Equation for LFO rate on Moog analogue synth

I have a Moog Voyager synth and it has an LFO with a rate setting. It is some sort of exponential scaling. On the front panel it goes from 0.2 to 50Hz, with 3Hz in the middle. These three values correspond with with the A to D values 0, 254 and 131 respectively on the LCD display. I was wondering if there is a likely equation that describes the circuit so I can approximate the rate (Hz) for each digital setting. I tried to fit an exponential curve and a quadratic curve to the values but neither worked out. Any ideas? In the meantime I may try a cubic spline.

 

On Sun, 29 Jun 2014, danwgrace@gmail.com wrote:

I have a Moog Voyager synth and it has an LFO with a rate setting. It
is some sort of exponential scaling. On the front panel it goes from
0.2 to 50Hz, with 3Hz in the middle. These three values correspond with
with the A to D values 0, 254 and 131 respectively on the LCD display.
I was wondering if there is a likely equation that describes the circuit
so I can approximate the rate (Hz) for each digital setting. I tried to
fit an exponential curve and a quadratic curve to the values but neither
worked out. Any ideas? In the meantime I may try a cubic spline.

Well, three points determine a quadratic so there isn't enough for a
cubic. The quadratic would be
..00142x^2-.16464x+.2
but
..2*16 = 3.2 ~=3
3*16=48 ~=50
so something like
..2*256^(x/256) =.2*2^(x/32)is suggested
for this
0->.2
131->3,414
254->49.03
and
127->3.13
maybe the 256s should be 255s or something but I'll bet
it's exponential and the marks are approximate. can you measue the LFO?
--record it AM modulating and look at the distance between peaks.

Ron