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

Nick Copeland nickycopeland at hotmail.com
Tue Aug 21 20:14:28 UTC 2012

On Tue, 2012-08-21 at 11:02 +1200, Jeff McClintock wrote:
>> > I think the most reasonable standard for an absolute 1/oct
>> > frequency unit is 0.0 = 440Hz?
>> My modular plugins use a reference of 440Hz. Also parameters are ranged
>> between 0.0 - 10.0 but can exceed that if need be. (in a modular synth,
>> everything needs to interoperate).
> So for frequency 5.0 is 440Hz (Middle-A). i.e. the middle of the range -
> 5.0, is the standard 'middle' key.
> While I initially thought negative values was really weird, on second
> thought having it centred about 0.0 is nicer, especially considering the
> relative uses
> Great idea though. Octaves are far more universal than western semitones,
> yet trivial to convert between.
   Aren't there a couple of misconceptions here or at least some potentialfor incompatibility?
The signal itself is a digital representation of a voltage modifier. Thevoltage has no semantic other than its value, it does not represent freqor gain or cutoff or anything - the semantics of what happens only occursat the 'sink/dst' of the signal.
Consider the following points:
a. 1/Oct is not a reference value, it is a definition for the effects of amodifier signal. The value of 0.0 should definitely not refer to A440.When a mod signal is applied to an osc then it modifies the settingof the osc. You set the osc to A440 and apply 0v then it has no affect.If you apply 1v it should output A880, that is 1/octave. If you want thesignal to be able to reduce the frequency then you transpose the osc downby a couple of octaves and then set the base mod signal at 2+ve to getback to a natural tuning, you can now reduce the frequence by 1/Octave too.
Now let's have a look at those negative values with another example:
b. It should not support negative values. Let's forget about the 1/Octfor a minute. This defines how a mod signal is applied: if it is appliedto an osc or filter coff it represents 1/Octave. It can also be applied toan amplifier though. If you accept negative values you will get phaseinversion and a relative signal gain, that is probably not the intentionof the modifier though: if you do not restrict the signal to +ve valuesthen the source of the mod signal may need to understand more aboutthe semantics of the sink - in the case that you have 0.0 to represent A440 then does it also have to represent 0dB (or -96dB) when itis applied to an amplifier for example. This is excessive since in anarbitrary system where rewiring is possible then sources can be audiosignals or can be mod signals so now they need to understand where they are patched to be able to deliver a correct reference. That is atbest difficult and at worst unmanageable.
The above is partially opinion but is based on analogue signal pathsfrom the old mono/mega synths. Agreed it might be time to move on buteither way, a modifier signal of 1/Oct is based on the 1v/Oct and inthat model the modifier signal has no reference regarding frequency,gain or anything else, that reference is a function of what it is beingused to modified.
Just to be complete, I have no objection to such signals having someimplied semantics. There will be some apps that do not have suchrestrictions and this will result in inconsistencies - this is a good thing though, probably, modifier signals and arbitrary routing wasalways used as a testbed to generate new sounds so bring them on.
Regards, nick
"I had to enter a password that needed eight characters. I used "SnowWhite and the Seven Dwarves",' Vine, 2011 Edinburgh Fringe (I think).
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