Encoding Frequency Modulation to Improve Cochlear Implant Performance in Noise

Abstract

Abstract-Different from traditional Fourier analysis, a signal can be decomposed into amplitude and frequency modulation components. The speech processing strategy in most modern cochlear implants only extracts and encodes amplitude modulation in a limited number of frequency bands. While amplitude modulation encoding has allowed cochlear implant users to achieve good speech recognition in quiet, their performance in noise is severely compromised. Here, we propose a novel speech processing strategy that encodes both amplitude and frequency modulations in order to improve cochlear implant performance in noise. By removing the center frequency from the subband signals and additionally limiting the frequency modulation's range and rate, the present strategy transforms the fast-varying temporal fine structure into a slowly varying frequency modulation signal. As a first step, we evaluated the potential contribution of additional frequency modulation to speech recognition in noise via acoustic simulations of the cochlear implant. We found that while amplitude modulation from a limited number of spectral bands is sufficient to support speech recognition in quiet, frequency modulation is needed to support speech recognition in noise. In particular, improvement by as much as 71 percentage points was observed for sentence recognition in the presence of a competing voice. The present result strongly suggests that frequency modulation be extracted and encoded to improve cochlear implant performance in realistic listening situations. We have proposed several implementation methods to stimulate further investigation.

Cochlear implants: A remarkable past and a brilliant future

“…During the past several years, increasing attention has been devoted to representing "fine structure" or "fine frequency" information with CIs (e.g., Smith et al, 2002;Nie et al, 2005;Wilson et al, 2005;Zeng et al, 2005;Hochmair et al, 2006;Arnoldner et al, 2007;Berenstein et al, 2008;Brendel et al, 2008;Buechner et al, 2008;Litvak et al, 2008;Bonham and Litvak, this issue). These recent efforts are reviewed and discussed in Wilson and Dorman (2008b, c).…”

“…Strategies still in the initial stages of development include those designed to provide a closer mimicking of the (intricate and interactive) processing that occurs in the normal auditory periphery (e.g., Wilson et al, 2005Wilson et al, , 2006Wilson et al, , and 2008, and additional approaches aimed at representing fine-structure information (e.g., Nie et al, 2005;Wilson et al, 2005;. Some of these newer strategies also are described in Wilson and Dorman (2008c), along with progenitors of the Fine Hearing and HiRes 120 strategies.…”

“…The possibility that only a small amount of the fine structure information is transmitted with envelope-based strategies, along with the findings of Smith et al (2002) and of Xu and Pfingst (2003) demonstrating the importance of the information, have motivated efforts to represent the information in other ways (e.g., Nie et al, 2005;Wilson et al, 2005;Hochmair et al, 2006;Arnoldner et al, 2007;Litvak et al, 2008;Bonham and Litvak, this issue). Indeed, many have assumed that little or no fine structure information is transmitted by the "envelope-based" strategies, as "only envelope information is presented."…”

The Role of Temporal Fine Structure Processing in Pitch Perception, Masking, and Speech Perception for Normal-Hearing and Hearing-Impaired People

“…Nie et al (2005) evaluated the potential contribution of TFS information to speech recognition in noise via acoustic simulations of a cochlear implant. They transformed the rapidlyvarying TFS into a slowly varying FM signal which was applied to the carrier in each band (which was already amplitude modulated by the speech envelope in that band).…”

Speech perception problems of the hearing impaired reflect inability to use temporal fine structure

“…The lack of ability to use TFS cues probably also limits the ability of people with cochlear implants to understand speech when background sounds are present. Improving the ability to use TFS should be a goal for designers of hearing aids and cochlear implants (42,43), and TFS speech, as used here, may provide an important tool in evaluating how well this goal is achieved and in the early diagnosis of sensorineural hearing loss.…”