~ Making a flute controlled mouse

There is something about surprising interfaces: clapping to switch on lights is more fun than a flipping a switch. Pressing a panic-button to order a pizza is more fun than ordering via an app. Recently I came across this surprising interface: a flute controlled mouse cursor for a first person shooter. I recognize a good idea when I see one, and immediately wanted replicate the idea and make it freely available. So I got to work:

What do we need for flute-based mouse? First we need a way to determine if a note is being played and if a note is produced, we need to be able to determine which note is being played by the musician. Next, we need to hijack and control a cursor via the detected note and trigger a click event when a specific note is played. Finally we need to play a flute, preferably a recorder, to move the mouse cursor in an obviously superior and relaxed fashion. It is not strictly required to use a recorder but a recorder is very much advised.

The note determination can be done by a fundamental frequency detector. A detector returns a frequency in Hertz and a confidence score which tells you how reliable the detection is. With some filtering, this is exactly what we need. If the frequency is close enough to a configured value, a note is detected. The confidence score tells us to either accept or ignore the detection. With this info it is possible to connect a note-detection to an action - like moving a cursor left or right, up or down.

Finally we need to move the mouse cursor. There are a few ways to do this.

🪈 An micro-controller-based solution - Pitch perfect pointer positioning - PiPePoPo

Fig: Flute-based web-browsing as envisioned by its developer.

A portable way to move a mouse cursor is to let a micro-controller impersonate as a standard mouse, a ‘USB Human Interface Device’. Once the micro-controller is attached via USB it registers as a mouse and allows to move the cursor and register click events. To build a flute-based mouse, the micro-controller then needs a microphone and a pitch estimator to finally send cursor events.

I based my project on an RP2040 - a micro-controller chip designed by Raspberry Pi - since it offers a simple way to present itself to an operating system as a mouse. Just include PluggableUSBHID.h and USBMouse.h and use the Mouse API to control the mouse. For me it only behaved as a standard mouse if Serial is not used at the same time: in other words the dual USB profile does not seem to work reliably. Sending mouse events from your code looks, for example, like ` Mouse.move(-4, 7)` to move the mouse minus four units in the horizontal and seven units in the vertical direction. Click events have a similarly straightforward API. The RP2040 also has a built-in microphone, which makes it ideal for audio applications, or so it seems.

Unfortunately, the RP2040 chip performs poorly for computationally heavy audio processing workloads. Such applications need to perform many floating point operations per second, but the RP2040 lacks a hardware floating point unit (FPU) which makes it relatively slow. When attempting to run a pitch-detection algorithm, the RP2040 was too slow to run the algorithm in real-time. After profiling the pitch estimation algorithm there was a clear place where most float operations occurred. Replacing those with much quicker fixed point operations makes the algorithm faster than real-time and usable on the RP2040.lt

To give a sens of the difference in speed between fixed point and floating point operations on the RP2040: with the default arduino build process, a million floating point operations take over 883 000 microseconds, a million fixed point operations take 8 microseconds. Fixed point operations are around 5 orders of magnitude faster!

I have named released the code under the name Pitch perfect pointer positioning or PiPePoPo for short. For the details, please do check the source code repository. Perhaps the most intresting, reusable component is the ANSI C implementation of the YIN pitch estimator, both in floating point and sped up with fixed point operations.

🪈 A browser extension - Pitch perfect pointer positioning - PiPePoPo

The hardware based solution works reliably but, evidently, it needs a piece of hardware. To make sure everybody can enjoy a solution in software is provided in this section in the form of a chrome browser extension.

Moving a cursor is not possible in a browser: if a pointer location could be modified it would open a whole range of possibilities for abuse. A surprisingly easy workaround, however, is to hide the actual cursor and show a replacement cursor-like icon. This fake cursor can be moved programmatically. With the position of this fake cursor known, a click event can be triggered and result in, for example, following a link.

To take this idea to its logical next step, I implemented a chrome browser plug-in for flute-based web-browsing. I also relased this on GitHub under the Pitch perfect Pointer Positioning or PiPePoPo brand. Check the installation instructions in the PiPePoPo repository. Perhaps most of interest is how audio processing is handled by a Web Audio API Audio Worklet.

Join the flute-based web-browsing revolution today and experience web browsing like never before and install PiPePoPo.

UGent and Code