A crucial part of frequency domain coding of audio signals is the appearance of pre-echoes. Consider the case that a silent period is followed by a percussive sound, such as from castanet or triangles, within the same coding block. Such an attack will cause comparably large instantaneous quantization errors. In ATC(Adaptive Transform Coding), the inverse transform of the decoder will distribute such errors over the block; similarly in SBC(Sub Band Coding), the decoder bandpass filters will spread such errors. In both mappings, pre-echoes occur and can become distinctively audible, especially at low bit rates with comparably high error contributions. By the time domain effect of pre-masking, pre-echoes can be masked if the time spread is of short length(in the order of few milliseconds).