PedalPi-WaveShape - Audio Distortion for the Pedal Pi

A waveshaping audio distortion written in C for the Electrosmash Pedal Pi, and using the Broadcom BCM2835. It utilizes some of the same waveshaper algorithms as Waveshape-Synth and WaveDist.

Table of Contents

• General Info
• Features
• Installation
• Algorithms
• References

General Info

This project makes things sound bad, but in a good way. It relies on a handful of waveshaping algorithms to produce differing kinds of overdrive and distortion. The harmonic ratios and the balance between even and odd harmonics varies by algorithm, with some sounding better on certain musical sources than others. There's no hard-and-fast rules here, so just use your ears.

The RectifierThreshold parameter can lend itself to some neat octave-doubling effects too!

Features

Three waveshaper algorithms from the Waveshape-Synth (cycled via the left/right buttons with the toggle switch in the 'up' position):

• Leaky Integrator
• Cubic Soft-Clip
• Soft-Knee Clip

Additionally, there is a selectable gain boost on the order of 10 dB (toggle down + left button), and a partial rectifier algorithm which can be turned on/off to add some additional harmonics (toggle down + right button).

To Do

• Get the basic UI functioning done
• Get audio passing with the waveshape processing applied done
  • leaky integrator done
  • soft-knee done
  • cubic done
• Create a 'gain boost' on/off control done
• Add a partial input rectification on/off control to create additional harmonics done
• Experiment with adding a Wet/Dry control
• Get this to run on every boot so that you don't have to ssh into the Pi on every boot and start it
• Add oversampling + filtering to reduce aliasing artifacts

Status: This project is considered complete and not actively maintained. Additional 'to do' items or new features are unlikely, although some bugs may be fixed.

Installation

The BCM2835 library is available here:

https://www.airspayce.com/mikem/bcm2835/

Download the BCM2835 library onto a Pedal Pi and install it with:

wget https://www.airspayce.com/mikem/bcm2835/bcm2835-1.70.tar.gz;
tar xvfz bcm2835-1.70.tar.gz;
cd bcm2835-1.70;
./configure;
make;
sudo make install

Next, download this repo onto a Pedal Pi and compile it with:
gcc -o WaveshapeDistortion -l rt WaveshapeDistortion.c -l bcm2835 -lm

then run:
sudo ./WaveshapeDistortion

Algorithms

This project uses the following algorithms for waveshaping and signal limiting:

 1) hard clip alg:        y[n] = 0.5 * (abs(x + Theshold) - abs(x - Threshold))
 2) soft clip alg:        y[n] = (1.5*x[n]) - (0.5*x[n]^3);
 3) leaky integrator alg: y[n] = ((1 - A) * x[n]) + (A * y[n - 1]);
 4) soft knee clip alg:   y[n] = x[n] / (K * abs(x[n]) + 1);
 5) cubic soft clip alg:  y[n] = (1.5 * threshold * HardClip(x[n]))
                                  - ((0.5 * HardClip(x[n])^3) / threshold);
 6) partial rectify alg:  y[n] = ((1 - R) * softclip(x[n])) + (|softclip(x[n])| * R);

References:

1. Aarts, R.M., Larsen, E., and Schobben, D., 2002, 'Improving Perceived Bass and Reconstruction of High Frequencies for Band Limited Signals' Proc. 1st IEEE Benelux Workshop MPCA-2002, pp. 59-71
2. Arora et al., 2006, 'Low Complexity Virtual Bass Enhancement Algorithm for Portable Multimedia Device' AES 29th International Conferance, Seoul, Korea, 2006 September 2-4
3. Gerstle, B., 2009, 'Tunable Virtual Bass Enhancement', [ONLINE] http:rabbit.eng.miami.edu/students/ddickey/pics/Gerstle_Final_Project.pdf
4. Yates, R. and Lyons, R., 2008, 'DC Blocker Algorithms' IEEE Signal Processing Magazine, March 2008, pp. 132-134

(C) 2017-2021, Winry R. Litwa-Vulcu