Research of Low Bit-Rate Speech Coding Technology

在本計劃中，針對音訊訊號的遮 罩效應 (masking effect) 進行研究，提出 一個新的後遮罩 (forward masking effect) 的模型，應用在音訊壓縮音質的改善 上。這個模型利用了人耳音訊系統中 接收感應器與神經刺激的效應，這些 效應通常在心理聲音學中後遮罩的原 因。其中，人耳中的非線性效應我們 以一個非線性電路的差分方程式來建 立模型。我們將這個模型加入MPEG Layer III 音訊壓縮架構當中的遮罩效 應，建立在時間頻率空間中的遮罩曲 面。加入這個模型我們可以在相同壓 縮比下得到比較好的音訊音質。在我 們的實驗中，主觀與客觀的音質測試 顯示我們可以比MPEG Layer III 的音訊 壓縮減少12%到25%所需的位元數。This paper presents a new forward masking model for perceptual audio coding. This model exploits adaptation of the peripheral sensory and neural elements in the auditory system, which is often deemed as the cause of forward masking. Nonlinearity of the ear is modeled by a nonlinear analog circuit with difference equations. We incorporate this model in the MPEG Layer III audio coding scheme and construct a masking plane in the frequency-time space. With some extra computations, the new audio coding scheme can improve the sound quality of the decoded audio signals. In our experiments, subjective and objective sound quality measurements show that, to achieve the same reconstructed sound quality, the new scheme requires 12% to 23% less bits than the original MPEG Layer III scheme