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If N-edo is an
equal division of the octave
, and if for any interval r, N(r) is the best N-edo approximation to r, then N is
with respect to a set of intervals S if for any two intervals a and b in S where ab is also in S, N(ab) = N(a) + N(b). Normally this is considered when S is the set of
q odd limit intervals
, consisting of everything of the form 2^n u/v, where u and v are odd integers less than or equal to q. N is then said to be
q limit consistent
. If each interval in the q-limit is mapped to a unique value by N, then it said to be
uniquely q limit consistent
of odd limits, with the smallest edo that is consistent or uniquely consistent in that odd limit. And
of edos, with the largest odd limit that this edo is consistent or uniquely consistent in.
An example for a system that is
consistent in a particular odd limit is
The best approximation for the interval of
(the septimal subminor third) in 25edo is 6 steps, and the best approximation for the
perfect fifth 3/2
is 15 steps. Adding the two just intervals gives 3/2 * 7/6 =
, the harmonic seventh, for which the best approximation in 25edo is 20 steps. Adding the two approximated intervals, however, gives 21 steps. This means that 25edo is not consistent in 7 odd-limit. The 4:6:7 triad cannot be mapped to 25edo without one of its three component intervals being inaccurately mapped.
An example for a system that
consistent in the 7 odd-limit is
: 3/2 maps to 7\12, 7/6 maps to 3\12, and 7/4 maps to 10\12, which equals 7\12 plus 3\12. 12edo is also consistent in the 9 odd-limit, but not in the 11 odd-limit.
One notable example:
is not consistent in the 15 odd limit. The 15:13 interval is very closer slightly closer to 9 degrees of 46edo than to 10 degrees, but the
15:13 (the difference between 46edo's versions of 15:8 and 13:8) is 10 degrees. However, if we compress the octave slightly (by about a cent), this discrepancy no longer occurs, and we end up with an 18-
-limit consistent system, which makes it ideal for approximating mode 8 of the harmonic series.
Generalization to non-octave scales
It is possible to generalize the concept of consistency to non-edo equal temperaments. Because octaves are no longer equivalent, instead of an odd limit we must use an integer limit, and the term 2^n in the above equation is no longer present. Instead, the set S consists of all intervals u/v where u <= q >= v.
This also means that the concept of octave inversion no longer applies: in this example, 13:9 is in S, but 18:13 is not.
consistent (TonalSoft encyclopedia)
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