Overvolting
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Overvolting is an arm of overclocking where the voltage flowing through computer components is increased slightly as to increase their overclocking ability.
For example, an overclocker (one who overclocks) will commonly increase the core voltage of their CPU as to satisfy the increased current demands of the CPU because of the higher speeds achieved through overclocking.
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Signal levels and overclocking
Digital circuits, such as those found in the processors of a personal computer, represent information as swings between two voltages. As a simplified example, high voltages can represent a "1", and low voltages can represent a "0".
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(Figure 1: a circuit running within its tolerances)
In this example signal diagram (fig. 1), there are two voltage levels that the signal must conform to in order to be considered valid. Voltages at or below the "low" level are valid, voltages at or above the "high" level are valid, but voltages in the no-man's-land between them are not valid.
The transistors inside the circuit can change their voltages very quickly, but not instantaneously. They have a particular rate at which their voltage changes. At low speeds, the transistors switch much faster than the circuit needs, resulting in the switching time (the time spent in the no-man's-land) being insignificant. However, as the signalling speeds increase up to the limit of the transistor's abilities, the transistor's frequency response begins to affect the circuit's performance.
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(Figure 2: an overclocked circuit)
When a circuit is overclocked (fig. 2), it processes the same data faster (notice that it takes less time to send the same six transitions), but the swings in voltage become less "clean". If the circuit is overclocked too much, the transistors will not be able to fully swing between the acceptable low and high levels. The resulting signal levels lie in the invalid zone, and the circuit becomes unstable.
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(Figure 3: an overclocked, overvolted circuit)
By increasing the circuit voltage (fig. 3), the transistors can switch more effectively, restoring signal levels to the valid range.
Depending on how much the system is overclocked past its tolerances, the effects can manifest themselves as display errors, errors in calculations (Prime95 tests for these in its torture test mode), general application instability, to an instant crash.
Demonstration using a door
A real-world demonstration of overclocking's effects can be performed using a door. Stand by the inside of a door, so that you can repeatedly open and close it. Start by opening and closing the door completely (from 90 degrees open to completely closed), as fast as you can. Now, increase the speed of the door to simulate the increase in clock speeds in a computer. As you move the door faster, you'll notice that the "open" and "closed" states get closer and closer together. At low speeds, the door's inertia is negligible and you can change its position with ease. Howerver, as the door's speed increases, it becomes noticeably harder to stop and reverse its direction each cycle. Eventually, you end up bouncing the door back and forth between your hands, which are likely spaced a few inches apart.
Misconceptions and modifications
It is a common misconception that increasing the voltage running through a component increases its "overclockability" simply because the increased voltage supplies more power. While it is true that the circuit draws more power, the real reason it becomes more stable when overvolted is because of the change in signal levels.
Increasing voltage causes a component to draw increased current. However, this increase in current results in increased power dissipation (heating).
Many overclockers can now increase the voltage to various components in their machines (such as the CPU, RAM and AGP bus) easily in the BIOS. However, some components cannot be overvolted in software, and special modifications ("mods") are commonly made by experienced overclockers to manually increase the voltage, especially in the case of modern video cards and the motherboard's northbridge. These modifications are affectionately known as "voltage mods" in the overclocking community.
Potential problems with overvoltage
Increasing the voltage running through a component invariably increases the heat generated by that component. This can lead to overheating and even damage to circuitry. It is often said that overvolting decreases the lifespan of a component, and the higher the voltage put through, the shorter the component's life will be. This is due to a combination of various factors, notably increased heat production and internal damage to the conductors from electromigration.