4 String Karplus-Strong MIDI Synth

Description

This is a MIDI controlled synth that implements 4 Karplus-Strong strings. This instrument's timbre is controlled by 3 pots:
  • Reflection filter Fc value (which determines how quickly harmonics are rolled off).
  • Damping factor pot which causes the waveform to decay linearly across the spectrum.
  • Pluck pulse filter Fc value. This controls filtering of the pluck pulse. The harmonics of this pulse are important for the starting timbre of the instrument at pluck time. The plucks can be hard and sharp or they can be soft and thump-like.

    Pitch wheel controls pitch bend which affects all four voices at once. Mod wheel controls the width of the excitation pulse.

    Unlike previous KS designs, this synth does not control pitch by changing the length of the delay line. Instead, the delay line is a constant length and pitch is controlled by fractional sample rate resampling.

    The synth with included MIDI controller is contained in one dsPIC IC.

    Voice assignment consists of logic using a "gates" memory which is used to determine which voices are available. Part of the algorithm for deciding which voice is selected includes scanning the string models for any that are already set to the pitch of a newly received note on message. This feature provides two things:
    1: A more realistic sound in that when the same pitch is played more than once on a real harp, the same string is reused. The synth does this whenever possible which results in fewer voices being stolen from another pitch. When a vibrating string is replucked, there is already wave information in the delay line. The pluck pulse modifies some of this data, so the new pluck can sound different just as it does with a real harp (or other struck or plucked string instrument).
    2: Voice theft for new notes is limited and occurs when no matching pitch string can be found and when all voices are already in use. Unused voices are always used before sounding voices are stolen. The code is written in assembly language.

    • Musical Examples of the 4 String Karplus-Strong Instrument

    • No Demo is available at this time.