3The processor's signal path

FOne chain, every implant

It is easy to get lost in the differences between manufacturers and strategies. The antidote is to remember that they all implement the same backbone: analyse the sound, reduce it to a few channels of amplitude over time, and paint that onto the electrodes. The strategies differ in the details of these stages, not in their order.[1998]

FTWalking the path

Sound enters at the microphone (increasingly directional, to favour the front). Pre-processing applies automatic gain control and noise reduction to tame the wide, noisy real-world signal. The filter bank splits it into frequency bands, one per channel. Envelope detectionextracts each band's slow amplitude, discarding the fast fine structure. Compression and mappingfit that envelope into the narrow window between the electrode's threshold and comfort levels. Finally, pulsatile stimulation delivers the current as brief, interleaved pulses — the coding strategy deciding exactly how.[2015]

CWhere the rest of the chapter fits

With the chain in view, the chapter's structure becomes a tour of its blocks. The filter bank and place code are Module 4; the envelope and what it discards, Module 5; the stimulation strategies — CIS, ACE, FSP — Modules 6, 8 and 9; the limit they all fight, channel interaction, Module 7; sharpening the field, Module 10; and the pre-processingfront end, Module 11. Each later module simply opens one box in this diagram.