A keyboard matrix circuit is a design used in most electronic
musical keyboard
A musical keyboard is the set of adjacent depressible levers or keys on a musical instrument. Keyboards typically contain keys for playing the twelve notes of the Western musical scale, with a combination of larger, longer keys and smaller, sh ...
s and
computer keyboard
A computer keyboard is a peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or keys to act as mechanical levers or electronic switches. Replacing early punched cards and paper tape technolog ...
s in which the key switches are connected by a grid of wires, similar to a
diode matrix
A diode matrix is a two-dimensional grid of wires: each "intersection" wherein one-row crosses over another has either a diode connecting them, or the wires are isolated from each other.
It is one of the most popular techniques for implementing a ...
. For example, 16 wires arranged in 8 rows and 8 columns can connect 64 keys—sufficient for a full five octaves of range (61 notes). By scanning these crossings, a keyboard controller can determine which keys are currently pressed.
Description
Without a matrix circuit, a 61-key keyboard for a
synthesizer,
electronic organ
An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since developed ...
, or
digital piano would require 62 wires to connect (one for each note, and a ground)—an awkwardly thick bundle of wiring. With a matrix circuit, any of 61 notes can be determined with only 16 wires. This is drawn schematically as a matrix of 8 columns and 8 rows of wires, with a switch at every intersection. The keyboard controller scans the columns. If a key has been pressed, the controller scans the rows—and in a manner analogous to the board game
Battleship, the controller determines the row-column combination at which a key has been pressed, and generates a note corresponding to that key. This process occurs so quickly that the performer is unaware of any delay.
[Dave Dribin]
"Keyboard Matrix Help"
2000.
There are at least two limitations with this system. The first is that it provides only a crude binary on/off signal for each key. Better electronic musical keyboards employ two sets of switches for each key that are slightly offset. By determining the timing between the activation of the first and second switches, the velocity of a key press can be determined—greatly improving the performance dynamic of a keyboard.
The second is that instruments with a matrix circuit can only play in a
monophonic
Monaural or monophonic sound reproduction (often shortened to mono) is sound intended to be heard as if it were emanating from one position. This contrasts with stereophonic sound or ''stereo'', which uses two separate audio channels to reproduc ...
fashion without the addition of a diode for each key crossing. The
diode prevents unwanted notes ("phantom keys") from being triggered, or intended notes from being masked (
"phantom key blocking").
Monophonic instruments and most low-cost computer keyboards reduce costs by leaving out most or all of those diodes. To avoid "phantom keys", the keyboard controller in modern low-cost computer keyboards will ignore further key presses once two keys (other than
modifier key
In computing, a modifier key is a special key (or combination) on a computer keyboard that temporarily modifies the normal action of another key when pressed together. By themselves, modifier keys usually do nothing; that is, pressing any of the , ...
s) have been pressed, which is known as
jamming.
The matrix circuit approach is also used in non-musical keyboards, such as in the keypads for calculators and the "QWERTY" alphabetic and numeric keyboards used to enter information into computers. The same matrix circuit approach is also used in many
pinball
Pinball games are a family of games in which a ball is propelled into a specially designed table where it bounces off various obstacles, scoring points either en route or when it comes to rest. Historically the board was studded with nails call ...
machines.
Often in pocket calculators the
multiplexed digit drive lines would be used to scan the keyboard as well, providing further savings.
[Stan D’Souza]
"Microchip AN529: Multiplexing LED Drive and a 4x4 Keypad Sampling"
1997.
See also
*
Charlieplexing
*
Crossbar switch
*
Polyphony (instrument)
References
{{Reflist, 30em
Synthesizer electronics
Computer keyboards
Switches