Common Options : 0.25%, 0.5%, Smart Clock, Disabled
All clock signals have extreme values (spikes) in their waveform that create EMI (Electromagnetic Interference). This EMI interferes with other electronics in the area. There are also claims that it allows electronic eavesdropping of the data being transmitted.
This BIOS feature controls spread spectrum clocking of the motherboard. It usually offers two levels of modulation - 0.25% or 0.5%. The greater the modulation, the greater the reduction of EMI. Therefore, if you need to significantly reduce EMI, a modulation of 0.5% is recommended.
Generally, frequency modulation via this feature should not cause any problems. However, spread spectrum clocking can interfere with the operation of timing-critical devices like clock-sensitive SCSI devices. If you are using such devices, you must disable this feature.
System stability may also be compromised if you are overclocking the motherboard, especially those that have been taken to extremes. Therefore, it is recommended that you disable this feature if you are overclocking the motherboard. Of course, if EMI reduction is still important to you, enable this feature by all means, but you may have to reduce the overclock of the various components to provide a margin of safety.
If you are not overclocking the motherboard, the decision to enable or disable this feature is really up to you. If you have electronic devices nearby that are affected by the EMI generated by your motherboard, or have sensitive data that must be safeguarded from electronic eavesdropping, enable this feature. Otherwise, disable it to remove even the slightest possibility of stability issues.
Some BIOSes also offer a Smart Clock option. This works differently from spread spectrum clocking, although it is usually offered as an option under the Spread Spectrum BIOS feature.
Instead of modulating the frequency of clock signals over time, Smart Clock turns off the PCI Express (or AGP), PCI and memory clock signals that are not in use. Therefore, EMI can be reduced without compromising system stability. As a bonus, using Smart Clock also helps reduce power consumption. If the option is available (and you do not sensitive data to protect), try Smart Clock before enabling spread spectrum clocking.
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