Welcome to HWC.
Well, laptop displays these days generally use a signalling protocol called LVDS (low voltage differential signalling - related to the TMDS used in DVI output). Older displays use a variety of different standards.
So assume it is LVDS, then you need a DVI to LVDS converter (essentially TMDS to LVDS). The only devicess I've seen that do such a thing are extremely expensive multipurpose units such as SGI's Multilink boxes - they are about $600! I haven't come a cross a standalone device designed for this purpose but imagine they would be extremely expensive also. Could be wrong - transforming one digital signal into another should just require a DSP of some sort.
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