Frequency scaling of speech signals by transform techniques

The general framework of short-time Fourier analysis, modification, and synthesis is used to describe in a unified way several known techniques for frequency scaling of speech signals. Subsequently, a frequency domain harmonic scaling technique is studied in detail with emphasis on improving its performance and its implementation efficiency. This technique is particularly attractive for 2:1 scaling by use of a sign tracking algorithm which avoids the need for explicit phase computation and unwrapping. The implementation efficiency is achieved by using the fast Fourier transform algorithm, embedded decimation and interpolation, and an extended version of a recently developed weighted overlap-add synthesis scheme. The improvement in quality is achieved by improved sign tracking and elaborate design and selection of the analysis and synthesis prototype filters (data windows). Results of computer simulations, for a variety of adverse acoustical environment conditions, indicate that the system is highly robust but its quality for clean speech is lower than with a time domain harmonic scaling technique which uses pitch information. In applications which do not permit pitch transmission, a hybrid scheme which combines the two techniques is found to yield a better quality than either system alone.