Does the 1.5m 75ohm rule apply to 110ohm?

sugabooger

Reflection starts from the beginning of transition (first knee).  Assuming 5ns/m it will take about 7.5ns to travel to the end of the 1.5m cable, reflect and another 7.5ns to come back - total of 15ns.  It will miss 12.5ns midpoint of 25ns transition and that is the whole point.  Reflections  often create staircases, that in presence of noise induce jitter.  There can be many more transitions in the cable going back and forth creating multiple step staircase patterns (Bergeron diagrams), but the first reflection in this case is the worst one.  Following reflections will affect next edge only and will be greatly reduced since next edge comes in more than 1us.                                               

As for speeding the edges - it should reduce noise induced jitter since noise with vertical crossing of the threshold point will affect  exact moment of level recognition less.  Slow transitions are the weakness of Toslink, making it susceptible to system noise, in spite of being immune to ambient electrical noise.  Some transports might have transitions as fast as 5ns, but you will really have to match characteristic impedance well. 

Except the 25nsec is not a set number, and neither is 5nsec/ft, both can be smaller or larger, not to mention there is hysteresis and settling time that pushes that transition values above/below 0V, which means that theoretical 12 and 15nsec could end up sitting on top of each other, which could happen anywhere from 1-2 meters system dependent. 1.5 meters may work, or it may be a worse case. There is simply not enough tolerance at 1.5m.


w.r.t fast edges, if the termination mismatch is not large, then fast edges will always be better. If the termination mismatch is large, you have the potential for again a worse case situation. It would be rarer, but still possible. The slow speed of TOSLINK couples with noise optical detectors and high gain. It is the worst of call conditions for jitter.


Of course, fortunately most system today are fairly jitter immune. Well $50 consumer DACs are relatively jitter immune. $5,000 audiophile DACs you are taking your chances.

So the answer is that 1.5 M is the best choice for AES digital cables if one doesn't know the input impedance of the receiving component?

@mesch   Input impedance of receiving component has nothing to do with it.  We assume 1.5m minimum for average system.  In some systems 0.5m might be enough, while others will require 2m.  The whole point was that longer cable might be better (counterintuitive).