Go to www.lexicon.com and click on support, knowledge base, then DC-1. You should get all the answers you need there.
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Version 1 was the base model, Pro logic. There's upgrades to that that give it THX and AC3 (DD).
Then Version 2 that has DD DTS THX.
Version 3 has DD DTS THX also but also has a different remote (backlit). Also refinments on the programs.
Version 4 is an upgraded version 3, more refinments to the programs.
But, here's the kicker, you can have a version 3 with DD and THX but no DTS. They had a ton of options depending on customer needs.