Harmonic model for MDCT based audio coding with LPC envelope

Conventional music coders, based on a modified discrete cosine transform (MDCT) suffer greatly when lowering their bit-rate and delay. In particular, tonal music signals are penalized by short analysis windows and the variable length coding of the quantized MDCT coefficients demands a significant amount of bits for coding the harmonic structure. For solving such an issue, the paper proposes a frequency-domain harmonic model aiming to amend the probability model of the variable length coding of the quantized MDCT coefficients. The new model was combined successfully with an envelope based arithmetic coding at rate lower than 10 kbps, and with a context based arithmetic coding at higher bit rates in the recent 3 GPP EVS (Enhanced Voice Services) codec standard. Objective and subjective quality tests indicate that the proposed harmonic model enhances the quality of music for low-delay audio coding.