Karplus-Strong Algorithm

At FITC Amsterdam I demoed my first and fairly simple implementation of a Karplus-Strong implementation. Together with KP, we were able to enhance it to sound more like a real guitar. I think, the result is already quite impressive, considering spending just a few hours. At some point I cannot hear a difference to a real guitar. This is even more impressive when you think of how it is accomplished.

A simple low-pass filtered feedback in a short delay line with the length of one period of the target frequency and some white noise do the first impression of a plugged string in common. For a guitar you need more tweaking, randomizing parameters and adding the guitar body resonance to get it more realistic. But the basic is done with an algorithm, discovered 1978!
I really like the elegance of the circuit. You basically bring in chaos (white noise contains all frequencies) and the system is going to stable itself and produces a nice waveform, perfectly suggesting a plugged string.

Naturally we are already planning an AudioTool plugin for synthesizing different guitars. But I am worried about the footprint of the algorithm. I stumbled over this comment which pointed out that the Karplus-Strong algorithm is patented. That is tough. At the time being this algorithm was a great accomplishment! But now where even Flash is able to playback code-generated audio data, I would think that someone may stumble over this method even by accident. It is better to commit our lawyer with this issue, but as the comment says, it is a really good example how bad code-patents are.

Listen and picture it for yourself. (Check the CPU!)

The implementation has a static set, but gives you some control on how the guitar is played. Nothing really magic. Definitely it would be a perfect addition to the AudioTool.

8 thoughts on “Karplus-Strong Algorithm”

  1. Hi there, I understand you are no longer freelancing, however I’m looking to have something developed along similar lines (instead of music, I’m just looking for something that would greatly assist hobbyist radio play and dub producers) — I was wondering if there are some people you could recommend who could help me get a better understanding of what to look for/plan, and what’s possible with PopForge? :)

    Thank you, and I forgot to say this, but faaaaaantastically beautiful work you’ve done with Hobnox. It’s absolutely brilliant.

  2. really excellent. the comment you’ve linked to then has a followup saying if there was a patent then it’s expired, so sounds like it’s fine to use. hurrah!

    \a

  3. Fritz is right: the patients (US4,649,783 and US4,622,877) belonging to the Karplus-Strong method have expired in 2006 and 2007.

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