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This video offers up a first look at Reason 5 and the Dr. Octo Rex Loop Player:
The upgraded Dr. Octo Rex loop player loads eight REX loops into one player and lets you switch between them on the fly. This is designed to make  arranging a breeze – load the drum loops into one player, the guitars into another and use the sequencer to select what loop to play in a pattern-like fashion.
With eight loops to switch between, the new Rex also comes ready for the experimental minded. Set the player to retrig the loops on the beat, on the bar or on the 16th note. Or program the loops manually like in the original rex player.
For each of the eight loops, the new rex player also comes with an expanded set of per-slice settings. Set pan, pitch, filter frequency and level, reverse slices, use multiple outputs, create alternating groups of slices and much more.
Propellerhead says that this is the first peek at Reason 5, but that they’ll have more Reason 5 and Record 1.5 previews throughout this week.