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

Abstract

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 timedomain 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 particuinrly 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. I. INTRODUCTION W AVEFORM coding techniques attempt to reproduce encoded signals at lower bit rates than PCM encoding by utilizing the temporal and spectral properties of the signal. Speech-specific coders have achieved sizeable reductions in bit rate by incorporating in the coder design known properties of both short-time and long-time (pitch period) correlations which are related to spectral envelope (forniants) arid fine structure (pitch harmonics) of speech, respectively. However, because they attempt to replicate the speech waveform, even the more complex forms of waveform coders, such as adaptive-predictive coders (APC) [1], [2] and adaptive transform coders (ATC) [3], [4], require bit rates typically at or above 9.6 kbits/s in order to have acceptable communication quality [5]. This range of bit rates is several times higher than the bit-rate range of vocoders which typically encode speech at 2.4 kbits/s and below [5]. Vocoders are based on a speech production model and achieve low bit rates by analyzing and Manuscript