Question: I just received the Western Digital Caviar Green (WD5000AADS). I noticed it has a jumper setting “Spread Spectrum Clocking”. I read somewhere this is a trade-off between speed and reduced electromagnetic noise.
Answer: Many hardware devices that put out enough electromagnetic interference, or EMI, to interfere with the reception of TVs and radios have a feature known as spread spectrum clocking (SSC). However, SSC is not well understood by many people. It is usually disabled by default. You have the option of turning it on or off, but it’s best to leave it disabled. ?Digital systems such as computers synchronize the activity of multiple elements through the use of a clock signal or a pulse. This signal usually exists in a narrow band of frequencies. If it’s intense enough at those frequencies, it can violate FCC rules about how much EMI a device rated for certain use can put out.
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SSC tries to get around this problem by shaping the emissions — i.e., varying the frequencies used — so that instead of the clock using one narrow frequency range, it’s spread out over a number of different ones at a much lower
Other resources
http://zone.ni.com/devzone/cda/tut/p/id/4154
http://wdc.custhelp.com/app/answers/detail/a_id/3641/~/what-is-spread-spectrum-clocking-(ssc)-and-does-it-need-to-be-enabled
http://spuder.wordpress.com/2011/08/04/ssc-spread-spectrum-clocking-hard-drives/