- Pre-echo
Pre-echo is an audio
compression artifact where a sound is heard before it occurs (hence the name). It is most noticeable in impulsive sounds frompercussion instrument s such ascastanet s orcymbal s.It occurs in audio compression algorithms which use
Fourier-related transforms – particularly themodified discrete cosine transform (MDCT) – such asMP3 , MPEG-4 AAC, andVorbis , and is due to theGibbs phenomenon .Explanation
It is a mathematical phenomenon, not primarily a
psychoacoustic one – formally, it is a defect in theimpulse response of the compression algorithm, resulting in the energy of atime domain transients being smeared backwards and forwards in time after processing in thefrequency domain due to the Gibbs phenomenon. The psychoacoustic component of the effect is that one hears only the echo preceding the transient, not the one following – because this latter is drowned out by the transient. Formally, forwardtemporal masking is much stronger than backwards temporal masking, hence one hears a pre-echo, but no post-echo.Mitigation
In an effort to avoid pre-echo artifacts, many sound processing systems use filters where all of the response occurs after the main impulse, rather than
linear phase filters. Such filters necessarily introduce phase distortion and temporal smearing, but this additional distortion is less audible because of strong forward masking.Avoiding pre-echo is a substantial design difficulty in transform domain
lossy audiocodec s such asMP3 , MPEG-4 AAC, andVorbis . It is also one of the problems encountered inDigital room correction algorithm s.Pre-echo cannot be eliminated by using better approximations in the frequency domain, but it can be ameliorated by using smoother summations, such as
Fejér summation orRiesz summation , or by usingsigma-approximation .Alternatively, one can use different encoding, such as
wavelet s, which do not exhibit such artifacts.Related effects
A similar effect occurs when magnetic tape is stored, and magnetism is transferred between adjacent windings.
ee also
*
Compression artifact
*Gibbs phenomenon External links
* [http://wiki.hydrogenaudio.org/index.php?title=Pre_echo Pre-echo] at [http://wiki.hydrogenaudio.org/ Hydrogenaudio Knowledgebase]
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