[LAD] Fader mapping - was - Ardour MIDI tracer

Fons Adriaensen fons at linuxaudio.org
Fri Aug 22 16:53:11 UTC 2014

On Fri, Aug 22, 2014 at 08:11:39AM -0700, Len Ovens wrote:
> It appears this: "On the sender side you transmit 0..127 which is
> just 7 bits of an ADC measuring the voltage from a linear fader or
> pot, there is no mapping at all." Is not true... or at least not
> usable as we would end up with some areas of our fade with giant
> steps from controler tick to controler tick. It seems that for
> proper information transfer we need .5db per step for 50 to 60 db
> travel no matter how it is mapped.

You have two mappings:

* Position (mm or degrees) to MIDI (0..127) by the control surface,
* MIDI (0..127) to gain (dB), or to whatever is being controlled,
  by the DAW or instrument.

Position to gain (or anything else), in other words what is 
experienced bu the user, results from the combination of those

This also means that the controller can't have any calibration
marks unless it knowns the mapping used by the receiver. This
is always true, regardless of the actual mappings used.

Having a linear mapping from postion or angle to 0..127 is in
almost all cases the only sensible thing to do. The accuracy
with which a user can set or move a fader or pot does not
depend on its position. Hence the resolution should not depend
on it. 

Such a linear mapping is very probably also what is assumed by
the receiver, which will have its mapping defined such that in
combination with the one from the controller you get the desired
result, e.g. a good audio taper for a fader.
For a fader you want small steps (0.5 dB or less) in the upper
region. At low gains larger ones (1 dB or so down to -60 and even
bigger ones below that) are perfectly OK, and allow to extend the
range and/or to have smaller steps in the upper region. 

What would feel very strange to a user is to have bigger _physical_
(mm or degrees) steps in any part of the range. Hence the linear
mapping to 0..127. There may be exceptions for some very specific
controllers (e.g. thrust levers in an airplane), but an audio fader
is not one of them.



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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