Tomas Jansson Toftgård

MDCT audio coding with pulse vector quantizers

This paper describes a novel audio coding algorithm that is a building block in the recently standardized 3GPP EVS codec [1]. The presented scheme operates in the Modified Discrete Cosine Transform (MDCT) domain and deploys a Split-PVQ pulse coding quantizer, a noise-fill, and a gain control optimized for the quantizers properties. A complexity analysis in terms of WMOPS is presented to illustrate that the proposed Split-PVQ concept and dynamic range optimized MPVQ-indexing are suitable for real-time audio coding. Test results from formal MOS subjective evaluations and objective performance figures are presented to illustrate the competitiveness of the proposed algorithm.

Harmonic Vector Quantization

Audio coding of harmonic signals is a challenging task for conventional MDCT coding schemes. In this paper we introduce a novel algorithm for improved transform coding of harmonic audio. The algorithm does not deploy the conventional scheme of splitting the input signal into a spectrum envelope and a residual, but models the spectral peak regions. The presented coding scheme is part of the recently standardized 3GPP EVS codec.

Linear prediction based comfort noise generation in the EVS codec

A Discontinuous transmission (DTX) system, which is widely adopted in speech codecs, is an important function for speech communication systems that can reduce the transmission bandwidth by at least a half. Within a DTX system, the comfort noise generation (CNG) plays a key role in the overall quality. Critical performance parameters with respect to the CNG including the transition quality from active to comfort noise (CN) frame, the quality of CN spectrum estimation, wider bandwidth rendering and the DTX efficiency have all been found to be very important. This paper describes a series of new technologies developed for the EVS codec aiming to address the performance of the CNG: A new hangover based CN analysis technique provides improved CNG transition quality. A new entropy based CN spectrum estimation technique and a new hybrid CNG scheme improve the CN spectrum estimation. Finally, a novel bandwidth extension technique for efficient rendering of high-frequency CN and a novel technique improving the DTX efficiency by controlling the DTX hangover length are described.