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SPDIF cable

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Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 2:07 pm
by Martin Walker
blinddrew wrote:Right.
Maybe. ;)
I shall read that again a couple of times. :)

Just make sure that you use 75 ohm 'digital' cables for S/PDIF, rather than an audio phono to phono cable, to avoid any dropouts/glitches/twilight zone weirdness ;)


Martin

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 2:28 pm
by Hugh Robjohns
blinddrew wrote:Right.
Maybe. ;)
I shall read that again a couple of times. :)

:D Try reading this explanation.

https://www.testandmeasurementtips.com/ ... impedance/

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 2:47 pm
by ManFromGlass
"calculated vectorially"
Gaahk - math alert! :shocked:
Although there were some bits I could understand and it is interesting stuff.
Do they actually do measurements when cable temperature is reduced to absolute zero for a baseline, or is that a modeled scenario?

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 4:10 pm
by Hugh Robjohns
ManFromGlass wrote:Do they actually do measurements when cable temperature is reduced to absolute zero for a baseline, or is that a modeled scenario?

No, it was just a thought experiment in order conveniently to remove the cable's inherent resistance so as to simplify the rest of the explanation about the charging of the cable's capacitance and energising the inductance...

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 4:14 pm
by MOF
Do they actually do measurements when cable temperature is reduced to absolute zero for a baseline, or is that a modeled scenario?
In theory there should be no resistance, not sure about impedance, at that temperature but apparently it’s thought that there would still be some movement of atoms, when in theory there shouldn’t be.

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 8:42 pm
by blinddrew
Somewhat amazingly I actually sort of understood that. Enough to answer what would have been my next question.
Thanks :)

Re: SPDIF cable

PostPosted: Sat Jan 18, 2020 9:44 pm
by Hugh Robjohns
:thumbup:

In practical terms, if the signal frequency is high enough (like digital audio, video, rf etc), or with a lower frequency signal if the cable is long enough (like a telephone cable between cities), the cable itself behaves as a 'transmission line', and we become concerned with the transfer of power from one end to the other.

In that situation, some strange things happen. Most notably for us, if the end of the cable isn't correctly terminated, it acts like a mirror and the signal is reflected back towards the source which can cause all manner of problems depending on the circumstances. So, it is vital that the line is terminated with a resistance which is equal to the cable's characteristic impedance, and the output impedance of the source must also match the characteristic impedance... And that's why we have the notion of 'matched impedance' interfaces for high-frequency systems today.

The old 600 Ohm thing you see with a lot of vintage gear (and modern emulations) stems from the way the old telephone systems employed a 600 Ohm matched impedance format, and early pro audio gear technology was developed from the world of telephony!

But for normal audio frequencies, the cables we use -- even really long studio cables -- are far too short to behave as transmission lines, and it just makes no sense to maintain matched impedance interfaces. Hence the adoption of voltage-matched interfaces in the 70s, where the source (output) impedance is kept as low as possible, and the input impedance is kept as high as practicable -- usually with a ratio between them of 1:5 or 1:10, or something of that sort of order.

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 10:44 am
by ConcertinaChap
Hugh Robjohns wrote:So for standard video coax cable that's around 75 Ohms, RF coax will be 50 or 75 Ohms. Mic cable, AES3, and cat5/6 cables are around 100-110 Ohms, and overhead telephone cables are around 600 Ohms.

I'll add one other to that list because I had lot of the stuff kicking around at one time. Ethernet cable (the old coax stuff that predates twisted pair) is also 50 Ohms. The connectors and T joints are identical to Wordclock ones but the terminators are also 50 Ohms rather than 75 Ohms as required by Wordclock.

CC

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 3:23 pm
by James Perrett
ConcertinaChap wrote:Ethernet cable (the old coax stuff that predates twisted pair) is also 50 Ohms. The connectors and T joints are identical to Wordclock ones but the terminators are also 50 Ohms rather than 75 Ohms as required by Wordclock.

While the connectors may mate together, they are different and it isn't advisable to mix them.

Image

(Picture borrowed from Wikipedia)

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 3:25 pm
by Sam Spoons
So how much would 50Ω coax degrade the transmission of word clock or SP/DIF data compared to the correct 75Ω Would 50Ω be better than audio coax?

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 3:59 pm
by Hugh Robjohns
As most cable lengths in typical studio applications are relatively short, it tends to be incorrect terminations that cause most problems through reflected signals, rather than losses in the cable itself. And, being digital, signal distortion due to incorrect cable impedance is not as great a problem as it would be for an analogue signal...

But since 75 Ohm cable is readily available and cheap, why would you not use the right stuff?

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 4:16 pm
by Sam Spoons
Only because I have a load of 'thin ethernet' cable, connectors and terminators knocking about and it seems a waste to just bin it.

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 5:26 pm
by wireman
Sam Spoons wrote:Only because I have a load of 'thin ethernet' cable, connectors and terminators knocking about and it seems a waste to just bin it.

I have a computer with a thin transceiver and I need a TX one if I want to try to get it working again. Asked a few people but even those are so old people have thrown them all out. Don't really want to buy one.

As for not correctly terminating, I once had systems just not work at particular locations on a thin ethernet network. Eventually someone turned up with a TDR to find extra termination that was causing reflections.

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 5:45 pm
by Sam Spoons
Not sure I understand #wireman, what do you mean by TX and transceiver in this context?

Re: SPDIF cable

PostPosted: Sun Jan 19, 2020 6:14 pm
by wireman
Sam Spoons wrote:Not sure I understand #wireman, what do you mean by TX and transceiver in this context?

I meant 10-baseT, AUI Ethernet transceiver. Been a while since I used those terms...