Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Trill Flex Raw Visualisation

This example visualises the raw readings from a Trill Flex sensor and at the same time displays discrete touches, which are retrieved through a CentroidDetection object. The rest of the settings are the same as the general-settings example.

The Trill sensor is scanned on an auxiliary task running in parallel to the audio thread.

Once the project is running click the GUI button to see the reading on each channel. From this GUI you can also adjust the setting of the sensor to best suit your needs.

There are two important sensor settings that you may want to adjust when working with Trill Flex: the threshold and the prescaler.

The threshold setting is simply the threshold above which to read and is for ignoring any noise that might be present in the lowest regions of the sensor reading. This only applies to DIFF mode and is a float between 0.0 and 0.625. For Trill Flex we recommend a value of 0.03 or above.

The prescaler setting equates to the sensitivity of the sensor. Technically, this value is a divider for the clock on the cypress chip and so it decides how long the chip charges the connected material for before taking a reading. There are 8 different settings for the prescaler. For Trill Flex we recommend a value of 4.

The rule of thumb when adjusting these values is:

When connecting different materials to the Trill Flex base, we recommend experimenting with the settings using this example. This example allows you to experiment with different settings from within the GUI which you can then hard code in your project once you're happy with the behaviour of the sensors.

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#include <Bela.h>
#include <libraries/Trill/Trill.h>
#include <libraries/Gui/Gui.h>
#include <libraries/Pipe/Pipe.h>
#include <libraries/Trill/CentroidDetection.h>
#define NUM_TOUCH 4 // Number of touches on our custom slider
Trill touchSensor;
Pipe gPipe;
Gui gui;
// Location of touches on Trill
float gTouchLocation[NUM_TOUCH] = {0.0};
// Size of touches on Trill
float gTouchSize[NUM_TOUCH] = {0.0};
// Number of active touches
int gNumActiveTouches = 0;
// Sleep time for auxiliary task
unsigned int gTaskSleepTime = 12000; // microseconds
// Time period (in seconds) after which data will be sent to the GUI
float gTimePeriod = 0.015;
int bitResolution = 12;
int gButtonValue = 0;
typedef enum {
} ids_t;
struct Command {
ids_t id;
float value;
#include <tuple>
std::vector<std::pair<std::wstring, ids_t>> gKeys =
{L"prescaler", kPrescaler},
{L"baseline", kBaseline},
{L"noiseThreshold", kNoiseThreshold},
{L"numBits", kNumBits},
{L"mode", kMode},
// This callback is called every time a new message is received from the Gui.
// Given how we cannot operate on the touchSensor object from a separate
// thread, we need to pipe the received messages to the loop() thread, so that
// they can be processed there
bool guiCallback(JSONObject& json, void*)
struct Command command;
for(auto& k : gKeys)
if(json.find(k.first) != json.end() && json[k.first]->IsNumber())
{ = k.second;
command.value = json[k.first]->AsNumber();
return false;
void loop(void*)
int numBits;
int speed = 0;
for(unsigned int i = 0; i < cd.getNumTouches() ; i++) {
gTouchLocation[i] = cd.touchLocation(i);
gTouchSize[i] = cd.touchSize(i);
gNumActiveTouches = cd.getNumTouches();
/* you could use compound touch instead
gTouchLocation[0] = cd.compoundTouchLocation();
gTouchSize[0] = cd.compoundTouchSize();
gNumActiveTouches = !(!gTouchSize[0]);
Command command;
// receive any command from the gui through the pipe
while(1 == gPipe.readRt(command))
float value = command.value;
case kPrescaler:
printf("setting prescaler to %.0f\n", value);
case kBaseline:
printf("reset baseline\n");
case kNoiseThreshold:
printf("setting noiseThreshold to %f\n", value);
case kNumBits:
numBits = value;
printf("setting number of bits to %d\n", numBits);
touchSensor.setScanSettings(speed, numBits);
case kMode:
printf("setting mode to %.0f\n", value);
bool setup(BelaContext *context, void *userData)
// Setup a Trill Flex on i2c bus 1, using the default address.
if(touchSensor.setup(1, Trill::FLEX) != 0) {
fprintf(stderr, "Unable to initialise Trill Flex\n");
return false;
// set it to DIFF mode: we will only use some pads from it, which we set up next
// specify which pads to use (and in which order), how many touches and
// the rescaling divisor for centroid size (depends on the shape and
// material of your sensing surface)
cd.setup(30, NUM_TOUCH, 3200);
gui.setControlDataCallback(guiCallback, nullptr);
return true;
void render(BelaContext *context, void *userData)
static unsigned int count = 0;
for(unsigned int n = 0; n < context->audioFrames; n++) {
// Send number of touches, touch location and size to the GUI
// after some time has elapsed.
if(count >= gTimePeriod*context->audioSampleRate)
gui.sendBuffer(0, touchSensor.getNumChannels());
gui.sendBuffer(1, touchSensor.rawData);
gui.sendBuffer(2, gNumActiveTouches);
gui.sendBuffer(3, gTouchLocation);
gui.sendBuffer(4, gTouchSize);
count = 0;
void cleanup(BelaContext *context, void *userData)