In both cases, two sets of files are required. The usual rom files, plus an additional large "supplemental" file (in the case of laserdisc games, this is the MPEG related files, in the case of hard drive based games, it is the CHD file).
But any similarity ends there. The MPEG files that Daphne uses represent a digital capture of a source that was originally analog (laserdisc). CHD files represent a collection of digital files that were digital originally. In fact, it's really not much different than simply being a very large rom file.
Rom files literally represent a dump of the data that was originally stored on a set of rom chips. A CHD file is the exact same kind of dump, but the data is much larger, so it was stored on a hard drive instead.
Obviously, a CHD file can represent any kind of digital data, while the MPEGs used by Daphne only represent video and audio footage.
-Jeff "etumor"
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