(1) Depending upon the width of each frequency band in a transform coding scheme, it is possible for the decoder to produce variable amounts of pre-echo. For example, a sudden transient that has its rising edge contained within a frequency band:. The codec quite correctly detects the transient’sits presence and codes the information within the appropriate band, and also uses that information to re-use bits from surrounding masked envelopes. Upon decoding, the output level is turned on the for complete duration of the coded frequency band, which means that the signal will be heard before the arrival of the actual transient. Such pre-echoes sound very much like analog tape print-through and may produce unacceptable results, particularly on material containing large numbers of sharp level excursions.
(2) In room acoustics, any early reflections of sound that occur within about 40 ms, the shortest time for which the ear can distinguish two non-simultaneous sounds. Some digital reverb devices include simulated pre-echoes as a part of their hall simulation program.