Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Finite Impulse Response Filter

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 cannot be run from within the IDE or the IDE's console.

See here how to use Bela with a terminal.

In this project an audio recording processesd through an FIR filter.

To control the playback of the audio sample, use your computer keyboard, by pressing:

'a' <enter> to start playing the sample

's' <enter> to stop

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

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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:
#define ENABLE_NE10_FIR_FLOAT_NEON // Define needed for Ne10 library
#include <Bela.h>
#include <cmath>
#include <ne10/NE10.h> // neon library
#include "SampleData.h"
#include "FIRfilter.h"
SampleData gSampleData; // User defined structure to get complex data from main
int gReadPtr; // Position of last read sample from file
// filter vars
ne10_fir_instance_f32_t gFIRfilter;
ne10_float32_t *gFIRfilterIn;
ne10_float32_t *gFIRfilterOut;
ne10_uint32_t blockSize;
ne10_float32_t *gFIRfilterState;
void initialise_filter(BelaContext *context);
// Task for handling input from the keyboard
AuxiliaryTask gTriggerSamplesTask;
bool initialise_trigger();
void trigger_samples(void*);
bool setup(BelaContext *context, void *userData)
// Retrieve a parameter passed in from the initAudio() call
gSampleData = *(SampleData *)userData;
gReadPtr = -1;
// Initialise auxiliary tasks
return false;
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->audioFrames; n++) {
float in = 0;
// If triggered...
if(gReadPtr != -1)
in += gSampleData.samples[gReadPtr++]; // each sample...
if(gReadPtr >= gSampleData.sampleLen)
gReadPtr = -1;
gFIRfilterIn[n] = in;
ne10_fir_float_neon(&gFIRfilter, gFIRfilterIn, gFIRfilterOut, blockSize);
for(unsigned int n = 0; n < context->audioFrames; n++) {
for(unsigned int channel = 0; channel < context->audioOutChannels; ++channel)
// ...and copy it to all the output channels
audioWrite(context, n, channel, gFIRfilterOut[n]);
// Request that the lower-priority task run at next opportunity
// Initialise NE10 data structures to define FIR filter
void initialise_filter(BelaContext *context)
blockSize = context->audioFrames;
gFIRfilterState = (ne10_float32_t *) NE10_MALLOC ((FILTER_TAP_NUM+blockSize-1) * sizeof (ne10_float32_t));
gFIRfilterIn = (ne10_float32_t *) NE10_MALLOC (blockSize * sizeof (ne10_float32_t));
gFIRfilterOut = (ne10_float32_t *) NE10_MALLOC (blockSize * sizeof (ne10_float32_t));
ne10_fir_init_float(&gFIRfilter, FILTER_TAP_NUM, filterTaps, gFIRfilterState, blockSize);
// 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 start playing the sample\n"
" 's' <enter> to stop\n");
rt_printf(" '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 (!gShouldStop){
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':
gShouldStop = true;
void cleanup(BelaContext *context, void *userData)
delete[] gSampleData.samples;