A DC blocking filter. This is used to correct any DC offset introduced into the signal path. Common culprits that introduce DC offset include reverberators, comb filters, and string resonators.

 # use an impulse as an excitation signal
 1 metro
 # feed it into some string resonators
 220 0.999 streson
 300 0.8 streson
 # remove any dc offset with dcblk