Learn How to Use PWM to Generate an Audio Signal From Your Raspberry Pi Pico

The new Raspberry Pi Pico is a fantastic development board that is surprisingly capable, especially when you consider how inexpensive it is. But one feature that the RP2040 microcontroller is lacking is a digital-to-analog converter (DAC), which means that it has no way of outputting a true analog signal. While audio files can certainly be stored digitally — an MP3 file is digital, of course — the actual audio signal that goes into your amplifier and then speakers is analog. Fortunately, analog signals can be simulated using PWM. In his newest video, Robin Grosset explains how you can use your Raspberry Pi Pico to generate a decent PWM audio signal.

Pulse-width modulation (PWM) is a technique used to create a pseudo analog signal from a digital source. Because the Raspberry Pi Pico’s RP2040 doesn’t have a DAC, it can only output digital signals through the GPIO pins. That means a pin can either be set HIGH (3.3V) or set LOW (0V), but it can’t be anything in-between. If it did have a DAC, as some microcontrollers do, it would be able to produce any voltage between fully-on and fully-off. But by using PWM, we can output a signal that looks a lot like an analog signal. This is possible because the RP2040 runs at a very fast speed (133MHz by default) and can switch a pin on and off very quickly. This gives us the ability to create something similar to the square waves you get from old-school synthesizers, but with much finer steps that are closer to a true analog wave.

Grosset will walk you through exactly how to achieve this in his video. You need very little hardware for this project: just the Pico, two capacitors, three resistors, an audio jack breakout board, and a few jumper wires. From there you can go to Grosset’s GitHub page to download the C library and code that you will to take an audio file and convert it into a PWM single. This code is pretty clever, because it uses interrupts to essentially buffer the next change to the GPIO pin. This does require that you overclock the RP2040 to 176MHz, but that probably won’t harm anything. In the video, Grosset demonstrates this with 11KHz audio samples, but you can use up to 44KHz. The actual audio files themselves have to be converted in C header files so that they can be read by the code, and Grosset has programmed a handy tool in Python to take care of that for you. The resulting audio output isn’t good enough to satisfy an audiophile, especially at 11KHz, but it is more than enough to be useful for playing sound effects or even voice feedback for your projects.