In this way even when mod delay is set to zero, pressing a new key (when no key is held) leads to a short interruption of the delay. The guess is that it the modulation is reset directly when a new key is played (to fix a prior problem with modulation not being directly set to zero when mod delay > 0) but applying the new (and in this case unchanged!) modulation depth requires synth_update(). That function is called at much lower update rate than the core 2kHz, depending on what is being played as low as 1/30 second.
In this way even when mod delay is set to zero, pressing a new key (when no key is held) leads to a short interruption of the delay. The guess is that it the modulation is reset directly when a new key is played (to fix a prior problem with modulation not being directly set to zero when mod delay > 0) but applying the new (and in this case unchanged!) modulation depth requires synth_update(). That function is called at much lower update rate than the core 2kHz, depending on what is being played as low as 1/30 second.