Hello,
I got the e-book "basic synth" which describes very thouroughly how to make many elementary parts of a software synthesizer, and how it relates to the math.

After trying to modify the basic sine patch with an envelope, the IDE keeps giving me an "underrun detected" and then the Bela disconnects from USB and I have to unpower it completely before it will work again.

My code looks like this:

#include <Bela.h>
#include <cmath>
#include <iostream>
using namespace std;

float frequency = 440.0;
float phase;
float inverseSampleRate;

float TWO_PI = M_PI * 2;
float freqRadians;
float phaseIncr;
int envelopeDuration = 3; // seconds
float totalSamples; 
float attackRate = 10;
float decayRate = 10;
float attackTime;
float decayTime;
float decayStart;
float envInc;
float peakAmp = 1;
float volume = 0;

bool setup(BelaContext *context, void *userData)
{
	inverseSampleRate = 1.0 / context->audioSampleRate; // time of one sample
	phase = 0.0;
	freqRadians = TWO_PI / context->audioSampleRate;
	phaseIncr = freqRadians * frequency;
	totalSamples = envelopeDuration * context->audioSampleRate;
	attackTime = attackRate * context->audioSampleRate;
	decayTime = decayRate * context->audioSampleRate;
	decayStart = totalSamples - decayTime;
	if (attackTime > 0) envInc = peakAmp / attackTime;
	
	return true;
}

void render(BelaContext *context, void *userData)
{
	unsigned int n = 0;
	
	for(unsigned int k = 0; k < totalSamples; k++){
		if(k < attackTime || k > decayStart) volume += envInc;
		else if(k == attackTime) volume = peakAmp;
		else if(k == decayStart) envInc = -volume/decayTime;
		
		
		float out = volume * sinf(phase);
		phase += phaseIncr;
		if((phase += phaseIncr) >= TWO_PI) phase -= TWO_PI;

		for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
			audioWrite(context, n, channel, out);
		}
		n++;
		if(n > 16) n = 0;
		
	} 
}

void cleanup(BelaContext *context, void *userData)
{

}

Can anyone see what I'm doing wrong maybe? I put the original for loop, that loops around the audio frames, inside a "larger" for loop that (should) loops around my envelope. I hear a constantly loud clicking, very low frequency, which changes it's timbral qualities slowly.. but not the tone or envelope i would expect.

Alternatively, if someone can point me to a simple well-working example of a simple envelope implementation - that would be helpful too.

Thanks