6Frequency Allocation and the Filter Map

FThe filter bank: how input frequency becomes electrode place

Frequency allocation determines how the analysed input spectrum is divided into bands and assigned to active channels, recreating an electrical version of the cochlea's normal frequency-to-place mapping. In the Nucleus default map, the full input range across all 22 channels spans about 188 to 7938 Hz. Nucleus channel bandwidths are deliberately uneven: low-frequency channels are narrow at about 125 Hz, mid channels about 250 Hz, and high channels range from roughly 375 up to 1000 Hz, mirroring the narrower auditory filters of the normal low-frequency cochlea. Channel 1 is the most basal contact and channel numbers rise toward the apex, so allocation maps high frequencies basally and low frequencies apically, matching cochlear tonotopy. In MED-EL software the audiologist can move the lower (70 to 350 Hz) and upper (3500 to 8500 Hz) frequency limits in 10 Hz and 500 Hz steps and choose among four psychoacoustically derived tables; in Advanced Bionics allocation is automatic and not user-adjustable.[2020][2009]

TPlace-frequency mismatch from shallow insertion

Tonotopy, the orderly frequency-to-place relationship, is the cochlea's first organising principle; an electrode placed basal to the site that naturally codes its assigned frequency creates a place-frequency mismatch. Default allocations push frequencies onto electrodes that sit more basally than the Greenwood place for those frequencies, because a one-turn lateral-wall insertion only reaches roughly the cochlear region that natively codes around 1000 Hz. Insertion angle and the default allocation jointly determine the spiral-ganglion place pitch each electrode actually stimulates, so the mismatch is systematic, not random. Electrical place pitch saturates and rate-pitch cues are limited to frequencies below roughly 300 Hz, so much of the frequency information must be carried by place, magnifying the cost of a place mismatch. Most recipients adapt to the initial mismatch over months, so default maps are generally retained rather than re-aligned electrode-by-electrode to the Greenwood place.[2020][2009][2015]

CFrequency importance and bandwidth trade-offs

Narrow mid-frequency bands resolve the formant frequencies of vowels and consonants, which is why several studies favour narrow mid-frequency allocation for better speech recognition. Restricting the upper input limit from about 10000 Hz to roughly 8000 Hz narrows the channel bandwidths across the speech range and improved speech recognition. There is a bandwidth trade-off: extending the upper limit captures more high-frequency speech energy but widens each band and worsens spectral resolution, while restricting it sharpens resolution but discards high-frequency cues. Adults are typically allocated out to at least about 6000 Hz (some clinicians cap at 6500 to 7000 Hz), whereas children are given a wider band out to at least 7000 to 8000 Hz because they need high-frequency cues to recognise and produce speech, especially female and child talkers. The Nucleus default frequency table widens the bandwidth for children, while a narrowed mid-frequency table is often chosen for adults.[2020][1995][2001]

CWhen (and whether) to reassign frequencies

Frequency allocation matters most when an electrode must be disabled; on disabling, the remaining channels widen their bands (the total input range stays the same), which can blur two close frequencies into one channel and degrade spectral resolution. Because disabling reshapes allocation and can cost performance, electrodes should be turned off only when justified, for example abnormal impedance, non-auditory side effects such as facial-nerve stimulation or pain, poor sound quality, or a need for far higher levels than neighbours. When a recipient has few usable channels, double-channel mapping routes two channels into one electrode to restore overall bandwidth. Manufacturers caution against haphazard manual editing of the filter table, but two accepted uses are setting the acoustic-electric crossover frequency for EAS users and recreating a legacy allocation when an upgraded recipient rejects the new defaults. Channel-to-electrode reassignment is rarely warranted, but is reasonable when a sweep reveals a non-tonotopic pitch order on a channel; for EAS, frequency allocation must be coordinated with preserved acoustic low-frequency hearing to avoid overlap.[2020]