Logarithmic midi control in tidal

Hi everyone,

I just worked out how to use midi input on effects and control busses thanks to this thread here: Tidal 1.7 control busses + midi control input = ❤️

This is a fantastic feature! I just have one query about it. Can anyone think of a way to map midi input onto a logarithmic range, so I could control tidal's filters with it?

From what I can gather, tidal automatically converts midi cc from a 1-127 range into a 0-1 range, which you can then multiply to get larger or smaller ranges. For example, # speed (8 * "^14") would allow midi cc 14 to control sample playback speed in a range of 0-8. However, if I wanted to control a low pass filter, to achieve a clean-sounding sweep of values, I'd have to somehow scale a cc input logarithmically between 0 and 10000.

Any ideas as to how to do this?

1 Like

Hi! What you're looking for is rangex, the exponential version of the range function. There's some documentation here, but the usage is pretty straightforward:

# lpf (rangex 1 10000 "^14")

This would take your CC 14 input and map it to the logarithmic range 1-10000 (note, you have to start your output range on a positive number because log(0) is undefined).

2 Likes

thank u so much! I had tried that but didn't know about having to start from a positive number, works perfect with the new effects busses too :slight_smile:

know about having to start from a positive number ...

let me (again) advertise the :doc command. It shows the info that you missed.

ghci> :doc rangex
rangex :: (Functor f, Floating b) => b -> b -> f b -> f b
  	-- Identifier defined at src/Sound/Tidal/UI.hs:1702:1
 `rangex` is an exponential version of `range`, good for using with
frequencies.  Do *not* use negative numbers or zero as arguments! 

I can see the mathematical beauty of the implementation
( Tidal/UI.hs at main · tidalcycles/Tidal · GitHub )

rangex from to p = exp <$> _range (log from) (log to) p

but it does go counter to everyday experience with logarithmic potentiometers (e.g., sliders on a mixing desk): they typically go from 0 to 1 (or somewhere), with 0 = total silence (and not: 0 = forbidden)

Supercollider has similar functions
( supercollider/MiscInlineMath.h at develop · supercollider/supercollider · GitHub ) (tidal's rangex is their expexp? they can addidionally specify the base of the logarithm = the skewness of the curve) but they have the same problem (division by zero, log of zero). Huh.