Your coax cable is a transmission line at digital-signal frequencies. In this circumstance there are reflections generated within the cable if the impedances at either end are not exactly 75 ohms. That is the case with all RCA-terminated cables and also with some that are not.
You want to avoid having the reflected wavefront arrive close enough to the incident wave that it registers on the DAC's clock as a (spurious and jitter-inducing) timing event. The 1.5m length places the reflection out of the danger range but a longer length might bring it back where you don't want it.
I would suggest asking Steve Nugent of Empirical Audio about your 5-meter coax S/PDIF cable. He posts here and at AA as "audioengr"
You want to avoid having the reflected wavefront arrive close enough to the incident wave that it registers on the DAC's clock as a (spurious and jitter-inducing) timing event. The 1.5m length places the reflection out of the danger range but a longer length might bring it back where you don't want it.
I would suggest asking Steve Nugent of Empirical Audio about your 5-meter coax S/PDIF cable. He posts here and at AA as "audioengr"