Performance of transform and subband coding systems combined with harmonic scaling of speech

In this study an approach for improving the performance of waveform coders, based on coding a frequency scaled speech signal, is examined and subjectively evaluated for specific subband and transform coding systems. The recently developed simple and efficient time-domain harmonic scaling (TDHS) algorithms are used to frequency scale the speech signal. The underlying frequency-domain model of the pitch-adaptive TDHS algorithms provides insight and guidelines for their use in this application, as outlined in this work. The subjective evaluation is based on an A-B comparison test involving 12 listeners and shows a meaningful improvement in quality for the waveform coders used at low bit rates. In particular, subband coding (SBC) combined with TDHS (SBC/HS) at 9.6 kbits/s was found to provide a quality equivalent to that of SBC alone at 16 kbits/s, i.e., a bit-rate advantage of about 7 kbits/s was realized. For the speech specific adaptive transform coder (ATC) used, the combined system (ATC/HS) achieves a bit-rate advantage of 4 kbits/s at 7.2 kbits/s. The SBC/HS system emerges as a particularly attractive method for speech encoding at the data rate of 9.6 kbits/s since its quality is comparable to that of ATC/HS (or SBC at 16 kbits/s). Yet, its complexity is lower than ATC and the system is amenable to real-time hardware implementation using current technology.