 Having access to ARP's schematics, Oberheim noticed that the ARP 2500 had a feature that allowed two notes to be played simultaneously (a capability of neither the 2600 nor other commercially available synthesizers at the time), and Oberheim designed a modification to the ARP 2600 that enabled it to do the same thing.Using two of these modified ARP 2600s, Oberheim and Grayson performed concerts together.Oberheim further expanded on the performance capabilities of 2-note polyphony in 1973, using his computer engineering experience to design the DS-2, one of the first digital-electronics-based music sequencers.