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Info: What is the SSC jumper on my hard drive and why is it disabled?

post #1 of 3
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This is my first ever faq. Someone asked this question on the forums so I decided to make a faq out of it. I got this info in the August 2006 Issue of Maximum PC on page 64 in the "Ask the Doctor" section. They asked a Western Digital engineer on what SSC is. Enjoy!
SSC, or spread-spectrum clocking, is a frequency-modulation technique used in the design of synchronous digital systems (a hard drive) to reduce the spectral density of electromagnetic interference (EMI) that hard drives generate. Because of its precise periodic nature, the system emits spectral energy that's concentrated in a vary narrow frequency and its harmonics. This creates emission-energy peaks that can interfere with the operation of neighboring electronics. SSC lowers peak spectral energy, thus EMI as well, by modulating the frequency of the clock signal by around .1-.5 percent.
However EMI emissions really didn't vary much with SSC or enabled or disabled when tested at a third party lab. SSC did though did marginally hurt performance because the intentional modulation of the clock frequency-however minor-eats away at the drive's valuable jitter-tolerance margin. The SATA spec says that a SATA drive must be capable of receiving a signal with SSC. Which basically means that it is optional. Most drives have SSC turned off and if it isn't I recommend that you do so. Western Digital says that there are probably no readily observable systems if it is hurting your drive.

Note: I dont know of any case where it there was any noticeable or permanent damage.
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post #2 of 3
you posted the same thing twice....edit it so it's right....

Interesting though...
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post #3 of 3
Noises on the various data buss's are pretty much completely eliminated by using differential op-amps running at very low voltages on all the data lines. This noise elimination method also eliminates the need for strong RF shielding around the computer and individual components.
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