Question about length of SPDIF COAX interface cable length

grm,  I assumed typical 25ns transition time, but perhaps it is different.  Maybe your source is swinging faster, especially at smaller capacitive load (shorter cable).   Whatever it is - ears are the best judge.  Same goes for different 1.5m cables - cheap cable can have by chance better impedance matching with your gear, than very expensive one, resulting in better sound.  

Hi kijanki, thanks for your kind assistance. In another room of my home where my Home Theater setup is located, I have connected the output from my DAC in my main system to the HT system DAC input. Using some return air ducts I got lucky and was able to install a 9M COAX cable I made myself. The cable is a 75ohm quad-shield terminated with 75ohm RCA style connectors. Anyway, my point is that this connection sounds great even at 9M (I think the max you can go is 10M) so something is going on with cable length with relationship to sound quality. Question is, is there a formula you can go by to determine optimal length of COAX cable? For example eliminating a transmission line by keeping a cable under 12", or in multiples of 1.5M which is my case here? I do believe not having a transmission line to be the best place to start but it did not work out for me with respect to SQ.

grm,  I'm not sure if there is any formula, and case of your 8" cable shows it.  What counts into calculation of transmission line effects is transition time that can vary between 5ns for dedicated transports to over 50ns for average gear.  In addition the highest slew rate is usually at the beginning of transition (first knee).  When calculating trace length on PCB it is a little easier, because logic family is known, PCB material is known, etc.
At the end, result is what counts and your ears are the best judge.

So I have a different type of question on cable length.
What do you recommend to do with all the excess cable?
For instance if 8” fits then 1.5m leaves a lot of spare cable to “hide”
Do you just hide it round the back as best as possible or do you coil it up in loops so as to look neat and tidy?
As I was taught that is potentially problematic as you could be fashioning an antenna of sorts.
Very curious!

Uberwaltz, In case of regular cable coiling doesn't matter, since cable is not a wire but the pair of them with opposite direction currents, that produce opposing magnetic flux.  That would be a common mode choke, and that's how non inductive bifilar resistors are wound.  I think, that magnetic field on the outside of the coax cable is zero, otherwise coiling coax would change its inductance, resulting in change of characteristic impedance (being roughly SQRT(L/C) ) and we would have huge, noticeable problem.
You can find similar conclusion here:

https://pages.uncc.edu/phys2102/online-lectures/chapter-7-magnetism/7-3-amperes-law/example-magnetic...

I don't think that coiling coax creates any antenna.  The fact the people usually coil coax to >12"dia. is likely only to avoid mechanical damage (often multiple layers of shield).