Temporal noise shaping has been defined in MPEG-4 AAC to control the pre-echo noise in attack signals. The module, which is especially important for the MPEG-4 Low Delay AAC due to the absence of window switching mechanism, can shape and control quantization noise spread to improve the quality under bit rate constraint. However, this paper illustrates that the TNS will introduce three artifacts. The first artifact is similar to the Gibbs phenomenon which has high noise level occurring at the edge of the attack signal. The second effect is the time-domain aliasing noise which has unusual noise at a distance from the attack time frame. The third is the noise spreading with the TNS filter orders. This paper will propose the efficient TNS method which shapes noise with good concerns on the above three artifacts. Also, we provide an efficient computing method to activate the TNS. Both subjective and objective tests are conducted to illustrate the improvement over existing TNS methods.
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