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Acoustic book

Digital processing

The digital processing allows doing many processing operations via computer. Some of these (such as amplification) are really simpler than the corresponding analogic operations.

Let’s now consider some of the most important processing operations.


Simple to understand, it is about incrementing the y- axis value of the wave.


One of the most interesting possibilities of digital processing is the time "stretching".
It consists in the modification of audio signal duration without changing its spectral composition.
It is an obviously impossible operation in analogic area, where duration and height are intimately linked.
When applied to a music piece, stretching changes its duration but not its spectral structure. Normally, one can think that stretching operates a time dilation (or narrowing) of the spectrum (encoded in frequency bands, as in the case of MP3 encoding), then returning in the amplitude/time space. Stretching can also be used to change spoken tracks by changing their expression.


As a complement to stretching, you can make a change in the height of a sound without changing its duration. This is also a typical digital effect.
Normally, the Pitch Shift makes a spectrum redefinition in the frequency/time space, then returning in the amplitude/time space.


The combination of Stretching and Pitch Shift allows you to optimize the mixing of different audio tracks, synchronizing them over time while controlling pitch.
However, these processing must be carefully applied to avoid side effects.
For example, a vowels frequency transposition without taking into any account the forming regions can lead to completely different sounds.


This feature allows you to reduce, or even eliminate, a specific noise from the signal.
The function requires a sampling of the noise, so as to be able to examine the spectrum and then subtract it from the spectrum of the signal.


When the noise is of impulsive kind, the elimination process is more complex because it must also identify the beginning and end instants of the disorder to be removed.

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