What is a pick up?
The sound of a musical instrument is quite complicated, because it is composed of a set of sinusoidal waves, called harmonic, out of phase one respect to each other.
The amplitudes of the harmonics are variable, but their frequency is always a whole multiple of a fundamental frequency called order one harmonic or fundamental which is the one with the lowest frequency.
When talking about sound, references are made to three sizes: intensity, height and timbre:
INTENSITY: Depends on the amplitude of the harmonic and grows as the vibration increases, is the trait that distinguishes strong sounds and weak sounds.
HEIGHT: Depends on the frequency of the sinusoidal components, is the trait that distinguishes acute sounds and low sounds.
TIMBRE: It distinguishes two sounds with the same intensity and same height. Think about two violins on which are played notes at the same frequency and with the same amplitude, the sound we perceive will be different from the two violins: it is said that they will have a different timbre.
How does it work?
The pick-up mounted on the guitar body below the ropes, is a wire coil wound on a ferromagnetic core (usually nickel or cobalt). Once the rope is pinched begins to vibrate, moving in this way within the magnetic field produced by the magnet below, causing a variation of flow within the magnetic field with the consequent production of induction voltage in the coil (Faraday's law) which is the electrical signal of the sound that we will amplify.
Pick-ups may consist of one or more coils, in the first case we talk about single-coil pick-up (pick up with one coil)
All pick-up coils are susceptible to interference from "electromagnetic radiation", this means they tend to pick up noises or buzz from other amplifiers or nearby electrical appliances, buzz neutralization is called "humbucking".
These pickups have two coils instead of one, these coils are connected in series (the current passes first through one and then through the other) but off phase with each other, so any roaming interference sent by a coil as a positive signal will be feeled as a negative signal from the other, so the two currents that slide in opposite directions, cancel each other, and the buzz is not transmitted to the amplifier.
To ensure that the two coils do not even erase the generated currents from vibrational strings, the poles of each coil have opposite magnetic polarities. So when the secondary coil inverts the signal of the disturbed magnetic field, it duplicates, instead of erasing, the electric pulse.
The humbucker not only reduces the unwanted interference, but has a distinctly different sound from single coil pickup, in fact the double coil structure of the humbucker causes an overall definition of lower sound and a lower response to high frequencies, which are cut off. So it is to be found in the fact that the Gibson mounts humbucking pickup while the Fender uses single coils the main reason for the two so many distinct timbre of their guitars. The Fender instrument timbre is brilliant and sharp, more "full" than the Gibson instruments.
OPTICAL PICK-UP: Based on the principle of the optical mouse, this type of Pick-up does not suffer from electromagnetic interference, both power amplifiers and power cables, as it does not exploit a magnetic field to capture the vibration of the string but an optical sensor.
PIEZOELECTRIC PICK-UP: These pick-ups use the piezoelectric characteristics of some crystals, ie the property of generating a potential difference when they are subject to mechanical deformation. These pickups are usually used to amplify the sound of semi-acoustic guitars. They are positioned close to the guitar deck and resonate with the vibrations of the resonance case that comes when the instrument is played.
