Dual-carrier processing to convey temporal fine structure cues: Implications for cochlear implants

Abstract

Speech intelligibility in noise can be degraded by using vocoder processing to alter the temporal fine structure (TFS). Here it is argued that this degradation is not attributable to the loss of speech information potentially present in the TFS. Instead it is proposed that the degradation results from the loss of sound-source segregation information when two or more carriers (i.e., TFS) are substituted with only one as a consequence of vocoder processing. To demonstrate this segregation role, vocoder processing involving two carriers, one for the target and one for the background, was implemented. Because this approach does not preserve the speech TFS, it may be assumed that any improvement in intelligibility can only be a consequence of the preserved carrier duality and associated segregation cues. Three experiments were conducted using this "dual-carrier" approach. All experiments showed substantial sentence intelligibility in noise improvements compared to traditional single-carrier conditions. In several conditions, the improvement was so substantial that intelligibility approximated that for unprocessed speech in noise. A foreseeable and potentially promising implication for the dual-carrier approach involves implementation into cochlear implant speech processors, where it may provide the TFS cues necessary to segregate speech from noise.

The role of periodicity in perceiving speech in quiet and in background noise

“…While a degraded spectrum is likely to interfere with the segregation of target and masker when both signals are processed together, as is the case in CI simulations, the spectrum of the maskers in Experiments 2 and 3 was always intact. As demonstrated by Apoux et al (2015), it is differences in TFS per se that appear to be crucial for segregating target and masker. Thus, the critical point in the current experiments is that two separate carriers were present throughout.…”