Team Members: Abagail (Abby) Che, Terrence Wong, Theodore Kaufman, Yongzhao Wu
Mentors: Prof. Phinney and Dr. Heilemann
Project Description
The Plug-in Playground is an interactive audio station that offers a hands-on way to introduce people to the Audio and Music Engineering (AME) program. Created as an introduction into the exciting world of audio engineering, this station invites prospective students, families, and other visitors to the 6th floor of CSB to explore both analog and digital audio effects in an intuitive and playful environment.
The station consists of two main components: modular audio effect modules and a user-friendly console. Users can rearrange the modules in any order, select from the input and output options, and instantly hear how different effects transform the audio signal. This “plug and play” design ensures the experience is fun, educational, and accessible to all ages and experience levels. By encouraging experimentation and discovery, the Plug-in Playground not only demonstrates technical concepts in audio engineering but also reflects the hands-on, innovative spirit of the AME program.
Instructions for Use
- Choose one of three input options: keyboard, microphone, or aux
- Choose one of two output options for playback: speakers or headphones
- Place up to three modules into the slots on the console and listen to how different modules affect the sound. Mix them around and be creative!
System Overview
Module Design
The 3D-printed module enclosure and base plate feature magnets for mechanical connection, an ergonomic contour for easy handling, and a pointed shape for seamless alignment and to show the direction of signal flow. The signal connection is made using self-aligning magnetic pogo connectors with a spring loaded female side and an angled male side. This design serves to prevent hot swapping issues by letting power and ground connect before the signal.
The PCBs and laser-cut acrylic top covers are mounted to the enclosure using brass inserts and M3 screws. The base plates are installed beneath the cutouts in the console. When modules is not connected, the effect slots are bypassed using SPDT magnetic reed switches, which are activated by magnets embedded in the module enclosures. Using the same mechanism, magnetic reed switches (SPST) are used to connect power through the pogo connectors to the effect modules once they are inserted into the baseplate. This prevents users from being able to externally short power and ground.
Analog Effects
In order to showcase analog circuit design as taught in the AME program, three analog audio effects were built: A 3-band semi-parametric active equalizer and two distinct varieties of distortion. These effects demonstrate design elements such as filtering, clipping, and virtual inductors. Using potentiometer knobs, the user can experiment with different parameters to control the sound.
The active EQ circuit splits the audio signal into three bands (shown in the figure below): low, mid, and high. The low and high bands use shelving filters with adjustable gain, while the mid band provides both a sweepable center frequency and adjustable gain. The filtered signals are summed and buffered to produce a stable output using op-amps.
The overdrive distortion uses diode-based soft clipping to add a warmer/richer distortion, and provides adjustable gain and output volume controls. The LED distortion circuit uses back-to-back LEDs with different forward voltages to produce asymmetrical clipping, with adjustable gain and volume controls. The LEDs light up when the signal clips, providing a visual cue alongside the audible distortion.
Digital Effects
The digital effects of the Plug-in Playground are designed to showcase digital signal processing techniques that are taught in the AME program. They are programmed in C++ on the Daisy Seed microcontrollers, which are placed into the modules. The Daisy Seed was chosen for its onboard analog-to-digital converter and digital-to-analog converter, as well as its ability to use potentiometers and switches to control digital parameters.
The effects on display are a Schroeder reverb and a toggleable tremolo and vibrato. Reverb is one of the most recognizable audio effects, and it is commonly implemented digitally. The Schroeder reverb is built with cascading comb filters and allpass filters. Tremolo and vibrato are two effects that are commonly mistaken for one another. Both use a low frequency oscillator (LFO) to modulate the signal; tremolo uses the LFO to change amplitude, while vibrato uses the LFO to change frequency. We combined these two effects into one module with a switch to toggle between them so that users can hear the differences for themselves.