ECE497 Project Makeshift Drums
Our project will communicate with several accelerators to make music. These sensors will be attached by the user to everyday objects like books or a table top and the Beagle Bone Black will use the sensors' outputs to play sounds. In this way, a user can play the drums, bongos, maracas, et cetera without a drum kit or other bulky equipment.
So far we have the accelerometer interface and signal conditioning portions of the project completed--it can detect strikes and reports them to the console. We have the Node.js portion partially complete but we need to iron out some kinks with our code to make it interact correctly.
We still need to work on the web interface and to make it play sounds after a strike.
Give step by step instructions on how to install your project.
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Our project allows the user to navigate to a port on the beaglebone. The bone then provides an interface which allows the user to select which instrument he or she wishes to play.
At this point, the user can shake, strike or otherwise agitate the ADXL345 accelerometer connected to the beaglebone. The beaglebone processes these movements and and will play sounds corresponding to the selected instrument through the browser accordingly. In this way the user can act like the beaglebone is a maraca, bongo, or other percussion instrument to emulate a real percussionist.
Include a YouTube demo.
Theory of Operation
When the program first runs it initializes the accelerometer to sample at 100Hz with a range of +/-2g. Whenever a sample is collected, the accelerometer sends an interrupt signal to one of the BBB's GPIO pins. Our code then uses the I2C protocol to retrieve that sample and any others it has collected in the meantime. The accelerometer has a FIFO queue which will collect up to 32 samples in the even that the BBB does not service the interrupt before the next samples are collected.
The BBB then conditions the signal so it can ignore any offsets (such as those due to the force of gravity) or small changes, like noise of the device being rotated. By changing a few parameters of our conditioning algorithm we can change how strong a strike or shake must be before the device detects it.
To condition the signal, we first considered the fact that the accelerometer is affected by the force of gravity. This provides an offset depending on the orientation with which the user is holding the accelerometer and affects our calculation. To factor this out, we found the derivative of the acceleration by subtracting each incoming sample with the previous one. By accumulating the subsequent values we eliminate this offset.
However, if we reorient the device the offset comes back--we merely calibrated the offset for the accelerometer's starting position. To continuously factor out this offset as the orientation changes we made the accumulator only add together the latest handful of samples. This essential creates a highpass filter. By changing the number of points accumulated at once we can affect how quickly a change must occur for it to not be filtered out. This helps us detect shakes or strikes much easier.
Next we simply accumulate our filtered acceleration values to get our velocity. When our velocity is equal to zero (and our acceleration is non-zero) we have found a shake or a strike. This is when our program reports the strike to the browser. We also take the acceleration value at that point and use that to change the volume of the sound we make.
Accelerometer interface -- Will Elswick
Signal conditioning -- Will Elswick
Node.js addon -- James Savage
Web browser interface -- James Savage
Inspiration/sarcasm -- James Savage
This project could easily be expanded to take inputs from multiple accelerometers. We didn't have any other ADXL345 accelerometers so we couldnt duplicate our inputs. With this, it we could easily modify our interface to recognize another accelerometer and assign to it its own unique sounds. This way we could have a whole percussion section or even a whole drumkit. Additionally we could add some composing tools to the interface so that the user could record and playback or loop his performance. By switching the instrument someone could create an entire rythmic track using only the one accelerometer we have.
Give some concluding thoughts about the project. Suggest some future additions that could make it even more interesting.