75R Digital Cable

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little eddy
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75R Digital Cable

Post by little eddy » Thu Apr 18, 2019 7:17 pm

After reading somewhere else, I am thinking of making up my own digital cable using a spare couple of meters of 4n silver run in PTFE sheathing that I have, a length of screen braid and using 'standard' RCAs.

One design I saw used an L-pad pair of resistors supposedly to match the cable impedance to the source and load 75R.

But isn't cable Zo=Sqrt (L/C), so is it folly thinking the performance can be optimised using a pair of resistors? But if I'm wrong, how do I determine the optimum values of the two resistors? I'm guessing I can get in the ballpark if I know or can measure the parameters of the cable I construct. And perhaps can be optimised with signal generator and scope.

Is the impact of a cable with different Zo going to noticeably affect performance of a lead from Squeezebox to Dac of less than 1m and guess max frequency of 96kHz?

Any advice would be appreciated.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: 75R Digital Cable

Post by Nick » Thu Apr 18, 2019 8:02 pm

96k Sample rate 24 bits stereo. 64 bits per frame = 6.144MHz bit rate, its bi-phase encoded so 12.288MHz signal rate.

https://en.wikipedia.org/wiki/AES3
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little eddy
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Re: 75R Digital Cable

Post by little eddy » Fri Apr 19, 2019 8:58 am

Nick wrote:
Thu Apr 18, 2019 8:02 pm
... so 12.288MHz signal rate.
Point taken. So at this frequency, if my 1m lead did not have a Zo of 75R, what is the risk of this introducing errors into the system?

Suppose one way to find out - not scientific and subjective but I have a pair of similar silver 4-conductor analogue interconnects so will give one of these a try, (once I get rid of this cold and my ears 'pop').
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: 75R Digital Cable

Post by Nick » Fri Apr 19, 2019 10:30 am

little eddy wrote:
Fri Apr 19, 2019 8:58 am
Nick wrote:
Thu Apr 18, 2019 8:02 pm
... so 12.288MHz signal rate.
Point taken. So at this frequency, if my 1m lead did not have a Zo of 75R, what is the risk of this introducing errors into the system?
No idea, in my limited experience the receivers are very good at extracting a signal, but there may be timing issues involved if you are using the extracted clock as the local reference. The main issue without having matched connections are reflections. The idea of a characteristic impedance is that the signal regards the connector as infinite in length tso there is no far end to reflect from. Of course this is all a bit irrelevant unless you use 75 ohm connectors (BNC) at both ends, and the transmitter and receiver PCB and terminations maintain the impedance. If you look at the lengths the RF boys go to you can see this is not trivial.

Having said that 12MHz is not a big deal, but thats just the data rate, trying to maintain a pulse rate at that frequency means that there will much higher frequency harmonics you need to keep in hand both WRT phase and amplitude.
Resistance isn't futile it's V / I.

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