What is the optimum rca digital cable length?????

Sean: One should not need a network analyzer to compare the results of various cables with a specific transport / dac combo. A decent quality scope with a high quality test disc containing multiple test tones / waveforms should allow one to visualize the various loading characteristics taking place.

The bad thing about this is that you might be able to find a cable that allows optimum power transfer with a high level of linearity that is not to your sonic preferences. Does one go with what they know is "technically correct" and hate their system or use what they like and regret not using what is "most accurate" ? I know which path i would take but i'd like to have the best of BOTH worlds : ) Sean
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Sean is absolutely correct. With perfect impedance matching shortest is best, as it usually is with all non-RF cables.
If the impedances are not perfect then I'm afraid you'll need about $50,000 of test equipment (an RF network analyzer) and the answer will be good only for your specific DAC and transport, and will vary if you change either. Not good, eh !?
It's probably easier to compare a few and pick one you like best.

I have not heard any degradation in using .5m and 1m lenghts for digital audio. However, per Ethernet specs 1.5m is correct but that is a local area network.

The 1.5 meter "magical length" was probably devised by the manufacturers so that they could charge more for a longer length cable. What is the truism about wire, it accounts for 20% of the sales in the industry and results in 80% of the profit.

This is a tough one that i'm currently working on trying to solve for my specific systems. I will only say that if the transport is exactly 75 ohms, the cable is 75 ohms and the DAC is 75 ohms, any length of cable is fine with the shortest length being the best. The same would be true if they were all 50 ohms, 110 ohms, etc... If any of the variables ( impedance of transport, cable, DAC ) are altered independently of each other, the line length will act as an impedance transformer and could either increase or reduce the amount of vswr / reflections that the transport sees depending on the length used.

Having said that, my thoughts are that anybody that can tell you that one length is better than another ( other than talking about line loss ) without knowing all of the factors that i previously mentioned is talking gibberish. I know that Kimber used to say that 1.5 meters was a "magic length", but i don't know how they arrived at that figure. Probably talked to the same guy that you did : ) Sean
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PS... most guys that work on RF can't answer this question without asking a bunch of complicated questions, let alone someone that works on audio gear being able to answer this question at all.