Quality Improvement of Very Low Bit Rate HE-AAC Using Linear Prediction Module

This paper proposes a new method of improving the quality of High Efficiency Advanced Audio Coding (HE-AAC) at very low bitrate under 16kbps. Low bitrate HE-AAC often produces obvious spectral holes inducing musical noise in low energy frequency bands, due to limited number of available bits. In the proposed system, a Linear Prediction (LP) module is combined with HE-AAC as a pre-processor to reduce the spectral holes. In its implementation, masking threshold of psychoacoustic model is normalized using the LP spectral envelope in prior to quantization of the LP residual. Also, in order to reduce the pre-echo, a block switching module is modified. Experimental results show that, at very low bitrate modes, the linear prediction module effectively reduces spectral holes, which results in reduction of musical noises of the conventional HE-AAC.