10ACE: Spectral Maxima Meet High Rate
The Advanced Combination Encoder fused SPEAK-style spectral-peak selection with CIS-style high-rate non-overlapping pulses, becoming the default strategy of the Nucleus system. ACE is the present-day commercial synthesis of the n-of-m lineage — and a useful baseline for understanding what comes after it.
TThe combination idea
ACE combines two principles: SPEAK-style spectral-peak (n-of-m) selection of the highest-energy bands and CIS-style high-rate non-overlapping interleaved stimulation ACE and fixed n-of-m designs are described as essentially identical, with n fixed across frames (unlike SPEAK's adaptive n) ACE selects 8-12 maxima from 10-12 FFT-based bands, or with a typically 22-band filter bank matching 22 physical electrodes ACE is the default / first-choice strategy for the Cochlear Nucleus system.[2006][2001]
CThe ACE signal chain
The chain is: mic to front-end to a band-pass filter bank to per-band envelope detection to per-frame selection of the n highest-amplitude bands (inactivating the rest) to compression and T/C-level mapping to high-rate interleaved biphasic pulses on the selected electrodes Pulses are delivered base-to-apex within each stimulation cycle, where the cycle is the inverse of the channel stimulation rate The default stimulation rate for Nucleus implants using ACE is about 900 pps On the Nucleus-24 the rate is selectable from 250 to 2400 pps per channel, up to a maximum of 14,400 pps across all channels.[2006][2001]
CThe Nucleus-24 platform
The Nucleus-24 has 22 intracochlear electrodes plus 2 extracochlear reference electrodes (a platinum ball under the temporalis and a plate on the receiver/stimulator) ACE has a higher rate and a larger peak-selection range than SPEAK The Nucleus-24 is also programmable with CIS using fixed amplitudes over 10-12 bands Recent comparisons generally show ACE superior to SPEAK and roughly comparable to CIS in challenging conditions.[2006][2001]
TWhy the lineage continues past ACE
ACE still uses raw-energy (amplitude) maxima selection, which may not match perceptual relevance ACE is envelope-based and does not encode temporal fine structure for pitch and music Selecting bands by raw energy motivated a psychoacoustic-masking selection rule (PACE/MP3000) Despite differing strategies, present commercial devices show no significant overall performance difference, underscoring that ACE is a strong but non-final synthesis.[2005][2004]
TDoes the high rate itself help, or just the n-of-m design?
ACE applies SPEAK-style n-of-m peak-picking at high per-channel rates; in the Nucleus 24 it is commonly run near 720-1800 pps/ch (Holden 2002). Despite the rate increase, controlled rate studies show no consistent group benefit of higher ACE rate: 720 vs 1800 pps/ch gave no conclusive advantage (Holden 2002), and 250/807 pps/ch were preferred over 1615 (Vandali 2000). Across implant types, speech recognition is largely flat with rate in quiet; only the lowest rates and non-interleaved high rates were significantly poorer (Friesen 2005). The only significant rate effect in one careful study was vowels in quiet, a small +5.23 pp rise from 600 to 4800 ppse, with consonants, words, and sentences unchanged (Shannon 2011). So ACE's clinical edge over SPEAK is attributed mainly to its faster temporal sampling and finer n-of-m updating rather than raw pulse rate per se; in children ACE beat SPEAK by +22.2 pp on words in quiet (Pasanisi 2002).[2002][2000]
TBy the numbers
What is the most accurate description of the ACE-versus-SPEAK difference?
What is the default stimulation rate for Nucleus cochlear implants using ACE?
How many electrodes does the Nucleus-24 have, and of what type?