Low-rate multimode multiband spectral coding of speech

At bit rates of 4 kbps and below, conventional time-domain algorithms such as CELP fail to retain high voice quality and robust performance against background noise as their waveform-matching ability is curtailed by the severely limited codebook space. Spectral coding, on the other hand, offers an effective parametric model, amenable for low-rate implementation. Instead of performing waveform matching, spectral coders preserve only the perceptually important spectral attributes of the speech signal. Spectral coding algorithms encompass a broad family of emerging low-rate speech coding techniques, the common goal being the representation of the short-term spectrum of input speech with a limited set of spectral parameters and the synthesis of the output speech with a set of sinusoids. Pitch, frequency-domain voicing information, and a varying number of spectral magnitudes are the usual parameters of spectral coders. In this paper, we present the enhanced multiband excitation (EMBE) coder as an illustration of this new generation of low-rate spectral coders. The distinguishing features of EMBE are: (a) signal-adaptive multimode spectral modeling and parameter quantization, (b) two-band signal-adaptive frequency-domain voicing decision, (c) a novel VQ scheme for the efficient encoding of the variable-dimension spectral magnitude vectors at low-rates, and (d) multi-class selective protection of spectral parameters from channel errors. A 4 kbps implementation of the EMBE spectral coding algorithm with 2.9 kbps source coding and 1.1 kbps for channel coding was specifically designed for satellite-based communication systems, targeting good voice quality at low bit rates and robust performance against channel errors. Fundamental concepts of the EMBE spectral coding algorithm, implementation details, and performance comparisons of the 4 kbps EMBE coder with earlier coders are reported.