[ardour-users] Manipulating Automation Tables

[Paul Davis]

On Wed, 2006-05-17 at 12:01 -0600, Neil Nelson wrote:
> I have a bus with gain automation on the bus slider and gain automation
> on the 'simple amp' plugin for that bus. My goal is to combine the
> tables for those two automations that are in the 'automation' directory
> and reduce or increase the resulting values by some constant db value.
> 
> I can see a way to combine the tables but do not immediately see what
> value needs to be applied to each row of the result to adjust the volume
> by a given db value. The table values appear to be logrithms. Most
> simply I am looking for the formula to convert a DB value to its
> automation table value. I have an estimate but the exact formula is
> coded and known.
> 
> DB     Table
> 1       0
> 1.9     1.25306
> 3.3     1.46225
> 3.7     1.53701
> 3.6     1.51181

from libs/ardour/ardour/dB.h:

static inline float dB_to_coefficient (float dB) {
	return dB > -318.8f ? pow (10.0f, dB * 0.05f) : 0.0f;
}

static inline float coefficient_to_dB (float coeff) {
	return 20.0f * fast_log10 (coeff);
}