Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Audio and analog passthrough: input to output

This sketch demonstrates how to read from and write to the audio and analog input and output buffers.

In render() you'll see a nested for loop structure. You'll see this in all Bela projects. The first for loop cycles through audioFrames, the second through audioInChannels (in this case left 0 and right 1).

You can access any information about current audio and sensor settings like this: context->name_of_item. For example context->audioInChannels returns current number of input channels, context->audioFrames returns the current number of audio frames, context->audioSampleRate returns the audio sample rate.

You can look at all the information you can access in BelaContext.

Reading and writing from the audio buffers

The simplest way to read samples from the audio input buffer is with audioRead() which we pass three arguments: context, current audio frame and current channel. In this example we have audioRead(context, n, ch) where both n and ch are provided by the nested for loop structure.

We can write samples to the audio output buffer in a similar way using audioWrite(). This has a fourth argument which is the value of to output. For example audioWrite(context, n, ch, value_to_output).

Reading and writing from the analog buffers

The same is true for analogRead() and analogWriteOnce().

Note that for the analog channels we write to and read from the buffers in a separate set of nested for loops. This is because they are sampled at half audio rate by default. The first of these for loops cycles through analogFrames, the second through analogInChannels.

By setting audioWrite(context, n, ch, audioRead(context, n, ch)) and analogWriteOnce(context, n, ch, analogRead(context, n, ch)) we have a simple passthrough of audio input to output and analog input to output.

It is also possible to address the buffers directly, for example: context->audioOut[n * context->audioOutChannels + ch].

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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 <algorithm>
int gAudioChannelNum; // number of audio channels to iterate over
int gAnalogChannelNum; // number of analog channels to iterate over
bool setup(BelaContext *context, void *userData)
// Check that we have the same number of inputs and outputs.
if(context->audioInChannels != context->audioOutChannels ||
context->analogInChannels != context-> analogOutChannels){
printf("Different number of outputs and inputs available. Working with what we have.\n");
// If the amout of audio and analog input and output channels is not the same
// we will use the minimum between input and output
gAudioChannelNum = std::min(context->audioInChannels, context->audioOutChannels);
gAnalogChannelNum = std::min(context->analogInChannels, context->analogOutChannels);
return true;
void render(BelaContext *context, void *userData)
// Simplest possible case: pass inputs through to outputs
for(unsigned int n = 0; n < context->audioFrames; n++) {
for(unsigned int ch = 0; ch < gAudioChannelNum; ch++){
audioWrite(context, n, ch, audioRead(context, n, ch));
// Same with analog channels
for(unsigned int n = 0; n < context->analogFrames; n++) {
for(unsigned int ch = 0; ch < gAnalogChannelNum; ch++) {
analogWriteOnce(context, n, ch, analogRead(context, n, ch));
void cleanup(BelaContext *context, void *userData)