In the past few months, I've been heavily involved with upgrading some CAT scanners that date from the late 70's. I've mostly been involved with the PDP-11 side, but bolted onto the PDP-11 (or, actually, the other way around) there's a custom piece of hardware (two dozen hex-height cards) that does all the heavy number crunching. Backprojection is what the process is called, and it's basically a deconvolution of the data measured by the individual detectors to make the pretty picture of your insides that a CAT scanner displays. The power and speed of this classic hardware is quite impressive, considering it's nearly 20 years old now. We've been timing other approaches to backprojection, and if we implement the same algorithm that the custom hardware is doing but on a modern 533 MHz Pentium III, we discover that the custom hardware is twice as fast as the Pentium. There are some other competing technologies, making use of DSP chips, which promise to be faster, but at a substantial development cost. Is this a fair comparison? Maybe not! The Pentium system costs roughly $2000, while the custom backprojection hardware was several hundred thousand dollars back in the late 70's. The PC boards that make up the backprojector make use of a unique technology: the PC boards themselves are just plain old two-layer boards, with the actual PC traces carrying only power and ground. All the actual wiring is done by fine machine-laid wiring between pads, with all the wire layers "potted" in a plastic compound. Obviously, reworking such a PC board is extremely difficult, but it gives impressive component densities for the time. The modern equivalent of this is multi-layer PC boards, naturally. Of course, there's another sort of power involved too: the custom backprojection hardware requires +5V at 350 Amps, and some installations have multiple backprojectors! -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology WWW: http://www.trailing-edge.com/ 7328 Bradley Blvd Voice: 301-767-5917 Bethesda, MD, USA 20817 Fax: 301-767-5927