Mapping: Difference between revisions

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A [[regular temperament]] is more than simply a set of pitches. It's a set of notes together with a ''consistent rule'' that maps any pitch of the relevant [[just intonation subgroup]] to a specific note from that set. (In fact, an abstract regular temperament is not a set of definite pitches at all! The pitches can vary, and the rule mapping JI pitches to notes is the thing that uniquely characterizes the temperament.) This consistent rule is known as the ''JI mapping'' or simply '''mapping'''. The mapping answers the question "how do I play this JI pitch as a note of this temperament?". The answer will be the "tempered version" of that JI pitch, which may be a very close approximation or a very distant approximation depending on the circumstances.

Naively, one might think that a simple rounding function might be suitable for a mapping: let the "tempered version" of each JI pitch simply be the tempered pitch that is closest to it. However, this (usually) does not result in a regular temperament at all! The reason is that, although this mapping assigns a tempered pitch to each JI pitch, it does not do so in a ''consistent'' way – some instances of the same JI interval are represented by different tempered intervals if they occur in different places. A regular temperament mapping always represents each JI interval by the ''same'' tempered interval (even if that tempered interval is not the closest tempered interval to the JI interval).

Naively, one might think that a simple rounding function might be suitable for a mapping: let the "tempered version" of each JI pitch simply be the tempered pitch that is closest to it. However, this (usually) does not result in a regular temperament at all! The reason is that, although this mapping assigns a tempered pitch to each JI pitch, it does not do so in a ''consistent'' way – some instances of the same JI interval are represented by different tempered intervals if they occur in different places. A regular temperament mapping always represents each JI interval by the ''same'' tempered interval, even if that tempered interval is not the closest tempered interval to the JI interval.

== A note on mathematical terminology ==

In mathematics generally, "mapping" is synonymous with "map" and "function". In RTT, "mapping" has the more specific meaning of a ~~[[Wikipedia:Linear_map~~|''linear'' mapping]], which is a function that can be represented by a matrix.

In mathematics generally, "mapping" is synonymous with "map" and "function". In RTT, "mapping" has the more specific meaning of a {{w|Linear map|''linear'' mapping}}, which is a function that can be represented by a matrix.

== Equal temperament mappings ==

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Furthermore, consider 12edo as an 11-limit temperament. What is its mapping matrix? It actually depends whether you consider 11/8 a "very sharp D" or a "very flat D#". This choice results in two different mappings, {{val| 12 19 28 34 41 }} and {{val| 12 19 28 34 42 }}. The latter has a more accurate 11/8, but the former has more accurate versions of other intervals, including 12/11. In the language of regular temperament theory, these are simply two different 11-limit temperaments that both happen to have 12 steps per octave. Phrases like "11-limit 12edo" are thus ambiguous because they don't specify the mapping, and therefore don't refer to a specific temperament.

(Strictly speaking, "5-limit 12edo" or even "3-limit 12edo" are also ambiguous, because {{val| 12 19 27 }}, for example, is a valid temperament even though it's much less accurate than {{val| 12 19 28 }}. In this temperament 5/4 would be represented as 3 steps of 12edo, or 300 cents. For practical purposes, of course, the ambiguity doesn't appear until higher limits.)

Strictly speaking, "5-limit 12edo" or even "3-limit 12edo" are also ambiguous, because {{val| 12 19 27 }}, for example, is a valid temperament even though it's much less accurate than {{val| 12 19 28 }}. In this temperament 5/4 would be represented as 3 steps of 12edo, or 300 cents. For practical purposes, of course, the ambiguity doesn't appear until higher limits.

== Linear temperament mappings ==

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-{{Beginner|Temperament mapping matrices}}
+{{Beginner|Temperament mapping matrix}}
 A [[regular temperament]] is more than simply a set of pitches. It's a set of notes together with a ''consistent rule'' that maps any pitch of the relevant [[just intonation subgroup]] to a specific note from that set. (In fact, an abstract regular temperament is not a set of definite pitches at all! The pitches can vary, and the rule mapping JI pitches to notes is the thing that uniquely characterizes the temperament.) This consistent rule is known as the ''JI mapping'' or simply '''mapping'''. The mapping answers the question "how do I play this JI pitch as a note of this temperament?". The answer will be the "tempered version" of that JI pitch, which may be a very close approximation or a very distant approximation depending on the circumstances.
-Naively, one might think that a simple rounding function might be suitable for a mapping: let the "tempered version" of each JI pitch simply be the tempered pitch that is closest to it. However, this (usually) does not result in a regular temperament at all! The reason is that, although this mapping assigns a tempered pitch to each JI pitch, it does not do so in a ''consistent'' way – some instances of the same JI interval are represented by different tempered intervals if they occur in different places. A regular temperament mapping always represents each JI interval by the ''same'' tempered interval (even if that tempered interval is not the closest tempered interval to the JI interval).
+Naively, one might think that a simple rounding function might be suitable for a mapping: let the "tempered version" of each JI pitch simply be the tempered pitch that is closest to it. However, this (usually) does not result in a regular temperament at all! The reason is that, although this mapping assigns a tempered pitch to each JI pitch, it does not do so in a ''consistent'' way – some instances of the same JI interval are represented by different tempered intervals if they occur in different places. A regular temperament mapping always represents each JI interval by the ''same'' tempered interval, even if that tempered interval is not the closest tempered interval to the JI interval.
 == A note on mathematical terminology ==
-In mathematics generally, "mapping" is synonymous with "map" and "function". In RTT, "mapping" has the more specific meaning of a [[Wikipedia:Linear_map|''linear'' mapping]], which is a function that can be represented by a matrix.
+In mathematics generally, "mapping" is synonymous with "map" and "function". In RTT, "mapping" has the more specific meaning of a {{w|Linear map|''linear'' mapping}}, which is a function that can be represented by a matrix.
 == Equal temperament mappings ==
@@ Line 34: / Line 35: @@
 Furthermore, consider 12edo as an 11-limit temperament. What is its mapping matrix? It actually depends whether you consider 11/8 a "very sharp D" or a "very flat D#". This choice results in two different mappings, {{val| 12 19 28 34 41 }} and {{val| 12 19 28 34 42 }}. The latter has a more accurate 11/8, but the former has more accurate versions of other intervals, including 12/11. In the language of regular temperament theory, these are simply two different 11-limit temperaments that both happen to have 12 steps per octave. Phrases like "11-limit 12edo" are thus ambiguous because they don't specify the mapping, and therefore don't refer to a specific temperament.
-(Strictly speaking, "5-limit 12edo" or even "3-limit 12edo" are also ambiguous, because {{val| 12 19 27 }}, for example, is a valid temperament even though it's much less accurate than {{val| 12 19 28 }}. In this temperament 5/4 would be represented as 3 steps of 12edo, or 300 cents. For practical purposes, of course, the ambiguity doesn't appear until higher limits.)
+Strictly speaking, "5-limit 12edo" or even "3-limit 12edo" are also ambiguous, because {{val| 12 19 27 }}, for example, is a valid temperament even though it's much less accurate than {{val| 12 19 28 }}. In this temperament 5/4 would be represented as 3 steps of 12edo, or 300 cents. For practical purposes, of course, the ambiguity doesn't appear until higher limits.
 == Linear temperament mappings ==