Temporal fine-structure coding (FSP/FS4)

Envelope-based strategies deliberately discard the fast temporal fine structure (TFS) — the carrier zero-crossings within each band — keeping only the slow envelope. TFS carries much of the information humans use for pitch, melody, lexical tone, and separating speech from a competing talker, so its loss is implicated in CI users' poor music perception and speech-in-noise performance. Fine-Structure Processing strategies (FSP, FS4, in MED-EL devices) attempt to restore low-frequency TFS by timing stimulation pulses to the phase (zero-crossings) of the band signal on the most apical (low-frequency) electrodes, rather than at a fixed carrier rate.

Evidence for benefit is mixed: some studies show improved pitch, tone-language, and music outcomes for subsets of users, others show no reliable advantage, and the interpretable rate range is capped by neural refractoriness and by facilitation/adaptation in the surviving auditory nerve (electric temporal pitch typically saturates around a few hundred Hz). Delivering true, place-appropriate fine structure remains an open problem tightly coupled to channel independence.