Tag Archives: karplus-strong

Caustic brings Karplus-Strong synth to the iPad

While performing my daily ritual of checking for current app updates, I was pleasantly surprised to see that multi-component mini production studio Caustic listed, among other things, the addition of “KSSynth.”

There could only be one thing that meant… and indeed, it is an actual Karplus-Strong synth implementation.

KS is a modeling method generally referred to as “plucked string synthesis,” similar to the pluck tones of FM but completely different in implementation.

Rather than being a strict physical model, however, the KS algorithm is basically an observation of the nature of plucked tones… it produces a short burst of complex inharmonic sound that rapidly decays to a much simpler wave, not by specifically modeling string mass and damper position and so on, but like this:

  1. Load a wavetable with a random signal.
  2. Start playing out the wavetable one sample at a time.
  3. Each time a sample is played, average it with the preceding sample, and place the result back in the wavetable.
  4. Rinse, repeat.

The first time through, the wavetable produces the noise it was loaded with. The second time, it produces the first-order-averaged signal, which is the same as having applied the simplest possible lowpass filter to the signal. The third time through, the signal is averaged (smoothed and amplitude-reduced) a little more… and so on, until the waveform decays to a quiet sine wave over time.

A good overview of the technique can be found here at Columbia.

It’s a remarkably simple little algorithm but it’s rarely implemented in commercial synths because it requires a specific architecture not normally part of the classic ‘basic patch.’ The Korg Prophecy had it, as part of its toolkit of monophonic doohickeys. And there is at least one app (Orphion) that appears to use it, though without surfacing any parameters.

The Caustic implementation, however, is a fully polyphonic KS synth with essential (though not extensive) parametric control, so you can tune it from kotos to wood blocks to impossible clavinets.

So this is rare, and a great find… and I’m happy to report that it’s a lot of fun.

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