preecho

Preecho is an auditory artifact in compressed audio signals, referring to a faint pre-transient signal that can be perceived just before a sharp onset. It is most noticeable as a subdued, ghostly energy preceding a transient such as a drum hit or a pluck, particularly at lower bitrates or with certain codecs. The term is commonly discussed in the context of lossy audio coding and perceptual encoding.

The effect arises in block-based transform codecs that operate on fixed time windows. When a transient occurs near a window boundary, the quantization noise and the way the encoder models transient information can cause energy to appear in samples preceding the actual onset. Because the encoder processes data in blocks and cannot know future samples during prediction, the reconstructed waveform may contain energy leakage that the listener perceives as pre-echo. This artifact is distinct from natural reverberation or other pre-transients caused by acoustics.

Preecho is most apparent with sharp, fast attacks and can be exacerbated by lower bitrates or longer transform blocks. Modern codecs mitigate it by using a mix of long and short blocks, improved temporal masking, and refined psychoacoustic models, though the artifact can still be heard under certain conditions. In practice, listeners may notice preecho on percussive sounds or when dynamic range is high and masking is weak.

In summary, preecho describes a coding artifact where a faint pre-transient signal precedes the intended transient, reflecting limitations in certain lossy audio compression schemes rather than a property of the original sound.