Question about length of SPDIF COAX interface cable length


Hi, maybe someone can assist me here.
I intend to connect my CD Transport to my DAC/Streamer by way of 75 ohm COAX cable. I have two cables to choose from, both the exact same make and model of cable with the exception of the length. One cable is exactly 1.5 meters, the other cable is approximately 8 inches tip to tip, both cables will fit between Transport and DAC, my question is which one to use? The cables are made by HAVE/Canare.

Thanks so much for you kind assistance!
grm
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Doesn't matter toss a coin.
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I would choose 8" for sure.  Cable becomes "transmission line" when propagation time (one way) is more than 1/8 of transition time (rule of thumb).  Transitions for typical transport are about 25ns.  Allowed travel time is about 25/8=3.125ns, equivalent to about 0.6m (assuming 5ns/m speed of electricity in the cable).  In reality it should be much less, because it includes length of connections inside of gear (including PCB traces) on both ends (source and DAC).  I would assume 12" to be maksimum, so 8" should be safe.  Designers use the same method to decide if traces on PCB need termination (impedance matching).  

When you need/have longer cable reflections inside (on characteristic impedance boundaries) can modify (disfigure) transition edge affecting timing (threshold point), causing timing jitter.  Threshold is roughly in the middle of transition at 12.5ns from the beginning of transition (knee).  It is equivalent to 2.5m travel both ways (to the end of the cable and back) equal to 1.25m cable.  1.5m cable should be long enough for reflections from the end of the cable to miss threshold point.   
I thought it was optical/toslink that needed to be a certain length to perform optimally.
i understand the ideal length of bnc or rca spdif electrical cabling is 1.25 to 1.5 m to avoid 'reflections' in key frequencies
kijanki,
Thank you for the detailed response. I was surprised that there did seem to be a perceptible difference between the two cables, I settled on the 8 inch cable. I too was under the impression that the 1.5 meter cable was the correct length to avoid internal reflections, now i'm not sure if this is the case.

Thanks to all who chimed in!
grm, I'm glad 8" works for you.  It is safer not to get reflections to start with then trying to manage them, IMHO.
Hi,

Not sure why this is the case, but I spent quite some time switching between my 8" cable and 1.5M cable. For some reason, I am finding myself preferring the sound of the 1.5M cable, where the 8" cable seems to sound a bit too bright.
Yes, the 1.5M particularly for SPDIF/COAX, DACs, etc. As already noted, and several references around about reduced reflections, and how reflections can sometimes be somewhat attenuated by the longer cable's added capacitance depending on the cable design used.

I tried it with .5M, 1.0M, 1.5M cables on my own DAC and prefer the 1.5M as well, fwiw. You already figured it out...   
That's surprising.  Brightness would suggest a presence of the jitter (that translates to added noise).  What about clarity/transparency and imaging?  It is remotely possible that some electrical noise is injected from the source, possibly thru the shield, while longer cable, having more shield inductance, impedes electrical noise injection more, but I'm grasping at straws.  Did you keep both gear pieces in the same physical location for both tests?  Did you notice it at the same time of the day?  Often people experiment after work and dinner around 6-7PM, when radio stations power is suddenly reduced by FCC regulations (better signal propagation at night).
I tried it with .5M, 1.0M, 1.5M cables on my own DAC and prefer the 1.5M as well..
I would also pick 1.5m from these choices.  8" (0.2m) shouldn't have reflections at all (not a transmission line).

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).
@kijanki

At the end, result is what counts and your ears are the best judge.

+1,000,000

Truly words to NEVER forget on this forum.
@jjss49   I tend to overanalyze things.   I'm sure nobody decided upon 1.5m based on electrical calculations, but rather on empirical experience and calculations came later to find explanation for that. 
I might be mistaken, memory not being what it used to be, but I seem to recall Steve Nugent at Empirical Audio who goes by Audiogon username:” Audioengineer”(?), might have done some measurements on this matter and suggested not using less than 1.5 meters.