The dynamic range of useful temporal fine structure cues for speech in the presence of a competing talker

Within an auditory channel, the speech waveform contains both temporal envelope (E(O)) and temporal fine structure (TFS) information. Vocoder processing extracts a modified version of the temporal envelope (E') within each channel and uses it to modulate a channel carrier. The resulting signal, E'(Carr), has reduced information content compared to the original "E(O) + TFS" signal. The dynamic range over which listeners make additional use of E(O) + TFS over E'(Carr) cues was investigated in a competing-speech task. The target-and-background mixture was processed using a 30-channel vocoder. In each channel, E(O) + TFS replaced E'(Carr) at either the peaks or the valleys of the signal. The replacement decision was based on comparing the short-term channel level to a parametrically varied "switching threshold," expressed relative to the long-term channel level. Intelligibility was measured as a function of switching threshold, carrier type, target-to-background ratio, and replacement method. Scores showed a dependence on all four parameters. Derived intensity-importance functions (IIFs) showed that E(O) + TFS information from 8-13 dB below to 10 dB above the channel long-term level was important. When E(O) + TFS information was added at the peaks, IIFs peaked around -2 dB, but when E(O) + TFS information was added at the valleys, the peaks lay around +1 dB.