Octave discriminator

In octave equivalence tunings, the Octave discriminator is to distinguish between the different octaves of a note.

Example the standard is to have A4 be 440Hz. However because xenharmonic tunings may deviate from the standard, other possibilites are available. Many tunings including Equal-step Tunings and many notation systems define relative intervals but not absolute pitch, leaving the absolute tuning and the octave discriminator implementation defined.

In case of A4, C4, etc. the octave discriminator is 4. The octave is applied on the frequency and its modifiers. Therefore, the flat of C5 would be Cb5.

Incrementing the octave discriminator will multiply the frequency by 2/1, whereas decrementing it will divide the frequency by 2/1. For example, if reference A4 is defined as 440Hz, and C is defined as A/(5/3), then C4 would be 264Hz, and C5 would be 528Hz. Depending on the convention, reference note may be other than A as well.