In what laboratory or think-tank...

...was the 1.5 meter length of digital cable established as the optimum length to use between Transport and DAC?

Is this a proven fact or just more cable BS?
It is actually been proven in double blind testing that this length of digital cable is lab tested and proven BS!
Somewhere in the Agon archives Audioengr provided the math to show why this length "could" be superior to shorter lengths. It primarily has to do with vswr / signal reflections / jitter. Sean
Sean...And its a proven fact that bumblebees can't fly.
I was browsing over at the Empirical Audio web site and, unfortunately, could not find the tech paper about this issue. I did notice that they sell 3 different lengths of digital cables (.5, 1.0 and 2.0 meter).

Respected cable manufacturer, Chris VenHaus, quotes the following on his website: "Optimum length for digital (SPDIF) Interconnects is >1.5 meter!"

Is this length issue only in regards to RCA and not to AES/EBU termination of the coax?

Thanks all for your imput on this topic. I am debating changing from the 1.0 length I have now but, as usual, am a bit skeptic. I use the AZ Mc2 AES/EBU model at present.
It was found that the probability wave collapsed for propogation of electrons in lengths longer or shorter than 1.500000000000000000000000000000000000 meters.
It was not that 1.5 meters in itself was a magic length, but a cable that was longer in length reduced the influence that signal reflections played on the loading characteristics of the source. Sean
Post removed 
If the transport has a true 75 ohm output impedance, the cable is 75 ohms and the input on the DAC is 75 ohms, you won't have any reflections. If you alter one of the three variables there, you will have reflections. As such, this can be system dependent.

Having said that, you can enter any impedance that you want into the above equation. That is, rather than 75 ohms, you can use 50 ohms or 110 ohms. So long as the impedance remains relatively constant along the length of the signal path, you won't run into major problems.

As a side note, there is a LOT to be said for impedance matching in an audio system, not just digital gear. Some manufacturers are smart enough to realize this and have developed systems from stem to stern that take this into account. Not only are the variables of interconnects drastically reduced, transient response and linearity are improved. Most of these designs are tall cash though as these folks are charging for their knowledge and skill. Then again, what requires "brain surgery" on the part of most audio engineers is something that folks working in RF knew long ago i.e. impedance mismatches are not beneficial to system performance. Sean
Chris Somivago, who created the groundbreaking Illuminati digital cables, might be a good person to ask. He is now with his new cable company called Stereovox.

Back in the day I think that the originator of the 1.5m magic number (or something like that) was originated with Marigo's digital cable. Maybe someone should email Ron hedrich and find out what his thoughts were on the subject...
Never heard a bit of difference with 75 Ohm cables from .5m - 3m lengths.
It was Jimmy the street dealer who unloaded a stash of wire long ago in the streets of Brooklyn. Jimmy never went to school but his Papa was from the Old Country, so Jimmy knew meters...and figured 1.5 was the shortest length he could move while still raking a sweet profit from that roll of Con-Ed cable.

You dig?