It was originally Jaeger's suggestion that the FM synthesis concept be extended to allow four simultaneous channels or voices of synthesis to be triggered with one key depression to allow the final synthesized sound to have much more harmonic series activity. This change greatly improved the overall sound design of the system and was very noticeable. 16-bit user sampling (originally in mono only) was added as an option in 1982. This model was succeeded by the ABLE Model C computer based PSMT in 1984 and then the Mac-based 3200, 6400 and 9600 models, all of which used the VPK keyboard.