3The Transcutaneous Link and Telemetry

TOne inductive RF link

Power and data both cross intact skin on a single inductive RF link. The amplitude-modulated carrier lies in the low-radiofrequency band, roughly 2.5 to 50 MHz depending on manufacturer — well below the 88–108 MHz of broadcast FM radio — with the audio envelope modulated onto it.[2020]

CForward and back telemetry

The link is typically bidirectional. Forward telemetry sends power and stimulus instructions in; back (reverse) telemetry returns implant status, measured electrode impedances, and intracochlear evoked potentials out to the processor.[2008]

TBack telemetry and NRT

Back telemetry is the basis of neural response telemetry (NRT) and lets the clinician record the electrically evoked compound action potential (ECAP) and verify the implant intraoperatively (cross-ref Ch.23 Objective Measures). Difficulty obtaining back telemetry intraoperatively is the first sign of a coupling or device problem and is investigated before closing.[2014]

CCoil-over-magnet coupling

The external coil is wire wound around its perimeter; current through it induces the magnetic flux that couples to the implanted coil. Poor coil-over-magnet alignment degrades both power transfer and the data link, which is why precise placement matters clinically.

TCarrier frequency as a constraint

The carrier frequency is itself a compatibility constraint: the original Nucleus CI22M used a 2.5 MHz carrier while all later Nucleus implants moved to 5 MHz, so CI22 patients require a special coil (a green-ringed N6 coil) to drive their older implant — a concrete example of hardware-bound limits (cross-ref Module 6).