Manuale d'istruzioni - Pizzicato 3.6 IT925 - Revisione del 14/11/2012

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The spectral analyser

Subjects covered:


What is an audio spectrum? [Professional] [Composizione Pro]

Sound is just a vibration of the air or of the matter in which it propagates (water, wall,...). This vibration starts for instance in a music instrument when the player makes his instrument vibrate, by picking up a string, hitting a drum,... This vibration is then transmitted to the air around the instrument and it comes to the auditor's ear and gets registered as a sound impression to him.

So, to represent a sound signal, you need to measure the air pressure at specific regular intervals and memorize these values. This is what is done when you register a concert with a microphone and a digital recorder. By playing this sequence back into a loudspeaker, you can reproduce the exact sound that was recorded. Because of the frequency band to which the ear is sensible, to have a good reproduction quality, you must sample the sound signal 44100 times per second, with a sufficient precision level. This series of values is called the time representation of the sound. It gives the pressure value at any given time of a music concert. Here is how it appears for instance in the audio editor window of Pizzicato:

A mathematician called Joseph Fourier, established in the 19th century that such a signal could always be decomposed as a sum of sine and cosine waves of fixed frequencies and amplitudes. Practically, this means that the sound of a full orchestral symphony may be reduced to the sum of a series of simple sine waves with specific amplitudes and phases, with no variation in time for the full duration of the symphony. When you think about it, it is quite fascinating! But do not change a few of the frequencies or amplitudes, as the symphony may become an obscure sound confusion.

This way of representing a sound of a given time duration is called the frequency representation, or the frequency spectrum of the sound. In theory, you can switch back and forth between the time representation and the frequency spectrum representation and lose no information in the process (which means, keep the sound exactly the same). In practice, you can not make this process with an infinite number of frequencies, so you have to quantize the frequencies you will analyse and this precision may affect the sound result.

Pizzicato lets you analyse a portion of a sound signal to see what it represents in terms of frequency. You can then edit these frequencies (delete some, add some others, modify the spectrum, filter it,...) and then synthesize the signal back to the audio window. This is a way of experimenting sound design and manipulations. We will see now how to do that practically.

The spectral analysis window [Professional] [Composizione Pro]

When you select some portion of a sound in the audio window (see the lesson on the audio editor window), you can right click the background of the window and reach the contextual menu item Spectral analysis...

Working on the harmonics of a sound [Professional] [Composizione Pro]

You can design a new spectrum based on the harmonic series and then synthesize the corresponding sound.

By enabling the first check box, Pizzicato will apply a filter on the spectrum, based on the note you select in the above dialog (you can also enter any frequency). This filter will act to display and use only the integer multiples of the note frequency. For instance, if you have a note of 220 Hz, Pizzicato will only enable the harmonics of this note, which are [220, 440, 660, 880,...].

The Inharmonic factor can be used to detune the all harmonic construction, as this number will be used to multiply frequencies. For instance, if you enter 1.1 for that value, the harmonic will become [220, 220+220*1.1=462, 462+220*1.1=704,...].

The Enlargment of harmonic peak will in fact extend slightly each harmonic. For instance, if the value is 1 and the precision is 1 Hz in the spectral window, there will be the presence of the above harmonics but for harmonic 440, there will also be 439 and 441 frequencies, etc...

By clicking OK, Pizzicato will filter out any frequency of the current spectrum that is not in these specifications. You can then use the pen, eraser and line drawing tools to design a spectrum and you will see that only the harmonics as specified in that dialog will appear. You can for instance draw a spectrum like this, around C1, with a peak enlargment of 4 :

You can then synthesize it back in the audio window and hear how it sounds like. Then modify, synthesize and hear again. You may combine this kind of manipulation with the design of a virtual and modular synthesizer to create and modify samples of your own sounds.


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