Actually, Jocko hasn't contradicted anything I've posted, or vice versa.
Well I started this thread with the statement "I have seen here that longer SPDIF cable length (2 meters or longer) is recommended (on properly terminated 75ohm cable) in order to minimise signal reflection problems. I believe the reason was along the lines of avoiding reflected signal showing up during the logic transistion of the SPDIF waveform.". This statement wasn't denied or clarified, just Lavry's statement was attacked. Your posting was the only one that gave any clarity! Notice there is no mention of return loss, it is assumed to be the same in each scenario! This whole return loss issue was raised by Jocko & then banged on about ad nauseum (a bit like this thread) by him with accusations that I don't understand it. When in fact it was assumed by the question that all variables were fixed except the length of the cable!
You still don't 100% understand what we're saying yet you're passing judgement. It's not as simple as "short cable, long cable". Given a CONSTANT RETURN LOSS, here's the cable length rules:
- Really short cable = ringing = signal integrity problems, creating jitter / bit errors / ... = bad.
Ok, this is new - Bogatin directly contradicts this "For example, if the rise time is 1 nsec, by keeping transmission lines shorter than 1 inch we will minimize reflections"
Altera directly contradict this too giving: "TDstub < (T10% to 90%)/3 Where TDstub = Electrical delay of the stub & T10% to 90% = Rise or fall time of signal edge
For a 1-ns rise-time edge, the stub length should be less than 0.5 inches."
- Cable with the length set so that reflections land on top of future transitions = SI + jitter, and it's data dependent jitter to boot. Bad.
- Cable sized to minimize both of these problems = Not as bad.
I know this so I'm not sure why you're saying I don't understand?
Now, if you improve the return loss but keep the cable the same, in each scenario:
the original question had nothing to do with return loss as I said already.
- Short cable rings less, less SI problems
- bad-length cable reflections are smaller, less reflection-induced SI/jitter.
Part of me would love to draw up a little Matlab/Octave program that given cable characteristics/length, driver slew rate and return loss at each end, would draw a simulated eye pattern showing how reflections mess it up. But I don't want to waste an unreasonable amount of time trying to explain something that might be hard to comprehend. I think I've already posted enough in this thread for people to figure that stuff out, and Jocko's got some good points also.
I thank you for the clarity of all your answers - they genuinely add something relevant but show me one point that Jocko has made which adds anything to the "original question" about cable length?
So yeah... read, comprehend, understand. Might be tough but there's enlightenment in there somewhere.
Please don't patronise me, I do read - on page 2 I quoted Bogatin's paper http://www.thefreelibrary.com/Riddle+me ... 0123202835
which pretty much summarises all that has been said - did anybody read it
I referenced an Altera paper http://www.altera.com/literature/an/an2 ... .oss=an224
jkeny, you are discussing with someone who makes a living out of this for many years, has the equipment to measure - and uses it - and knows this stuff for decades.
I'm sorry if I am asking questions, it's how I learn. To be told just to accept what Jocko says because he has worked in the field for many years is a bit like the fanclub syndrome over on DIYA i.e don't question anything & like a good Borg you will be assimilated
If he has all this knowledge why not just give a direct, informative answer - what's with all the semantic games? Look at Gmarsh's posts for example? It would save all this 4 pages of bullshit!
Don't make me say that you don't deserve to be clarified and helped - well, I just did.
Probably the answers you've got here were not what you were expecting, seems like you have your mind made up on this.
Why not go to some other place where people tell you what you want to hear?
No I don't have my mind made up on this - this is a your & Jocko's mistake here - I came to ask a question about long Vs short SPDIF cable - I got some answers - I came back with some questions about those answers & you accuse me of making up my mind?
If you read my posts I know that, like all things. it's not a simple black & white answer, neither long or short is correct & the details of why are given in this thread & I've summarised it already.