Under normal operating conditions, the clock signal from the PCI and memory clock is evenly distributed among all slots present on a mainboard. Unfortunately, each slot and the traces leading to it have their own inductance, impedience and capacitance, resulting in weakening and dilution of the original clock signal which, therefore arrives at its target with reduced driving strength. Consequently, the signal is much stronger at those slots where it is needed with greatly reduced EMI as side effect. Spread Spectrum Modulation was invented to reduce interferences of high order harmonics of the bus frequency. The theory is that, because every wave form generates higher order harmonic waves or Obertoene, accumulation of the latter can result in interference with the original signal.
Most current boards employ the centered modulation.
This is, at least, the official version of Spread Spectrum Modulation. In reality, there are different reasons for its implementation. EMI, on the other hand can cause interferences with other devices and is, therefore, subject to regulation by the FCC which limits the signal amplitude according to its guidelines. Any device exceeding the maximum allowable signal strength will not gain approval by the FCC and can, therefore, not be marketed. Basically, the testing device is a radio receiver and the testing is done by sweeping its receiving frequency through the frequency range of interest and measuring the interference with the video and audio signals.