Method and apparatus for modifying the play time of digital audio tracks

Abstract

A method an apparatus for modifying the play time of digital audio tracks. For each data block of samples taken from a plurality of such blocks comprising an audio stream or track, the original block data plus a time shifted copy of the block data are superimposed, in a manner dependent on either desired expansion or desired contraction of play time, to create an overlap region. The overlap region is selected at a position of best match as resulting from a normalized correlation over the overlap, the super position being weighted by a linear cross-fading function. All signal crossovers (signal transitions of opposite signal sign) in a block are located. A position at which to superimpose a copy of the data block with the data block, and the length of the data block, are determined based upon the location of said crossovers within a given data block. Additionally, as the data blocks are processed, any companding overage (companding in excess of a selected nominal companding amount) or underage (companding less than a selected nominal companding amount) are monitored and necessary adjustments are made so that the results in a final audio track length are very precise. Accordingly, the play time of an audio track or stream is either expanded or compressed in time while both preserving its original pitch and reproducing the sound at a very high quality level basically free of noise, distortion, and audible artifacts.