Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Keypress to WAV file playback

This scripts needs to be run in a terminal because it requires you to interact with Bela using your computer's keyboard. Note that it CAN NOT be run from within the IDE or the IDE's console.

See here how to use Bela with a terminal.

This program shows how to playback audio samples from a buffer using key strokes.

An audio file is loaded into a vector data. This is accessed with a read pointer that is incremented at audio rate within the render function: out += data[gReadPtr++].

Note that the read pointer is stopped from incrementing past the length of the data. This is achieved by comparing the read pointer value against the sample length which we can access as follows: data.size().

You can trigger the sample with keyboard input:

'a' <enter> to start playing the sample

's' <enter> to stop

'q' <enter> or ctrl-C to quit

Monitoring of the keyboard input is done in a low priority task to avoid interfering with the audio processing.

____ _____ _ _
| __ )| ____| | / \
| _ \| _| | | / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/ \_\
#include <Bela.h>
#include <sys/types.h>
#include <libraries/AudioFile/AudioFile.h>
#include <string>
#include <vector>
std::string fileName; // Name of the file to load
int gReadPtr; // Position of last read sample from file
std::vector<float> data;
bool initialise_trigger();
void trigger_samples(void*);
AuxiliaryTask gTriggerSamplesTask;
bool setup(BelaContext *context, void *userData)
// Retrieve the argument of the --file parameter (passed in from main())
fileName = (const char *)userData;
data = AudioFileUtilities::loadMono(fileName);
if(0 == data.size()) {
fprintf(stderr, "Unable to load file\n");
return false;
gReadPtr = -1;
if(initialise_trigger() == false)
return false;
// Start the lower-priority task. It will run forever in a loop
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->audioFrames; n++) {
float out = 0;
// If triggered...
if(gReadPtr != -1)
out += data[gReadPtr++]; // each sample...
if(gReadPtr >= data.size())
gReadPtr = -1;
for(unsigned int channel = 0; channel < context->audioOutChannels; channel++)
// ...and copy it to all the output channels
audioWrite(context, n, channel, out);
// Initialise the auxiliary task
// and print info
bool initialise_trigger()
if((gTriggerSamplesTask = Bela_createAuxiliaryTask(&trigger_samples, 50, "bela-trigger-samples")) == 0)
return false;
rt_printf("Press 'a' <enter> to trigger sample,\n"
" 's' <enter> to stop the current playing sample\n"
" 'q' <enter> or ctrl-C to quit\n");
return true;
// This is a lower-priority call to periodically read keyboard input
// and trigger samples. By placing it at a lower priority,
// it has minimal effect on the audio performance but it will take longer to
// complete if the system is under heavy audio load.
void trigger_samples(void*)
// This is not a real-time task because
// select() and scanf() are system calls, not handled by Xenomai.
// This task will be automatically down graded to "secondary mode"
// the first time it is executed.
char keyStroke = '.';
fd_set readfds;
struct timeval tv;
int fd_stdin;
fd_stdin = fileno(stdin);
while (!Bela_stopRequested()){
FD_SET(fileno(stdin), &readfds);
tv.tv_sec = 0;
tv.tv_usec = 1000;
// Check if there are any characters ready to be read
int num_readable = select(fd_stdin + 1, &readfds, NULL, NULL, &tv);
// if there are, then read them
if(num_readable > 0){
scanf("%c", &keyStroke);
if(keyStroke != '\n'){ // filter out the "\n" (newline) character
switch (keyStroke)
case 'a':
gReadPtr = 0;
case 's':
gReadPtr = -1;
case 'q':
void cleanup(BelaContext *context, void *userData)