub> + ''g''<sub>3</sub> = (''g''<sub>3</sub> − ''g''<sub>2</sub>) + (''g''<sub>1</sub> − ''g''<sub>2</sub>) = (chroma ± ε) != 0 by choice of tuning.
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| By applying this argument to 1-steps, we see that there must be 4 step sizes in some tuning, a contradiction. Thus ''g''<sub>1</sub> and ''g''<sub>2</sub> must themselves be step sizes. Thus we see that an even-cardinality, unconditionally MV3, AG scale must be of the form ''xy...xyxz''. But this pattern is not unconditionally MV3 if ''n'' ≥ 6, since 3-steps come in 4 sizes: ''xyx'', ''yxy'', ''yxz'' and ''xzx''. Thus ''n'' = 4 and the scale is ''xyxz''. This proves (3).
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| In case 2, let (2, 1) − (1, 1) = ''g''<sub>1</sub>, (1, 2) − (2, 1) = ''g''<sub>2</sub> be the two alternants. Let ''g''<sub>3</sub> be the leftover generator after stacking alternating ''g''<sub>1</sub> and ''g''<sub>2</sub>. Then the generator circle looks like ''g''<sub>1</sub> ''g''<sub>2</sub> ''g''<sub>1</sub> ''g''<sub>2</sub> ... ''g''<sub>1</sub> ''g''<sub>2</sub> ''g''<sub>3</sub>. Then the combinations of alternants corresponding to a step are:
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| # ''kg''<sub>1</sub> + (''k'' − 1)''g''<sub>2</sub>
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| # (''k'' − 1)''g''<sub>1</sub> + ''kg''<sub>2</sub>
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| # (''k'' − 1)''g''<sub>1</sub> + (''k'' − 1) ''g''<sub>2</sub> + ''g''<sub>3</sub>,
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| if a step is an odd number of generators (since the scale size is odd, we can always ensure this by taking octave complements of all the generators). The first two sizes must occur the same number of times. This proves (2).
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| (The above holds for any odd ''n'' ≥ 3.)
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| For (1), we now only need to see that SGA + odd cardinality => unconditionally MV3. But the argument in case 2 above works for any interval class (unconditional MV3 wasn't used), hence any interval class comes in at most 3 sizes regardless of tuning.
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| For (4), assume S is aX bY bZ, a odd. If b = 1, there’s nothing to prove. So assume b > 1. If Y’s and Z’s don't alternate perfectly, then (ignoring X's) you have two consecutive Y's somewhere and two consecutive Z's somewhere else. Assume that g_pf = iX + jW with j >=2. (If this is not true, invert the generator, since b > 1.)
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| In aX bY bZ, consider (i+j)-steps (representing the generator) with: (1) the maximum possible number of Y’s (at least 2 more than the # of Z's), (2) the maximum maximum number of Z’s (at least 2 more than the # of Y's), or (3) an intermediate number of Y’s and Z’s between the two or (4) (the preimage of) the imperfect generator. Since a + 2b >= 5, are at least 4 perfect generators, so there must be at least one of each of (1), (2), and (3), giving a contradiction to MV3. [Let T be the subword consisting only of Y's and Z's. If T has a substring of length j that's not contained in a perfect generator, you can go somewhere else to find it, since the numbers of Y's and Z's change one at a time and reach a maximum and a minimum somewhere, guaranteeing that intermediate values are reached "on the other side".] <math>\square</math>
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| === Proposition 2 ===
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| Suppose that a periodic scale satisfies the following:
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| * is generator-offset (chain lengths off by 1)
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| * has odd length ''n''
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| Then the scale is SGA.
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| ==== Proof ====
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| Assume that the generator is a ''k''-step and ''k'' is even. (If ''k'' is not even, invert the generator.) On some tonic p we have a chain of ceil(''n''/2) notes and on some other note ''p' = p'' + offset (not on the first chain) we'll have floor(''n''/2) notes.
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| We must have gcd(''k'', ''n'') = 1. If not, since ''n'' is odd, gcd(''k'', ''n'') is an odd number at least 3, and the ''k''-steps must form more than 2 parallel chains.
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| By modular arithmetic we have ''rk'' mod ''n'' = ''k''/2 iff ''r'' = ceil(''n''/2) mod ''n''. (Since gcd(2, ''n'') = 1, 2 is multiplicatively invertible mod ''n'', and we can multiply both sides by 2 to check this.) This proves that the offset, which must be reached after ceil(''n''/2) generator steps, is a ''k''/2-step, as desired. (If the offset wasn't reached in ceil(''n''/2) steps, the two generator chains either wouldn't be disjoint or wouldn't have the assumed lengths.) <math>\square</math>
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| == Open conjectures ==
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| === Conjecture 3 ("MV3 Structure Theorem") ===
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| If a non-multiperiod 3-step size scale word is
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| # unconditionally MV3,
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| # has ''odd'' cardinality, and
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| # is not of the form ''mx my mz'', ''xyzyx'' or ''xyxzxyx'',
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| then it is SGA. (a converse to Theorem 1)
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| If a non-multiperiod 3-step size scale word is
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| # unconditionally MV3,
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| # has ''even'' cardinality, and
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| # is not of the form ''mx my mz'',
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| then it is of the form ''W''(''x'', ''y'', ''z'')''W''(''y'', ''x'', ''z'') for some word ''W'' in 3 variables.
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| === Conjecture 4 ===
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| An SGA scale is always ''monotonic-well-formed'' (MWF). That is, the results of equating L with m, equating m with s, and setting s = 0 are all mosses
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| This would follow from the fact that MV3s are LQ (thus project to mosses upon eliminating one step size), and Conjecture 5.
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| === Conjecture 5 ===
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| An SGA scale is always pairwise-well-formed. That is, the result of identifying any two step sizes of an SGA scale is always a non-multiperiod mos.
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| == Falsified conjectures ==
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| [[Category:GO scales| ]]<!--Main article-->
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| [[Category:Theory]] | |