spread spectrum clocking on a hard drive?

[yan816]

spread spectrum clocking on a hard drive? Can anyone tell me what it does and is it worth enabling it. Thanks.

[kalniel]

Which hard drive are you talking about? I've heard about spread spectrum, I've heard about clocking and I've heard of hard drives.. but never in relation to each other..

[yan816]

Western Digital Wd2500KS SATAII hard drives have a set of pins you can put a jumper on that will enable spread spectrum clocking, but what does it actually do and does it effect performance?

[DataMatrix]

http://en.wikipedia.org/wiki/Spread_...nal_generation

Spread-spectrum clock generation (SSCG) is used in the design of synchronous digital systems, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal and because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency and its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the Federal Communications Commission (FCC) in the United States, JEITA in Japan and the IEC in Europe).

To avoid this problem, which is of great commercial importance to manufacturers, spread-spectrum clocking is used. This consists of using one of the methods described in the Spread-spectrum telecommunications section in order to reduce the peak radiated energy. The technique therefore reshapes the system's electromagnetic emissions to comply with the electromagnetic compatibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a simple modification to the equipment.

Spread-spectrum clocking has become more popular in portable electronics devices because of faster clock speeds and the increasing integration of high-resolution LCD displays in smaller and smaller devices. Because these devices are designed to be lightweight and inexpensive, passive EMI reduction measures such as capacitors or metal shielding are not a viable option. Active EMI reduction techniques such as spread-spectrum clocking are necessary in these cases, but can also create challenges for designers. Principle among these is the risk that modifying the system clock runs the risk of the clock/data misalignment.

Many personal computers have a BIOS setting to turn spread-spectrum clocking on or off. See external links at the bottom of this article.

It is important to note that this method does not reduce the total energy radiated by the system, and therefore does not necessarily make the system any less likely to interfere with sensitive equipment such as television and wideband receivers. It spreads the energy over a large frequency band which effectively reduces the electrical and magnetic field strengths that are measured within a narrow window of frequencies. Spread-spectrum clocking works because the EMI receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all of its energy at one frequency, then this energy would fall into a single frequency band of the receiver, which would register a large peak at that frequency. Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits. The usefulness of spread-spectrum clocking as a method of actually reducing interference is often debated, but it is probable that some electronic equipment with sensitivity to a narrowband of frequencies will experience less interference, while other equipment with broadband sensitivity will experience more interference.

FCC certification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, some BIOS writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This may be considered a loophole, but is generally overlooked as long as the default BIOS setting provided by the manufacturer has the spread-spectrum feature enabled.

In other words, it's used for reducing interference, not overclocking (I've heard that it has a bad impact on performance).

[schmunk]

Turn it off, unless you're more interested in avoiding radio waves than you are in performance.

[Clunk]

[DataMatrix]

IT'S TO STOP ALIENS FROM CONTROLLING YOUR PC!!!