This paper presents a new technique to reduce the preecho in a low bit-rate audio codec based on Adaptive Transform Coding (ATC). It allows the dynamic windowing to adapt the block size of the transform to the characteristics of the audio signal. The block size is changed from 2048 to 512 input samples when there is an attack in the audio signal found by a look-ahead detector. This approach can handle the case of closely spaced transient events. Apart from the start block or stop block required between the long and short windows, a new intermediate transition window type is proposed. It results in improved average segmental SNR (signal-to-noise ratio) by about 5 dB over the fixed-block-size ATC.
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