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Rollover (key)
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Rollover is the ability of a computer keyboard 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 keys such as Shift or Control, the user will intentionally hold down the modifier key(s) while striking and releasing another key.
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Encyclopedia
Rollover is the ability of a computer keyboard 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 keys 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.
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.
Most music keyboards use isolation diodes in their keyboard matrix to implement full N-key rollover, making them immune to both key ghosting and key jamming.
3-key rollover
A keyboard with "3-key rollover" can reliably detect 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 holds down two and attempts to strike a third, the keypress may be misinterpreted depending on what key is pressed.
3-key rollover is considered essential for English touch typing.
Key jamming and ghosting
Key ghosting occurs on matrix keyboards for certain combinations of 3 keys. When these three keys are pressed simultaneously a fourth keypress is erronously 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.
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.
See also
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