[LAD] Plugin 1/oct frequency controls (AMS/MCP/VCO)

David Robillard d at drobilla.net
Mon Aug 20 04:27:12 UTC 2012

Hi all,

I am porting to LV2 some AMS-influenced plugins (mainly those by Fons)
which have odd 1/Oct frequency ports.  I understand why it is sometimes
convenient to use octaves rather than the more typical Hz for frequency,
but after some digging to figure out how to precisely describe this
unit, I discovered the central frequency is middle C, i.e. C4, i.e.
around 262Hz.

For hosts to be able to use such plugins effectively, I need to
precisely describe this unit (and then other plugins can implement to
spec and they will all get along).  We already have an 'octaves' unit,
but no base frequency is defined.  I can add one, but I am not sure
about this strange choice.

Nobody tunes anything based on middle C, with its odd frequency of
261.62556... writing this in a spec gives me pause.  I suspect it
evolved from MIDI code in AMS where the 60 of middle C looks as
reasonable as anything else, but when you try to actually
define/document the unit it looks silly.

I think the natural central frequency to use is A440, at precisely a
nice round 440.0 Hz, so 0.0 is A4=440.0Hz, 1.0 is A5=880.0Hz, -1.0 is
A3=220Hz, and so on.  Whenever a default or center or tuning frequency
is needed, you use A4/440Hz...

tl;dr: I think the most reasonable standard for an absolute 1/oct
frequency unit is 0.0 = 440Hz


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