A Sparse Auditory Envelope Representation with Iterative Reconstruction for Audio Coding

Modern audio coding exploits the properties of the human auditory system to efficiently code speech and music signals. Perceptual domain coding is a branch of audio coding in which the signal is stored and transmitted as a set of parameters derived directly from the modeling of the human auditory system. Often, the perceptual representation is designed such that reconstruction can be achieved with limited resources but this usually means that some perceptually irrelevant information is included. In this thesis, we investigate perceptual domain coding by using a representation designed to contain only the audible information regardless of whether reconstruction can be performed efficiently. The perceptual representation we use is based on a multichannel Basilar membrane model, where each channel is decomposed into envelope and carrier components. We assume that the information in the carrier is also present in the envelopes and therefore discard the carrier components. The envelope components are sparsified using a transmultiplexing masking model and form our basic sparse auditory envelope representation (SAER). An iterative reconstruction algorithm for the SAER is presented that estimates carrier components to match the encoded envelopes. The algorithm is split into two stages. In the first, two sets of envelopes are generated, one of which expands the sparse envelope samples while the other provides limits for the iterative reconstruction. In the second stage, the carrier components are estimated using a synthesis-by-analysis iterative method adapted from methods designed for reconstruction from magnitudeonly transform coefficients. The overall system is evaluated using subjective and objective testing on speech and audio signals. We find that some types of audio signals are reproduced very well using this method whereas others exhibit audible distortion. We conclude that, except for in some specific cases where part of the carrier information is required, most of the audible information is present in the SAER and can be reconstructed using iterative methods.