Mediant hull

The mediant hull or Farey hull of a set of equal-step tunings for a given scale structure is the tuning range defined as the set of all tunings that can be obtained by traversing a finite or infinite path on the corresponding scale tree. We write the mediant hull of tunings by putting double bars || between the tunings. For example, 5L 2s(7||12), the mediant hull of 7edo diatonic and 12edo diatonic, is soft-of-basic diatonic.

Formally, given a 2-step scale structure S and a set of edo tunings T₁, ..., T_r, given by generators m₁/n₁, ..., m_r/n_r for S, the mediant hull or Farey hull of T₁, ..., T_r is [math]\displaystyle{ T_1 \mid\mid T_2 \mid\mid \cdots \mid\mid T_n = \operatorname{cl}\bigg\{\dfrac{a_1 m_1 + \cdots + a_r m_r}{a_1 n_1 + \cdots + a_r n_r} : (a_1, ..., a_r) \in \mathbb{Z}^r_{\geq 0} \setminus (0, 0, ..., 0) \bigg\} }[/math]

where [math]\displaystyle{ \operatorname{cl} }[/math] denotes the topological closure (specifying the generator is enough, given the period, since the period doesn't change upon taking the mediant). The generalization to higher-rank scale structures is obvious.

Notation

Properly, one should write out the generators or step ratios of all the input edo tunings, for example 5L 2s (7\12||11\19) or 5L 2s ((2:1)||(3:2)) Less carefully, we can also use just the edo numbers: 5L 2s (12||19) (but there is potential for confusion when an edo has multiple tunings of a given scale structure).

This works similarly for rank-3 and higher scales: for example, diasem(7||26||31) specifies the mediant hull of the 7edo (L:m:s = 1:1:0), 26edo (L:m:s = 4:2:1) and 31edo (L:m:s = 5:2:1) tunings for diasem.