Guitar Signals

There are two types of current: direct (DC) and alternating (AC). In stompboxes (at least the ones that aren’t purely passive) you usually have both: DC from a battery or a power adapter and AC from your guitar’s pickups.

Whereas the DC signal has a mostly constant voltage level, the AC signal oscillates between a positive and a negative peak at a given frequency. AC signals encode information about the pitch in the period length and about the volume in the peak voltage of every up and down swing.

A guitar signal is never just a clean sine wave, it’s a mix of multiple waves of different period lengths. Waves other than the fundamental frequency (the note you picked) are called harmonics. This complex mix of frequencies produces a sound unique to the instrument — the timbre.

Amplitude = Volume

The larger the peak-to-peak amplitude and voltage, the higher the volume of the signal. The low-volume signal on the left can be amplified to resemble the signal on the right, without changing the frequency itself.

A typical guitar pickup produces around 100 mV of output, although the initial pick can reach 1 V for a very short time. Those voltages are of course highly dependent on the type of pickup (single coil or humbucker), the number of windings, whether it’s passive or active, its height, and other factors.

Frequency = Pitch

The number of up and down swings per fixed time interval determines the pitch. A long wave period produces a low pitch, a shorter period produces a higher pitch. The higher the frequency of oscillation between voltage peaks the higher the pitch.

Both signals shown above represent the same note, one octave apart. The right signal’s pitch is an octave higher as its frequency has doubled while maintaining the same amplitude.

Compression

Effect pedals modify audio signals. While a boost would amplify — i.e. increase the amplitude, including maybe some tone shaping — a compressor limits the amplitude and compresses it down to stay below a configurable threshold.

Clipping

Clipping too limits the amplitude, but also changes the waveform — thereby distorting the signal. An amplifier pushed beyond its capabilities is said to be clipping when it runs out of headroom and can no further amplify the signal.

Different tubes, transistors and op-amps have varying clipping characteristics and will produce different kinds of overdrive and distortion. A common way to mimic this effect is to use clipping diodes that cut off any excess voltage beyond their specific forward voltage.