DIYHiFi.org

How - the cable loss is insignificant!
we have a reflection caused by any impedance mis-match encountered by the signal along the cable - normally these mismatches will be found at both ends of the cable. The mismatch will determine the amount of the signal reflected. Each trip to the end of the cable will reflect only the amount of the signal determined by this reflection coefficient. So we know the reflection is reduced as it bounces from end to end. So what! How has this got anything to do with cable length?

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There is a crack in everything, that's how the light gets in!

Gmarsh is still the only one who has posted anything sensible here & his posts are being ignored - does anybody read? Even Mr Motivator has ignored them preferring semantic evasions instead.

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There is a crack in everything, that's how the light gets in!

Exactly. A 0.3 wave cable is better than a 0.25 wave cable. But I didn't say that, I hoped people would take my math/calculation method/whatever, think about what goes on in the time domain and figure that out for themselves.

As I said, relatively prime is good.

Also, apologies for confusing people with the 0.9 VF. A recent project where an old power line was converted to a radio transmission line has warped my brain a little.

And all I said was that neither longer or shorter cables are better - it is mis-information to make either statement but Mr Motivator decided that I am saying shorter is better & Lavry is correct! Reading the posts is usually a pre-requisite to responding!

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There is a crack in everything, that's how the light gets in!

PRECISELY!!!!!!!!!!!!! That is what I am trying to get the "Design it for me" crowd to understand.

Instead, I am accused of not reading what you wrote.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

No, because you have not taken the time to determine by how much it has been reduced.

If you can't reduce it, you move it!

Wise up, bub...............how friggin't hard is this?

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

I said this on page 2 & I'll repeat it again So what are you saying or do you still just want to play semantic games?

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There is a crack in everything, that's how the light gets in!

I have better things to do than argue with someone is not willing to listen. Go ask Lavry to 'splain it to you. He is obviously the final authority on everything. My 40 years of being a real engineer is totally bogus to him, so let him brainwash you some more. I am obviously an idiot, and so is anyone who agrees with me. Make sure that you add GM, CG, CH, and a bunch of other folks who don't want their name dragged into this to that list.

I am done. If you can't find the answer, then maybe someone can point it out for you. It is there.

If anyone else has serious questions, please feel free to ask them.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

I believe this is called projection!
There you go again - haven't you read anything I wrote or are you just playing games?

I already found the answer back on page 2 - Gmarsh post - it contradicts both you & Lavry!

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There is a crack in everything, that's how the light gets in!

Actually, Jocko hasn't contradicted anything I've posted, or vice versa.

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.
- 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.

Now, if you improve the return loss but keep the cable the same, in each scenario:

- 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.

So yeah... read, comprehend, understand. Might be tough but there's enlightenment in there somewhere.

In that case how do you decide whether or not to terminate ? Presumably, somewhere there lurks a rule of thumb.

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.
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?

PS: please don't insist because I think you won't get more answers from Jocko - he has helped you a lot already.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein

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!

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."
I know this so I'm not sure why you're saying I don't understand?

the original question had nothing to do with return loss as I said already.

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?

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
and others
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!

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.

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There is a crack in everything, that's how the light gets in!

Short cable bad - edge gets clobbered - Bang Bang Bang Bang Bang Bang Bang Bang
Long cable good - edge gets clobbered - Bang..............................................Bang

Wow! Is that all you have gleaned from this thread ? Now there's someone who has made their mind up already & has been impervious to the contents of this thread. Don't put me in that same mould!

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There is a crack in everything, that's how the light gets in!