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Homework #2: Delay, Feedback, Moving Average, and Polyphony

Due date: 2013.10.15 11:59:59pm (or thereabout), Tuesday.


Specification (part 1 of 3): Delay + Feedback + Moving Average

  • create a program that is capable of real-time audio input/output (suggestion: use the skeleton framework from HW1)
  • craft the framework for a delay by implementing a circular buffer:
    • think about how to implement a delay element that delays the input signal by some integer number of samples
    • think about how to implement this efficiently, without shifting all the elements for each sample
      • an array of samples
      • two pointers: one for reading, and another for writing, initialize them
      • advance the pointers accordingly, writing and reading into the corresponding locations via the pointers
    • test with with the microphone (with varying delay lengths)
  • now add feedback, with some attenuation factor (<1)
    • take each output sample of the delay, multiply it by attenuation factor (<1), and add it into the input
    • test with the microphone (with varying delay lengths)
  • next, modify the feedback by adding a 2-point moving average filter into the feedback loop: simply average the current output sample with the previous output sample.
  • final test: on demand (e.g., via console input), load the array with white noise, and let it run

Mus256a karplus strong.png

Specification (part 2 of 3): Polyphony

  • abstract the above into something you can reuse
  • design an interface for straightforward control
  • add RtMIDI support with polyphony, so we can play our construction above
  • NOTE: If you do not have a MIDI keyboard, you can download a virtual midi keyboard to trigger your synthesis

OSX = such as Midi Keys

LINUX = run "vkeybd &" in the terminal open a keyboard. Open "qjackctl" in terminal. Click Connect. Go to the ALSA patch screen and connect the virtual keyboard to your RtMidi application.

WINDOWS = You can install vmpk

Specification (part 3 of 3): Sonic/Musical Statement

  • use the framework above, algorithmically generate and/or play a sonic/musical statement
  • record it turn it in


  • have fun with it!!!
  • your code should compile and run on the CCRMA machines
  • comment your code and have consistent style
  • you are welcome to work together, but you must do/turn in your own work (you'll likely get more out of it this way)
  • some considerations:

Possible Classes

  • KarplusSynth - a class to hold both the delay line and one zero filter. This should be the main synthesis object that wires up the onezero and delayline objects. You'll have a limited number of these (say 16 of them) to do polyphony.
  • MidiEvent - a class to represent the concept of the midi note velocity and pitch
  • MidiEventQ - some data structure to store the incoming MidiEvents and drain them when needed (produced in the midi callback and consumed in the audio callback)


  • 1) source code to the project in a single zip file (*.h, *.cpp, *.c makefile, etc.).
  • 2) online page for your project (should be viewable at http://ccrma.stanford.edu/~YOURID/256/hw2/). It should include:
    • links to your files of various kinds
    • instructions on building the project (for example, anyone in the class should be able to download
    • a short README text section that:
      • conveys your ideas/comments in constructing each program
      • describes any difficulties you encountered in the process
      • lists any collaborators
  • 3) submit via email (TBA)