Recent trends in speech and audio codec standardization include scalability and extending the signal bandwidth beyond wideband (WB) to superwideband (SWB). In this paper we introduce a SWB extension for the ITU-T G.718 WB codec. In the SWB extension the high frequency content is generated utilizing the quantized MDCT domain coefficients of the WB core, which enables low additional delay. The proposed implementation is scalable with 4 kbps layers. In the first layer two different coding modes are used depending on the input signal type. The proposed SWB extension is evaluated with listening tests and complexity analysis.

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The paper introduces a new coding methodology of the spectral modified discrete cosine transform (MDCT) coefficients of an audio signal. A lattice quantizer is used for each spectral sub-band, having the dimension equal to the size of the respective sub-band. The information that needs to be encoded consists of lattice codevector indexes, side information relative to the number the bits on which the indexes are represented and the integer exponents of the sub-band scaling factors. The nature of the side information, together with the parameterization of the quantization resolution allows the use of the method for a large range of bitrates e.g. for 44.1kHz sampled mono files, from 128 kbits/s down to 16 kbits/s. Subjective listening tests show similar performance of the proposed method to the advanced audio coding (AAC) codec for high bitrates (128 kbits down to 64 kbits/s) and clearly better performance for lower bitrates.

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