Limitations of the Envelope Difference Index as a Metric for Nonlinear Distortion in Hearing Aids.

OBJECTIVES: The envelope difference index (EDI) compares the envelopes of two signals. It has been used to measure nonlinear distortion in hearing aids, but it also responds to linear processing. This article compares linear and nonlinear processing effects on the EDI.
DESIGN: The EDI for spectral tilt and peak clipping distortion is computed to illustrate the effects of linear and nonlinear signal modifications. The EDI for wide dynamic-range compression is then compared with that obtained for linear amplification for a set of standard audiograms to show the expected range of EDI values for linear and nonlinear hearing aid processing. The EDI for hearing aid amplification and compression is also compared with a measure of time-frequency envelope modulation distortion for the same conditions.
RESULTS: The EDI is shown to be as sensitive to linear amplification as it is to nonlinear processing. The EDI values for spectral tilt can exceed those for peak clipping, and the EDI values for linear amplification exceed those for wide dynamic-range compression for four of the nine audiograms considered. The agreement of the EDI with a nonlinear envelope distortion measure is shown to depend on the long-term spectra of the signals being compared when computing the EDI.
CONCLUSIONS: The accuracy of the EDI as an indicator of nonlinear distortion for sentence materials can be improved by equalizing the long-term spectrum of the processed signal to match that of the unprocessed input. However, the EDI does not have a clear interpretation because of the confound between linear and nonlinear processing effects and the lack of an auditory model in calculating the signal differences.