Analog/analog-input/render.cpp

Connecting potentiometers

This sketch produces a sine tone, the frequency and amplitude of which are modulated by data received on the analog input pins. Before looping through each audio frame, we declare a value for the frequency and amplitude of our sine tone; we adjust these values by taking in data from analog sensors (for example potentiometers) with analogRead().

• connect a 10K pot to 3.3V and GND on its 1st and 3rd pins.
• connect the 2nd middle pin of the pot to analogIn 0.
• connect another 10K pot in the same way but with the middle pin connected to analogIn 1.

The important thing to notice is that audio is sampled twice as often as analog data. The audio sampling rate is 44.1kHz (44100 frames per second) and the analog sampling rate is 22.05kHz (22050 frames per second). Notice that we are processing the analog data and updating frequency and amplitude only on every second audio sample, since the analog sampling rate is half that of the audio.

if(!(n % gAudioFramesPerAnalogFrame)) {
    // Even audio samples: update frequency and amplitude from the analog inputs
    frequency = map(analogRead(context, n/gAudioFramesPerAnalogFrame, gSensorInputFrequency), 0, 1, 100, 1000);
    amplitude = analogRead(context, n/gAudioFramesPerAnalogFrame, gSensorInputAmplitude);
}

/*
 ____  _____ _        _
| __ )| ____| |      / \
|  _ \|  _| | |     / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/   \_\
http://bela.io
*/
 
#include <Bela.h>
#include <cmath>
 
float gPhase;
float gInverseSampleRate;
int gAudioFramesPerAnalogFrame = 0;
 
// Set the analog channels to read from
int gSensorInputFrequency = 0;
int gSensorInputAmplitude = 1;
 
bool setup(BelaContext *context, void *userData)
{
 
        // Check if analog channels are enabled
        if(context->analogFrames == 0 || context->analogFrames > context->audioFrames) {
                rt_printf("Error: this example needs analog enabled, with 4 or 8 channels\n");
                return false;
        }
 
        // Useful calculations
        gAudioFramesPerAnalogFrame = context->audioFrames / context->analogFrames;
        gInverseSampleRate = 1.0 / context->audioSampleRate;
        gPhase = 0.0;
 
        return true;
}
 
void render(BelaContext *context, void *userData)
{
        static float frequency;
        static float amplitude;
 
        for(unsigned int n = 0; n < context->audioFrames; n++) {
                if(gAudioFramesPerAnalogFrame && !(n % gAudioFramesPerAnalogFrame)) {
                        // read analog inputs and update frequency and amplitude
                        // Depending on the sampling rate of the analog inputs, this will
                        // happen every audio frame (if it is 44100)
                        // or every two audio frames (if it is 22050)
                        frequency = map(analogRead(context, n/gAudioFramesPerAnalogFrame, gSensorInputFrequency), 0, 1, 100, 1000);
                        amplitude = analogRead(context, n/gAudioFramesPerAnalogFrame, gSensorInputAmplitude);
                }
 
                float out = amplitude * sinf(gPhase);
 
                for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
                        audioWrite(context, n, channel, out);
                }
 
                // Update and wrap phase of sine tone
                gPhase += 2.0f * (float)M_PI * frequency * gInverseSampleRate;
                if(gPhase > M_PI)
                        gPhase -= 2.0f * (float)M_PI;
        }
}
 
void cleanup(BelaContext *context, void *userData)
{
 
}