Steve, what is the length at which one begins to get reflections?
You will always get reflections. The question is whether the first reflection, which is the highest amplitude, will reach the destination at the critical time when the edge is sampled by the receiver.
It depends on the edge-rate. Faster edge-rates will cause the receiver to detect the edge earlier, allowing the use of a shorter cable. The first reflection can arrive earlier without affecting the detection of the edge.
1.5m is just a good rule-of thumb because it will cover edge-rates from about 20nsec to 1nsec. The round-trip flight-time of a 1.5m cable is about 13-14nsec. If the edge is past the half-way mark after this time is elapsed, then it will not be affected by the first reflection.
My S/PDIF output, when driving a low-loss cable achieves about 4nsec risetime, so there is plenty of time for the receiver to detect the edge before the first reflection. If there are other impedance discontinuities within the receiving DAC, then there may be other reflections that return earlier, such as those from circuit board traces or wiring that is not 75 ohms. In this case, the faster the risetime is, the better.
Steve N.
