These days, I was asked how the Theremin, that strange instrument that consists of two antennas connected to a box that produces sounds similar to "angelic voices" ..
A famous example of using This instrument is short for Star Trek (first series).
So .. How does a Theremin?
The physical principle on which the operation of the instrument is the fact that the human body has its own impedance, so it can act as an electrical conductor, interacting with electrical circuits with which it comes into contact.
More specifically, with regard to the theremin specifically, if we take a ' antenna, which is a wire connected to an electrical circuit through a single terminal and approach it with your hand, create a variable capacitor , which presents as first armature antenna itself, as our second hand (actually, our whole body) and the air as a dielectric.
Electrically this is possible because both the electric circuit which is part of the antenna, and our bodies are connected with the earth then, although the electrical quantities involved are very small, there is still passing charges.
varying the distance between the hand and the antenna will change the ability of this "virtual capacitor" that abbbiamo just created. If this capacitor is connected to an oscillator , varying capacity, will also change the oscillation frequency of the latter.
That said, let's to analyze how the Theremin produces sound output as well as how to interact with this sound, the musician ...
tool allows you to manage two properties of the note produced: the frequency (pitch) and amplitude (volume).
To manage the theremin pitch uses a principle known as beat. Mixing two signals at two distinct frequencies are obtained by two other signals, characterized by having the first frequency equal to the sum of two frequency sources, and the second frequency equal to the difference.
We are interested in only the second signal product with a trick: if we generate a signal at a fixed frequency and then we create a second variable frequency, which deviates from the first between 20 Hz and 20 kHz, we obtain an output signal that falls in the spectrum audible by the human ear.
said .. how do you make the second signal to interact with the musician?
To do this we create two oscillators centered on the same frequency. Mixing the signals produced by these oscillators will get a signal at zero frequency in sleep.
connecting the tuning capacitor of the second oscillator, an antenna, we can vary the oscillation frequency (as described above). Properly calibrating the oscillator, we can ensure that the slip rate remains in the range 20 Hz - 20 kHz, obtained after mixing a low frequency signal.
We will now deal with the volume ...
is always to create the oscillator connected to the antenna, which then generate a variable signal in frequency as the pitch.
It changes the use made of this signal.
In this case, you use a circuit that frequency-voltage converter that allows you to create a voltage proportional to the frequency of the input signal.
This tension will be used to modulate the amplitude of the signal produced from the first antenna using a circuit called VCA (Voltage Controlled Amplifier).
Sarà poi necessario tarare l'oscillatore in modo che quando la mano sia vicina all'antenna il volume in uscita risulti nullo, e che questo aumenti man mano che ci si allontani dall'antenna.
Tutto qui..
Personalmente ne sto realizzando una versione pratica.. seguirà un articolo in cui spiegherò come realizzarlo.
Alla prossima!
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