Stefan Antoszko's Pink Noise Generator Packs a TI MSP430 and "a Handful of Resistors and Capacitors"
Mimicking the pink noises found in nature, this soothing project was an experiment in simulation and calculation.
Computer engineering student and electronic audio hobbyist Stefan Antoszko has built a "pink noise" generator, using a Texas Instruments MSP430G2231 microcontroller as a white noise source and building a three-stage filter to achieve the desired pink output.
"I used an MCU [Microcontroller Unit] as a white noise source using a 32-bit Linear Feedback Shift Register (LFSR)," I passed it into a three-stage high-shelf filter with offset frequencies to approximate a -3dB slope which is characteristic of pink noise. My main goal from this project was to learn how the values for [others'] shelf filters were chosen so I decided to derive the equations for them by hand."
To start, Antoszko had to get the TI MSP430 to generate white noise — choosing to use a linear feedback shift register, in which a combination of higher bits are XORed together and then pushed to the front of the register. "Since the MSP430 is a 16-bit MCU, initially I tried the 15-bit loop where you XOR the first and 15th bit and push that to the front," Antoszko notes. "However, this had a really short cycle length and I could clearly hear the sequence repeating."
Settling on a 32-bit variant with two 16-bit registers, Antoszko had a suitable source of white noise — but desired pink noise, also known as fractional noise and likened to the sound of a waterfall. To get that required filtering the microcontroller's output, using "a handful of resistors and capacitors."
For the experience, Antoszko calculated what each stage required by hand — taking into account how each prior stage has already affected the signal — and came up with a three-stage filter using five resistors and five capacitors.
"[The MSP430] is well suited for this application because I only need one shift register, one output pin, and a 1MHz clock is more than sufficient," Antoszko concludes. "Additionally, it operates at a low Vcc voltage [of] <3.3V which comes in handy if I want plug the final signal right into Line In."
The full project write-up is available on Antoszko's Hackaday.io page.