Rollover, blocking and ghosting

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These inter-related terms often come up in discussions of a keyboard's ability to register multiple keys at once. Often confusion exists over what exactly they correspond to, which is exacerbated by overzealous marketing departments using them in misleading ways, or using the terms interchangeably.

Ghosting

Ghosting is a potential issue where certain combinations of three or more keys can cause an additional key to be registered. Ghosting is a rare problem, and is a sign of a serious design flaw in the keyboard.

Many manufacturers misuse the term ghosting to refer to Blocking, or use the term 'anti-ghosting' to refer to a gaming optimized matrix.

Blocking

Blocking is a means of preventing ghosting where the controller is programmed to detect combinations of key presses that can cause ghosting, and ignore all of the keys that could be the result of ghosting. Blocking is a simple and common way of solving the ghosting problem, but leads to dropped/ignored key presses, which is problematic for gamers.

Game Optimized Matrix

This term, and others like it, are used by manufacturers to describe a keyboard whose switching matrix has been designed in such a way to prevent blocking occuring on certain commonly used gaming combinations. It does not eliminate blocking, it merely tries to avoid it. In many instances, it is little more than marketing hype, and does not provide a tangible benefit over a regular keyboard. However, on some keyboards such as the Microsoft Sidewinder X4, the quality of optimiziation means that it is almost as good as a NKRO keyboard for most usage.

See controller matrix traces for specific matrixes and their quirks.

Key Rollover

Key Rollover is a measure of how many keys can be simultaneously registered by the keyboard. There are a number of types of rollover that keyboards can have, and any limitations they have arise from different problems and have different effects.

n-Key Rollover

n-Key Rollover, usually called NKRO, refers to a keyboard that suffers from neither blocking nor ghosting. In a true NKRO keyboard, any number and or combination of keys can be pressed simultaneously, and all will be registered correctly.

A Technical Explanation of NKRO

Implementing NKRO involves preventing ghosting from happening on the electrical level, there are two ways in which this can accomplished -

• The keyboard can be built using non-electrical switching elements - such as capacitive switches, or hall effect switches.
• Insert diodes along the the switching matrix's columns to ensure that current can only flow in the correct direction.

Both of these were common practice up till the mid eighties, where cost cutting led to blocking becoming the standard way to prevent ghosting.

Interface-limited NKRO

Certain peripheral interfaces such as USB are not designed to support true NKRO. This, a USB NKRO keyboard will not suffer from blocking or ghosting, but is only capable of sending a limited number of keys over the interface at once. Additional keys pressed beyond the limit will generally cause some of the other keys to be dropped. USB is usually limited to 6 regular keys and 4 modifier keys. Some keyboards are able to cheat this limit, but sometimes at the expense of quirky behavior or incompatibility with certain operating systems.

2KRO, 6KRO and others

Sometimes, key rollover is described using a number. This practise is somewhat confusing because it means different things in different contexts. 2KRO is often used to describe a keyboard with blocking, and is meant to be read as "any combination of two keys are guaranteed to work, and some combinations of three or more keys will work, but others will not". 6KRO is often used to describe an interface-limited NKRO keyboard, and is meant to be read as "any combination of up to six keys are guaranteed, but no combinations of seven or more will work".

Some rollover quantities greater than 6, such as 12KRO, 18KRO or 22KRO are used to refer to heavily optimized matrices, or interface-limited NKRO keyboards that have got around the 6 key limit.

Transposition Errors

Transposition errors are when a number of keys entered in rapid succession are registered in the wrong order. For example, typing A then B then C, and the keyboard registering ACB or BAC. As with ghosting, this is the result of a design flaw in the controller, and is not considered acceptable failure. These errors have been spotted in certain low quality mechanical keyboards.

Testing Key Rollover

• Microsoft Applied Sciences: Keyboard Ghosting Explained!