Physically modeled guitar strings using the Karplus-Strong algorithm with some extensions by Jaffe & Smith.

The string model is built from primitive digital filters like white noise, delay, fader, lowpass & allpass connected by patch cables that form a feedback loop. The patch configuration is described in ks_string.v. Individual filters are implemented in ks_noise.v, ks_delay_*.v, ks_fader.v, ks_feedback.v, ks_tuning.v and ks_dynamics.v. You may wish to follow the original articles for more details:

• Karplus, K. and Strong, A., 1983. Digital synthesis of plucked-string and drum timbres. Computer Music Journal, 7(2), pp.43-55.
• Jaffe, D.A. and Smith, J.O., 1983. Extensions of the Karplus-Strong plucked-string algorithm. Computer Music Journal, 7(2), pp.56-69.

Strings can be set up, tuned, plucked or muted via the wishbone bus. The default build contains a single string, but it can be reconfigured for multiple strings with sympathetic resonsances by editing defines.v.

Being a fully digital design audio is output via the I2S protocol, so you'll need a DAC chip ("soundcard") such as CS4344, UDA1334A or MAX98357A to get analog output.