Dave Smith Instruments PROPHET 12 KEYBOARD - Manual (Page 37)

27 Prophet 12 Operation Manual Delay
For chorus, start with a value around 53 (17.164 milliseconds).
For ping-pong effects, use a square LFO routed to Delay Pan. Some
amount of DC routed to Delay Pan (in the modulation matrix) will place
the ping-pong in the desired position in the stereo field. Both the LFO
and the Delay must have Sync on, and the LFO should also have Wave
Reset on, to keep everything in sync.