2From Sound to Nerve: The End-to-End Signal Path

FThe seven-stage path

The signal path runs: sound → microphone → processor → coding strategy → RF coil → skin → implant → stimulator → electrode array → spiral ganglion / cochlear nerve, roughly seven transduction stages from acoustic input to neural excitation.[2020]

TThe one software stage

The microphone converts sound to an electrical signal that is preamplified, then the processor filters and analyses it according to the chosen coding strategy, producing per-channel envelope information. This is the one stage that is pure software (cross-ref Ch.8).[2008]

FPower and data across the skin

The processor modulates the per-channel data onto a radiofrequency carrier and drives the external coil; current in that coil creates magnetic flux that crosses the skin and induces a current in the internal coil — power and data on the same inductive link.[2009]

TDecoding and stimulating

The internal stimulator decodes which electrode to fire, with what current amplitude, pulse width and timing, then delivers charge-balanced biphasic current pulses to the selected contact, exciting the nearby auditory-nerve fibres.

FTonotopy, base to apex

Tonotopy is exploited directly: high-frequency channels drive basal electrodes and low-frequency channels drive apical electrodes, mapping the processor's filter bank onto the cochlea's natural place-frequency map (cross-ref Ch.1 Auditory Physiology).

TThe device sets the limits

The device sets the limits the strategy must live within — number of electrodes, maximum stimulation rate, current compliance and telemetry capability — so a processor upgrade often means newer strategies running on an unchanged internal array.