Application of the phase vocoder to pitch-preserving synchronization of an audio stream to an external clock

The phase vocoder is usually presented as a high-quality solution for time-scale modification of signals, Its main advantages versus the cheaper time-domain techniques include the high-quality of the output for a wide range of types of input signals (speech, music, noise), and the possibility to perform very large factor modifications (e.g., four-fold time-stretching or more). In this paper, we present two applications that require such extreme modification factors: we call the first one pitch-preserving audio scrubbing, in which a user can move a pointer along an audio track and hear the sound at the corresponding location without any pitch alteration. Because the user controls the playback location (and therefore the playback speed), and can very well stop at a given location, the required time-scale modification can involve a very large-factor. The second application consists of synchronizing an audio stream to a video stream, while avoiding pitch alteration. For extreme slow-motion playback, the time-scaling operation required to preserve the pitch can also involve a very large factor. We address theoretical and practical issues related to pitch-preserving synchronization of an audio track. Techniques are discussed to allow freezing time in the phase-vocoder and avoid problems associated with very large factor modifications.