[Linuxsampler-devel] Envelopes

[Steve H. <S.W...@ec...>]

Hi all,

Benno and I were discussing envelope generation last night, I think that
the right way to generate an exponential envelope (I checked some synths
outputs too and it looks like this is that way its done) is to feed a
constant value into a LP filter with different parameters for each stage.

I suspect that, in general you need to calculate the amplitude of the
envelope for each sample, to avoid stairstepping (zipper noise).

I expect Paul Kellett knows the right approach, so should be able to say
if we're barking up the wrong tree.

Example code:

#include <math.h>
#include <stdio.h>

#define EVENTS 5

#define ENV_NONE    0
#define ENV_ATTACK  1
#define ENV_DECAY   2
#define ENV_SUSTAIN 3
#define ENV_RELEASE 4

void lp_set_par(double time, double *a, double *ai) {
        *a = exp(-5.0 / time); // The 5.0 is a fudge factor
        *ai = 1.0 - *a;
}

int main() {
        unsigned int event_time[EVENTS] = {0, 100, 200, 400, 900};
        unsigned int event_action[EVENTS]= {ENV_ATTACK, ENV_DECAY, ENV_SUSTAIN,
                ENV_RELEASE, ENV_NONE};
        unsigned int i, event = 0;
        float env_input = 0.0f;
        double env = 0.0f;
        double a, ai;

        float attack_level =  1.0f;
        float sustain_level = 0.5f;
        float release_level = 0.0f;

        for (i=0; i<1000; i++) {
                if (i == event_time[event]) {
                        switch (event_action[event]) {
                                case ENV_ATTACK:
                                        env_input = attack_level;
                                        break;
                                case ENV_DECAY:
                                        env_input = sustain_level;
                                        break;
                                case ENV_SUSTAIN:
                                        env_input = sustain_level;
                                        break;
                                case ENV_RELEASE:
                                        env_input = release_level;
                                        break;
                        }
                        lp_set_par((double)(event_time[event+1] -
				event_time[event]), &a, &ai);
                        event++;
                }

                env = env_input * ai + env * a;
                printf("%g\n", env);
        }

        return 0;
}