Great observation flecom, you are entirely correct! We describe DigiSynth as being 'IO hardware agnostic' as it will use any compliant ASIO multi-channel audio device for realtime input/output, and most higher-end soundcards usually ship with an ASIO driver these days. (It also gives us the option to cross-compile for other operating systems as well in the future, as DigiSynth is not written as a Windows-specific application)
For a down-and-dirty solution, a modified soundcard would absolutely work and be a very inexpensive route to go. The motivation for Lightpipe I/O primarily stems from the software being built around very high end internal show authoring needs over the past few years when it was an in-house tool. For many installations the content is specifically required to be running multiple scanners played back from a stand-alone hard-drive deck such as the Alesis HD24, and thus the preference was to keep all production entirely digital through the authoring pipeline up until the final stage of playback. Doing the Lightpipe connections allow for this and many other cool possibilities such as multi-km fiber signal distribution, which we use on many larger installs. Your average user just doesn't need that kind of stuff, though!
To date Digisynth has been used near-exclusively with our Optical Showlink Transcoder product series, which was designed from the ground up to provide very precise response for high-speed color modulation and signal processing, as well as proper differential ILDA input/output, all in a separate box far away from on-board computer system noise. It allows for a lot of additional functionality you will not get from a sound card conversion: full soft-patch channel mappings, calibration outputs, level control, geometry correction, DMX/MIDI/Shutter, sub-encoding, additional ILDA signal lines (for controlling things like intensity, beam-brush, additional colors, etc) and the ability to handle bidirectional ILDA signal conversion (yes, DS can work with live analog inputs as a source!) simultaneously all in a 1U package.
Be that as it may, these are a lot of Transcoder features that the casual user will simply not need and would be silly (and expensive) overkill. A modified soundcard will work great for many casual users, but for an adjustable ILDA plug 'n play solution the Transcoder fits the bill for a lot of other permanent installations. The external conversion factor also allows running DigiSynth from a laptop or other portable system where a soundcard modification simply may not be possible, but a USB or Firewire connection is.
We are always working on ways to reduce cost, and the modified sound-card route is an excellent suggestion for this. We are also kicking around the idea of an extremely inexpensive USB->ILDA device that would take the place of a normal soundcard modification, but would maintain the same quality of signal output from the Showlink as well as provide dedicated audio output channels.
Like most things, there are a lot of tradeoffs! I'm a big fan of using the most appropriate tool for the job. Then again, I've been known to use my best flat-head screwdriver for prying open paint-cans because it was handy and 'just works' without going and digging for a proper paint-can opener.
-- Matt