Euromasters summer school 2005

Tutorial 7: Auditory processing of speech in noise

Speech and other sounds sources are encoded as a time-varying pattern of spikes in the auditory nerve. However, it is not yet certain which features of this pattern are responsible for conveying information in speech. On the one hand, the average firing rate in each fibre can be considered to be an internal representation or an auditory spectrum. Alternatively, the time intervals between spikes also reflects locally-dominant spectral components and can be processed to produce an interval-based spectrum. The situation is made more complex by the presence of a number of important nonlinearities in the processing chain within the cochlea. One of these ensures that the rate response saturates at moderate to high signal levels. Consequently, it is difficult to understand how a rate-based representation can encode stimuli such as vowels at high intensities, or with added noise, since most of the auditory nerve fibres will show a saturated rate response, leading to a flat spectral representation and an absence of formant peaks. An interval-based representation, on the other hand, shows no such saturation.

The purpose of this tutorial is to build computational models for both rate and interval based processing, and to evaluate their response to speech in various levels of background noise. Results will be compared with physiological data. Various components of an auditory model will be provided as MATLAB functions. If time permits, a number of different approaches to the estimation of dominant frequencies can be explored, including Seneff's generalised synchrony detector, Ghitza's ensemble interval histogram and Cooke's instantaneous frequency strands.

The project is suited to either individual or paired study. Some basic familiarity with MATLAB is needed for at least one member of the team.

The tutorial will begin with a presentation on speech processing in the peripheral auditory system.

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Location of tutorial files

Notes

• You need to disable the KDE sound server, see here
• start Matlab right away to make sure you get a license (there are a fixed number available on the network)
• leave Matlab running at all times, including lunchtime, so that you keep the license
• You can view license usage here