Got it from Xilinx Signal Integrity glossary : Pre emphasis is magnitude boosting of high frequency spectral components before launching the signal (wave) onto the Transmission Line. Transmission Lines embedded in most standard PCB materials (FR4, Rogers 43xx, Nelco and Rogers) suffer varying degrees of dispersion and loss in the 1 gigahertz spectrum. This is mostly due to conductance losses (leakage from the copper trace to any other conducting structure) and Skin Effect. Dispersion is a phenomenon whereby spectral components travel at different velocities. The waveform looks smeared when it arrives at the receiver. Both of these "characteristics" play into a diminished and poorly received signal. By boosting the high freq. spectral components, the magnitude of these components can be diminished as the wave travels through the Transmission Line, but since it starts out larger than the lower frequency components, the composite signal arrives at the receiver looking the way it was intended. Pre Emphasis is done by simply increasing the maximum amplitude of the signal for one bit period. If the signal is 1 bit in duration, the amplitude is allowed to rise to a value which is some percentage greater in magnitude. At this point, if the signal is to stay at the same logic state, the driver sends a decreased magnitude signal, or nominal logic level. Every time a transition occurrs, the greater magnitude level is used. For all times after this that the same level is to be transmitted, the nominal magnitude is used. --Regards Dorababu -----Original Message----- From: Miller [mailto:lincj@xxxxxxxxxxxxxxxxxxx] Sent: Friday, July 05, 2002 5:38 PM To: hpc@xxxxxxxxxx Cc: SI_LIST (E-mail) Subject: [SI-LIST] Re: TO define the "Preemphasis" You can also check xilinx's Multi-Gigahertz Signaling FAQ http://www.xilinx.com/xlnx/xil_prodcat_product.jsp?title=si_gigfaq Best Regards Miller Lin ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu