Hey does anybody know what fsb/agp spread spectrum does?

I got this from Rojak's BIOS Guide:

When the motherboard's clock generator pulses, the extreme values (spikes) of the pulses create EMI (Electromagnetic Interference). The Spead Spectrum feature reduces the EMI by modulating the pulses so that the spikes of the pulses are reduced to flatter curves. It does so by varying the frequency slightly so that the signal doesn't use any particular frequency for more than a moment. This reduces the amount of interference that will affect the other electronics in the area.

The BIOS usually offers two different levels of modulation - 0.25% or 0.5%. That's the amount of modulation (or jitter) from the baseline signal. The greater the modulation, the greater the reduction of EMI. Therefore, if you need to significantly reduce EMI in the surrounding area, a modulation of 0.5% is recommended.

In most conditions, frequency modulation via this feature shouldn't cause any problems. However, system stability may be slightly compromised in certain situations. For example, enabling Spread Spectrum may cause improper functioning of timing-critical devices like clock-sensitive SCSI devices.

Spread Spectrum can also cause problems with overclocked systems, especially those that have been taken to extremes. The slight modulation of frequency may cause the processor or any other overclocked components of the system to fail, leading to very predictable consequences. Of course, this depends on the amount of modulation, the extent of overclocking and other factors like temperature variation, etc... As such, the problem may not readily manifest itself instantly.

Therefore, it is recommended that you disable this feature if you are overclocking your system. The risk of crashing your system isn't worth the reduction in EMI. Of course, if EMI reduction is important to you, enable this feature by all means but reduce the clock speed a little to give this feature some "space" to modulate safely. If you are not overclocking, the decision to enable or disable this feature is really up to you. But if you ask me, unless you have EMI problems, it's best to disable this feature to remove the possibility of stability issues.

Some BIOSes also offer a Smart Clock option. Instead of modulating the frequency of the pulses over time, Smart Clock turns off the AGP, PCI and SDRAM clock signals that are not in use. Thus, EMI can be reduced without compromising system stability. As a bonus, using Smart Clock also help to reduce power consumption. The degree of EMI and power reduction will depend on the number of free (empty) AGP, PCI and SDRAM slots. But generally, Smart Clock won't be able to reduce EMI as effectively as simple frequency modulation.

Still, if your BIOS comes with this Smart Clock option, you should select it over the 0.25% or 0.5% options if you need some EMI reduction. It will allow you to reduce EMI without any risk of compromising stability.

Hey thanks BigAkita!