Te ARP-87 delay by Walrus Audio has a ton more than meets the eye, with the X knob changing functions depending on which Algorithm is selected. Trails Mode and Momentary Functions are also super cool features.
From the manufacturer’s web page:
The ARP-87 is a compact, feature-rich delay, packed with a large pallet of inspirational delay tones. Featuring four main algorithms, digital, analog, lo-fi, and slap back, the ARP-87 can cover some serious ground in the world of echoes. The Digital algorithm boasts pristine, crystal clear repeats great for rhythmic riffs. The Analog setting works well to add warmth and dimension to chords and lines without getting in the way. The Lo-Fi algorithm, with its adjustable frequency range on the repeats, goes from warm, warped, and murky, to strait AM radio. Finally, the Slap setting makes it easy to dial in a great slap back echo perfect for smoking chicken pickin’ runs. With other features like our smart bypass switching, and momentary knob ramp, the ARP-87 provides endless creative opportunities that are waiting to be explored.
The X knob changes functions depending on which Algorithm you are using. It adjusts a specific parameter in each algorithm. In Digital, Analog, and Slap modes, the knob will control modulation depth. In Lo-fi mode, the knob will control Filter width. – More info here.