Continuous Interleaved Sampling (CIS)

CIS is the foundational modern coding strategy (Wilson and colleagues, 1991) that made high open-set speech understanding routine. Sound is band-pass filtered into a fixed number of channels equal to the number of active electrodes; each band's envelope is extracted (rectify plus low-pass, or Hilbert), compressed nonlinearly to fit the narrow electric dynamic range (often only about 6-20 dB from threshold to comfortable loudness), and used to amplitude-modulate a fixed-rate biphasic pulse carrier. Crucially the pulses are interleaved — no two electrodes fire at the same instant — which sharply reduces the electric-field summation that otherwise makes channels interact.

CIS's virtues are simplicity and robustness; its limits are that it conveys only the slow envelope (fixed carrier rate, discarded fine structure), so temporal pitch and periodicity cues are weak. Nearly all later strategies (ACE, HiRes, FSP) are elaborations on the CIS skeleton — changing channel selection, carrier rate, or fine-structure handling — rather than departures from it.