audio signal processing Audio signal processing is a subfield of signal processing that is concerned with the electronic manipulation of audio signals. Audio signals are electronic representations of sound waves— longitudinal waves which travel through air, consist ...

, pre-echo, sometimes called a '' forward echo'', (not to be confused with

reverse echo Reverse echo and reverse reverb are sound effects created as the result of recording an echo or reverb effect of an audio recording played backwards. The original recording is then played forwards accompanied by the recording of the echoed or rever ...

) is a

digital audio Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, samp ...

compression artifact A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the media's data so that it beco ...

where a sound is heard before it occurs (hence the name). It is most noticeable in impulsive sounds from

percussion instrument A percussion instrument is a musical instrument that is sounded by being struck or scraped by a beater including attached or enclosed beaters or rattles struck, scraped or rubbed by hand or struck against another similar instrument. Exc ...

s such as

castanet Castanets, also known as ''clackers'' or ''palillos'', are a percussion instrument (idiophone), used in Spanish, Kalo, Moorish, Ottoman, Italian, Sephardic, Swiss, and Portuguese music. In ancient Greece and ancient Rome there was a similar ...

s or

cymbal A cymbal is a common percussion instrument. Often used in pairs, cymbals consist of thin, normally round plates of various alloys. The majority of cymbals are of indefinite pitch, although small disc-shaped cymbals based on ancient designs soun ...

s. It occurs in transform-based audio compression algorithms – typically based on the

modified discrete cosine transform The modified discrete cosine transform (MDCT) is a transform based on the type-IV discrete cosine transform (DCT-IV), with the additional property of being lapped: it is designed to be performed on consecutive blocks of a larger dataset, where ...

(MDCT) – such as

MP3 MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...

, MPEG-4 AAC, and

Vorbis Vorbis is a free and open-source software project headed by the Xiph.Org Foundation. The project produces an audio coding format and software reference encoder/decoder ( codec) for lossy audio compression. Vorbis is most commonly used in con ...

, and is due to quantization noise being spread over the entire transform-window of the codec.

Cause

The psychoacoustic component of the effect is that one hears only the echo preceding the

transient ECHELON, originally a secret government code name, is a surveillance program (signals intelligence/SIGINT collection and analysis network) operated by the five signatory states to the UKUSA Security Agreement:Given the 5 dialects that us ...

, not the one following – because this latter is drowned out by the transient. Formally, forward

temporal masking In audio signal processing, auditory masking occurs when the perception of one sound is affected by the presence of another sound.Gelfand, S.A. (2004) ''Hearing – An Introduction to Psychological and Physiological Acoustics'' 4th Ed. New York, ...

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 In signal processing, linear phase is a property of a filter where the phase response of the filter is a linear function of frequency. The result is that all frequency components of the input signal are shifted in time (usually delayed) by the s ...

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 In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data si ...

audio

codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or ...

s such as

, MPEG-4 AAC, and

. It is also one of the problems encountered in

digital room correction Digital room correction (or DRC) is a process in the field of acoustics where digital filters designed to ameliorate unfavorable effects of a room's acoustics are applied to the input of a sound reproduction system. Modern room correction syst ...

algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...

s and frequency domain filters in general (

denoising Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...

by spectral subtraction, equalization, and others). One way of reducing "

breathing Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cellu ...

" for filters and compression techniques using piecewise Fourier-based transforms is picking a smaller transform window (short blocks in MP3), thus increasing the temporal resolution of the algorithm at the cost of reducing its frequency resolution. To better reproduce transient and eliminate pre-echo- lossy audio compression software such as open source Vorbis encoder (oggenc from vorbis-tools)- impulse noisetune or and bit reservoir can be used as an advanced option (-advanced-encode-option).

References

{{Reflist

External links

Pre-echo
a
Hydrogenaudio KnowledgebasePre-echo detection & reduction, Massachusetts Institute of Technology, 1994
Acoustics

Cause

Mitigation

See also

References

External links