Rollover (key)
Encyclopedia
Rollover is the ability of a computer keyboard
to correctly handle several simultaneous keystrokes.
s such as Shift or Control, the user will intentionally hold down the modifier key(s) while striking and releasing another key. Rapid typists may sometimes inadvertently press a key before releasing the previous one. Certain unusual forms of keyboarding require multiple keys to be struck or held down simultaneously- for example, Braille2000 keying requires that as many as six keys be struck at once. Some computer games have interfaces requiring holding down keys (other than the usual modifier keys) while pressing and releasing other keys.
Most music keyboards use isolation diodes in their keyboard matrix
to implement full n-key rollover (sometimes abbreviated NKRO), making them immune to both key ghosting and key jamming.
However, to reduce cost and design complexity, most keyboards do not isolate all keys in this way. Instead, they use a matrix of key switches, without any isolation diodes, that assumes that only a limited number of keys will be held down at any given time.
"Multi-key rollover" is considered essential for quality keyboards and for English touch typing
. This is where the most common key sequences have been studied and keys in the same common sequence are placed in the electrical switch matrix such that three keys down cannot produce a fourth "phantom" key by shorting out the matrix. The simplest way to accomplish this is to put all keys in the same common sequence on the same X or Y line of the switch matrix. As long as the electronics does not see more than two keys on different X and Y lines, which would create a phantom key, it will continue to process the next key depressed. This typically produces 4- to 5-key rollover for the most common key sequences.
keyboards for certain combinations of 3 keys. When these three keys are pressed simultaneously a fourth keypress is erroneously registered by the keyboard controller
. Modern keyboards detect ghosting and instead of registering a fourth key will ignore the third key, which is known as jamming. Which keys jam when pressed together differs between brands and models of keyboards.
Most music keyboards and some high-end computer keyboards use an isolation diode with each keyswitch in their keyboard matrix
.
Such keyboards can correctly read any combination (chording
) of keys pressed in any order and released in any order -- they are immune to both key ghosting and key jamming.
Key jamming is most often noticed when using a keyboard to play action-oriented computer games. The original Star Control
game included a utility to test for key jamming and help the player determine the best key mapping for his/her keyboard, since during gameplay it was common for each of the two players to be pressing three or four keys at the same time.
Most modern action-oriented computer games and console emulators use the control, alt, and shift keys by default, since they are always designed to be pressed in combination with other keys, and thus usually circumvent key jamming.
Computer keyboard
In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...
to correctly handle several simultaneous keystrokes.
Normal typing
During normal typing on a conventional computer keyboard, only one key is typically being pressed by the user at any given time; each key is released before the next key is struck. However, this is not always the case. When using modifier keyModifier key
In computing, a modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination....
s such as Shift or Control, the user will intentionally hold down the modifier key(s) while striking and releasing another key. Rapid typists may sometimes inadvertently press a key before releasing the previous one. Certain unusual forms of keyboarding require multiple keys to be struck or held down simultaneously- for example, Braille2000 keying requires that as many as six keys be struck at once. Some computer games have interfaces requiring holding down keys (other than the usual modifier keys) while pressing and releasing other keys.
n-key rollover
Certain high-end keyboards have "n-key rollover". This means that each key is scanned completely independently by the keyboard hardware, so that each keypress is correctly detected regardless of how many other keys are being pressed or held down at the time.Most music keyboards use isolation diodes in their keyboard matrix
Keyboard matrix (music)
Most electronic keyboards used in synthesizers, electronic organs, and digital pianos use a keyboard matrix circuit to connect the switches for each key. In this matrix circuit, the rows and columns are made up of wiring...
to implement full n-key rollover (sometimes abbreviated NKRO), making them immune to both key ghosting and key jamming.
However, to reduce cost and design complexity, most keyboards do not isolate all keys in this way. Instead, they use a matrix of key switches, without any isolation diodes, that assumes that only a limited number of keys will be held down at any given time.
Multi-key rollover
A keyboard with "2-key rollover" can reliably detect only any two keys used simultaneously; in other words, a user can hold down any key on the keyboard and press a second key, and be sure that the keypress is correctly detected by the computer. However, if the user has two keys depressed and attempts to strike a third key, the third keypress may create a "phantom key" by shorting out the switch matrix. This is not acceptable for quality keyboards because there are many cases when more than two keys need to be depressed at the same time, or when more than two keys are depressed because of fast typing ("rolling over" more than two keys)."Multi-key rollover" is considered essential for quality keyboards and for English touch typing
Touch typing
Touch typing is typing without using the sense of sight to find the keys. Specifically, a touch typist will know their location on the keyboard through muscle memory. Touch typing typically involves placing the eight fingers in a horizontal row along the middle of the keyboard and having them...
. This is where the most common key sequences have been studied and keys in the same common sequence are placed in the electrical switch matrix such that three keys down cannot produce a fourth "phantom" key by shorting out the matrix. The simplest way to accomplish this is to put all keys in the same common sequence on the same X or Y line of the switch matrix. As long as the electronics does not see more than two keys on different X and Y lines, which would create a phantom key, it will continue to process the next key depressed. This typically produces 4- to 5-key rollover for the most common key sequences.
Key jamming and ghosting
Key ghosting occurs on matrixKeyboard matrix (music)
Most electronic keyboards used in synthesizers, electronic organs, and digital pianos use a keyboard matrix circuit to connect the switches for each key. In this matrix circuit, the rows and columns are made up of wiring...
keyboards for certain combinations of 3 keys. When these three keys are pressed simultaneously a fourth keypress is erroneously registered by the keyboard controller
Keyboard technology
There are many types of keyboards, usually differentiated by the switch technology employed in their operation. Keyboards are defined by the number of highly durable switches that are incorporated into the system...
. Modern keyboards detect ghosting and instead of registering a fourth key will ignore the third key, which is known as jamming. Which keys jam when pressed together differs between brands and models of keyboards.
Most music keyboards and some high-end computer keyboards use an isolation diode with each keyswitch in their keyboard matrix
Keyboard matrix (music)
Most electronic keyboards used in synthesizers, electronic organs, and digital pianos use a keyboard matrix circuit to connect the switches for each key. In this matrix circuit, the rows and columns are made up of wiring...
.
Such keyboards can correctly read any combination (chording
Chording
- Musical keyboards :In music, more than one key are pressed at a time to achieve more complex sounds, or chords.- Computer Keyboards :Chording, with a chorded keyboard or keyer allows one to produce as many characters, as a QWERTY keyboard, but with fewer keys and less motion per finger, and is...
) of keys pressed in any order and released in any order -- they are immune to both key ghosting and key jamming.
Key jamming is most often noticed when using a keyboard to play action-oriented computer games. The original Star Control
Star Control
Star Control is a science fiction computer game that was developed by Toys for Bob and published by Accolade in the early 1990s. Star Control still enjoys a cult following...
game included a utility to test for key jamming and help the player determine the best key mapping for his/her keyboard, since during gameplay it was common for each of the two players to be pressing three or four keys at the same time.
Most modern action-oriented computer games and console emulators use the control, alt, and shift keys by default, since they are always designed to be pressed in combination with other keys, and thus usually circumvent key jamming.