standard format now universally used across most digital electronic musical instruments. On the simplest example of an electronic keyboard, MIDI messages would be sent when a note is pressed on the keyboard, and would determine which note is pressed and for how long. Additionally, most electronic keyboards now have a "touch sensitivity", or "touch response" function which operates by an extra sensor in each key, which estimates the pressure of each note being pressed by the difference in time between when the key begins to be pressed and when it is pressed completely. The values calculated by these sensors are then converted into MIDI data which gives a velocity value for each note, which is usually directly proportional to amplitude of the note when played. MIDI data can also be used to add digital effects to the sounds played, such as reverb, chorus, delay and tremolo. These effects are usually mapped to three of the 127 MIDI controls within the keyboard's infrastructure – one for reverb, one for chorus and one for other effects – and are generally configurable through the keyboard's graphical interface. Additionally, many keyboards have "auto-harmony" effects which will complement each note played with one or more notes of higher or lower pitch, to create an interval or chord. DSP effects can also be controlled on the fly by physical controllers. Electronic keyboards often have two wheels on the left hand side, generally known as a pitch bend and a modulation wheel. The difference between these is that the pitch bend wheel always flicks back to its default position – the center – while the modulation wheel can be placed freely. By default, the pitch bend wheel controls the pitch of the note in small values, allowing the simulation of slides and other techniques which control the pitch more subtly. The modulation wheel is usually set to control a tremolo effect by default. However, on most electronic keyboards, the user will be able to map any MIDI control to these wheels. Professional MIDI controller keyboards often also have an array of knobs and sliders to modulate various MIDI controls, which are often used to control DSP effects. Most electronic keyboards also have a socket at the back, into which a foot switch can be plugged. The most common function is to simulate the sustain on a piano by turning on and off the MIDI control which adds