Audio Signal Processing and Coding

6.9 PRE-ECHO DISTORTION

An artifact known as pre-echo distortion can arise in transform coders using perceptual coding rules. Pre-echoes occur when a signal with a sharp attack begins near the end of a transform block immediately following a region of low energy. This situation can arise when coding recordings of percussive instruments such as the triangle, the glockenspiel, or the castanets for example (Figure 6.21a). For a block-based algorithm, when quantization and encoding are performed in order to satisfy the masking thresholds associated with the block average spectral estimate, time-frequency uncertainty dictates that the inverse transform will spread quantization distortion evenly in time throughout the reconstructed block (Figure 6.21b). This results in unmasked distortion throughout the low-energy region preceding in time the signal attack at the decoder. Although it has the potential to compensate for pre-echo, temporal premasking is possible only if the transform block size is sufficiently small (minimal coder delay). Percussive sounds are not the only signals likely to produce pre-echoes. Such artifacts also often plague coders when processing “pitched” signals containing nearly impulsive bursts at the beginning of each pitch period, e.g., the “German male speech” recording [Herr96]. For a male speaker with a fundamental frequency of 125 Hz, the interval between impulsive events is only 8 ms, which is much less than the typical analysis block length. Several methods ...