I suspect the SD5's riff feature far more sophisticated than any similar feature on an older Roland arranger. The riffs are produced as "digital audio" loops, but not merely memorizing the chords you're playing, but also interpreting what you're playing (such as fills) in your right hand as well. The keyboard then produces a repeating audio loop 'based' on your playing style, allowing you to solo over it. I don't recall remembering any Roland arranger ever having this digital loop creation capability, but then again, I could be wrong. I trust AJ will be back here soon, and write a much better explanation of the feature than I possibly can.

Scott