Hello,

/*
 ____  _____ _        _    
| __ )| ____| |      / \   
|  _ \|  _| | |     / _ \  
| |_) | |___| |___ / ___ \ 
|____/|_____|_____/_/   \_\

http://bela.io

C++ Real-Time Audio Programming with Bela - Lecture 4: Parameter control
wavetable-slider: oscillator example using a browser-based GUI for control
*/

#include <Bela.h>
#include <libraries/Gui/Gui.h>
#include <libraries/GuiController/GuiController.h>
#include <cmath>

const int gWavetableLength = 512;	// The length of the buffer in frames
float gWavetable[gWavetableLength];	// Buffer that holds the wavetable
float gReadPointers[2] = {0, 0};    // Position of the last frame we played

// Browser-based GUI to adjust parameters
Gui gui;
GuiController controller;

bool setup(BelaContext *context, void *userData)
{
	    // Generate a sawtooth waveform (a ramp from -1 to 1) and
    // store it in the buffer. Notice: generating the wavetable does 
    // not depend on the audio sample rate (why not?)
    for(unsigned int n = 0; n < gWavetableLength; n++) {
	    gWavetable[n] = -1.0 + 2.0 * (float)n / (float)(gWavetableLength - 1);
    }

    // Set up the GUI
    gui.setup(context->projectName);
    controller.setup(&gui, "Wavetable Controller");	

    // Arguments: name, default value, minimum, maximum, increment
    controller.addSlider("Frequency", 220, 55, 440, 0);
    controller.addSlider("Amplitude", -20, -40, 0, 0);
        controller.addSlider("Detune", 0, 0, 0.05, 0);

    return true;
}

void render(BelaContext *context, void *userData)
{
    float frequency = controller.getSliderValue(0);	// Frequency is first slider
    float amplitude = controller.getSliderValue(1);	// Amplitude is second slider	
    float detune = controller.getSliderValue(2);    // Detune is our third slider

    float amplitude = powf(10.0, amplitudeDB / 20); // I RECEIVE AN ERROR HERE

    float frequencies[2];
    frequencies[0] = centreFrequency * (1.0 + detune); // I RECEIVE AN ERROR HERE
    frequencies[1] = centreFrequency * (1.0 - detune); // I RECEIVE AN ERROR HERE, TOO

    for(unsigned int n = 0; n < context->audioFrames; n++) {
	    float out = 0;
	
	    for(unsigned int i = 0; i < 2; i++) {
	        // The pointer will take a fractional index. Look for the sample on
	        // either side which are indices we can actually read into the buffer.
	        // If we get to the end of the buffer, wrap around to 0.
	        int indexBelow = floorf(gReadPointers[i]);
	        int indexAbove = indexBelow + 1;
	        if(indexAbove >= gWavetableLength)
		    indexAbove = 0;
	 
	        // For linear interpolation, we need to decide how much to weigh each
	        // sample. The closer the fractional part of the index is to 0, the
	        // more weight we give to the "below" sample. The closer the fractional
	        // part is to 1, the more weight we give to the "above" sample.
	        float fractionAbove = gReadPointers[i] - indexBelow;
	        float fractionBelow = 1.0 - fractionAbove;
	
	        // Calculate the weighted average of the "below" and "above" samples
                out += amplitude * (fractionBelow * gWavetable[indexBelow] +
    				    fractionAbove * gWavetable[indexAbove]);
    				
    	        gReadPointers[i] += gWavetableLength * frequencies[i] / context->audioSampleRate;
    	        while(gReadPointers[i] >= gWavetableLength)
    	            gReadPointers[i] -= gWavetableLength;
	    }
        
	        for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
		    // Write the sample to every audio output channel
		    audioWrite(context, n, channel, out);
	        }
    }
}

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

}

I think I am missing something from Course 4 online on Youtube.com.

Seth

P.S. If you see where I am making errors, please let me know. Oh...

Bela stopped
redefinition of 'amplitude' column: 8, line: 54
use of undeclared identifier 'amplitudeDB'; did you mean 'amplitude'? column: 31, line: 54
use of undeclared identifier 'centreFrequency' column: 19, line: 57
use of undeclared identifier 'centreFrequency' column: 19, line: 58