Scoping sensor input

This example reads from analogue inputs 0 and 1 via analogRead() and generates a sine wave with amplitude and frequency determined by their values. It's best to connect a 10K potentiometer to each of these analog inputs. Far left and far right pins of the pot go to 3.3V and GND, the middle should be connected to the analog in pins.

The sine wave is then plotted on the oscilloscope. Click the Open Scope button to view the results. As you turn the potentiometers you will see the amplitude and frequency of the sine wave change. You can also see the two sensor readings plotted on the oscilloscope.

The scope is initialised in setup() where the number of channels and sampling rate are set.

scope.setup(3, context->audioSampleRate);

We can then pass signals to the scope in render() using:

scope.log(out, in1, in2);

This project also shows as example of map() which allows you to re-scale a number from one range to another. Note that map() does not constrain your variable within the upper and lower limits. If you want to do this use the constrain() function.

____ _____ _ _
| __ )| ____| | / \
| _ \| _| | | / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/ \_\
The platform for ultra-low latency audio and sensor processing
A project of the Augmented Instruments Laboratory within the
Centre for Digital Music at Queen Mary University of London.
(c) 2016 Augmented Instruments Laboratory: Andrew McPherson,
Astrid Bin, Liam Donovan, Christian Heinrichs, Robert Jack,
Giulio Moro, Laurel Pardue, Victor Zappi. All rights reserved.
The Bela software is distributed under the GNU Lesser General Public License
(LGPL 3.0), available here:
#include <Bela.h>
#include <cmath>
#include <Scope.h>
Scope scope;
float gInverseSampleRate;
float gPhase;
bool setup(BelaContext *context, void *userData)
// setup the scope with 3 channels at the audio sample rate
scope.setup(3, context->audioSampleRate);
// 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;
// Check that we have the same number of inputs and outputs.
if(context->audioInChannels != context->audioOutChannels ||
context->analogInChannels != context-> analogOutChannels){
printf("Error: for this project, you need the same number of input and output channels.\n");
return false;
gInverseSampleRate = 1.0 / context->audioSampleRate;
gPhase = 0.0;
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->audioFrames; n++) {
// read analogIn channels 0 and 1
float in1 = analogRead(context, n, 0);
float in2 = analogRead(context, n, 1);
// map in1 to amplitude and in2 to frequency
float amplitude = in1 * 0.8f;
float frequency = map(in2, 0, 1, 100, 1000);
// generate a sine wave with the amplitude and frequency
float out = amplitude * sinf(gPhase);
gPhase += 2.0 * M_PI * frequency * gInverseSampleRate;
if(gPhase > 2.0 * M_PI)
gPhase -= 2.0 * M_PI;
// log the sine wave and sensor values on the scope
scope.log(out, in1, in2);
// pass the sine wave to the audio outputs
for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
audioWrite(context, n, channel, out);
void cleanup(BelaContext *context, void *userData)