What it does
Live Guitar Sympathetic Strings, Tarab
Thirteen sympathetic strings instead of six, tuned to an E minor pentatonic scale across three octaves (E1 B1 E2 G2 A2 B2 D3 E3 G3 A3 B3 D4 E4) rather than to the six open guitar strings. Named after the tarab strings of a sitar, which sit under the playing strings and are tuned to the scale rather than to the instrument.
Because the bank follows a scale, far more of what you play finds a string to wake. Play in E minor and the answer comes back in tune and in key; play outside it and only the notes you share with the scale reply. Each string rings for minutes, so chords accumulate behind you: by the loop-gain formula around 40 s at the top E and past five minutes at E1. A bank of thirteen decays as a mixture, so what you hear runs somewhat shorter than the per-string figure.
HOW IT IS BUILT. Thirteen independent loops. Each is a summer feeding a delay line whose length sets the pitch, then a lowpass inside the loop so high harmonics decay faster than the fundamental as on a real string, then a gain just under one returning to its own summer. The thirteen are added only at the output and that sum is never fed back, so the strings cannot gang up into a howl and nothing self-oscillates. Every loop gain is below unity, so the board is silent until you play.
WHAT TO REACH FOR. String coupling sets how hard your playing drives the bank. Each string's Gain sets how long that string rings; they are equal here, so the LOW strings ring longest as on a real instrument. Raising a Gain towards 1.0 lengthens the ring steeply; at or past 1.0 the string becomes an oscillator that never stops, which is not what this board is for. The Lowpass Frequency sets how bright the ring stays. Note the delay lengths compensate for that filter's own delay, so a large change to Lowpass Frequency will pull the strings slightly out of tune.
And the delay lengths are counted in SAMPLES, so the tuning above is right at 44.1 kHz and shifts with the device rate -- every string goes about +147 cents sharp at 48 kHz -- until the lengths are derived from the sample rate at load time.