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spdif rca extension cable

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spdif rca extension cable

Postby jodaki » Tue Sep 17, 2019 12:48 pm

I have a proprietary focusrite spdif cable with a 9-pin on one end and a single rca on the other. It is too short and I need a longer one. 2m ideally.

I cant find such a cable on any of the sites I normally use - not even focusrite have them. So I thought Id try to find a female to male rca extension that will work with spdif.

Perhaps my search terms are wrong but this seems equally hard to find.

I tried a normal phono cable (f->m) but I get pops and clicks and it Im guessing appearances aside they are different beasts (rca phono and rca spdif).

Im wondering if perhaps an rca spdif cannot be extended past 1m.

Im out of ideas but if theres a way I know someone here will know it....
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Re: spdif rca extension cable

Postby Tim Gillett » Tue Sep 17, 2019 1:29 pm

It might be worth trying a few different RCA f to m cables. I think they varied in quality and the cheaper ones could go faulty. You could also try the video cable (yellow plugs) used on composite video and audio leads for connecting VCR's to TV's. It might have less signal losses per metre.
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Re: spdif rca extension cable

Postby Hugh Robjohns » Tue Sep 17, 2019 1:31 pm

jodaki wrote:I cant find such a cable on any of the sites I normally use - not even focusrite have them.

I'm not surprised. You'll either need to make it yourself, or have it made for you by a custom cable company.

So I thought Id try to find a female to male rca extension that will work with spdif.

A video cable extension with RCA-phono plugs could work, but it's not ideal and still might not be reliable.

I tried a normal phono cable (f->m) but I get pops and clicks and it Im guessing appearances aside they are different beasts (rca phono and rca spdif).

The cable requirements are different. SPDIF is much more like video than audio in terms of the signal frequencies and the cable's characteristic impedance. Standard audio phono cables will probably have far too high capacitance which will wreck the digital signal's eye height and introduce massive amounts of jitter. Both of those will cause the receiver to struggle to recover the signal -- hence the clicks and pops.

Im wondering if perhaps an rca spdif cannot be extended past 1m.

The original S/PDIF specification did, indeed, restrict it to 1m... but with decent cable it should manage two metres without any real trouble. I've certainly run S/PDIF over 3 or 4 metres without problems in the past... but it is dependent on good cable and good send/receive equipment.

If I were you, I'd use this as an opportunity to learn how to solder your own cables. But otherwise you'll need to have it specially made.
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Re: spdif rca extension cable

Postby James Perrett » Tue Sep 17, 2019 1:45 pm

Hugh Robjohns wrote:I've certainly run S/PDIF over 3 or 4 metres without problems in the past... but it is dependent on good cable and good send/receive equipment.

My computer with S/PDIF connections is in another room so I've run signals even further than that and also with a BNC patchbay in line as well. However I only use video cable (RG59) for my connections.
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Re: spdif rca extension cable

Postby ef37a » Tue Sep 17, 2019 4:01 pm

I am with Hugh, roll your own,
https://www.amazon.co.uk/Phono-Plugs-Au ... B071HPJFSF

They should accept UHF low loss TV coax. Yes, has a solid core but you are not going to swing the F'rite round your head?

Amazon also sell a gold plated RCA phono coupler.

This should work, I did it some years ago and found I could send S/PDIF reliably from a MD recorder to an M-A 2496 sound card over 10mtrs of low loss (the MD fed a cheapo optical to coax converter switch)

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Re: spdif rca extension cable

Postby jodaki » Tue Sep 17, 2019 7:27 pm

Great thanks, Ill try a video cable with f/m adapter first - I didnt think of that..

I make a lot of my own jack and xlr cables so Im happy to have a go at a 9-pin connector. The current focusrite plug is moulded so I cant unscrew it to see how the connections work. Ive asked FR for the wiring scheme for the ISA 430 plug.
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Re: spdif rca extension cable

Postby Hugh Robjohns » Tue Sep 17, 2019 8:34 pm

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Re: spdif rca extension cable

Postby jodaki » Wed Sep 18, 2019 3:33 pm

Haha Im still waiting for focusrite to tell me where to find that document :) I dont know how you do it Hugh - thanks.

I bought a 3m 75ohm spdif cable and will cut the end off and wire the dsub on. Hopefully plain sailing.
Thanks again all.
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Re: spdif rca extension cable

Postby ef37a » Wed Sep 18, 2019 4:25 pm

jodaki wrote:Haha Im still waiting for focusrite to tell me where to find that document :) I dont know how you do it Hugh - thanks.

I bought a 3m 75ohm spdif cable and will cut the end off and wire the dsub on. Hopefully plain sailing.
Thanks again all.

Oh! It downloaded fine for me and I often have trouble with such things. I sometimes have to leave F****Firefox and fire up Good old IE11 so I can use "Save Target As". Why can't all browsers do that?

Anyway, why are you not just using the quality video cable and an RCA phono with coupler?

Just remembered, a few years ago I emailed Neutrik and asked them if they had an RCA plug that was of 75 Ohm impedance (like a Belling TV plug or 75R BNC) "No such thing" they said but in practice standard plugs are close enough.

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Re: spdif rca extension cable

Postby Hugh Robjohns » Wed Sep 18, 2019 4:30 pm

The 9-pin break out cable is really intended for the four-channel ISA preamps, so onthe 430 the outputs are duplicated, and the Mark2 version manual explains how that works for the single and double wire formats on page 14:

https://customer.focusrite.com/sites/customer/files/downloads/isa-430mkii-user-manual-english.pdf

Basically, in single wire more -- which is all you'll ever need with modern equipment -- all four outputs of the standard breakout cable carry the same thing : channels 1&2. In two-wire mode the first and third (black and yellow) carry channel 1, and the second and fourth carry channel 2 (red and blue).

So in your case, you'll just need to wire your S/PDIF cable into output 1, which is pin 9 (signal) and 5 (screen) in the D-sub.

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Re: spdif rca extension cable

Postby jodaki » Wed Sep 18, 2019 5:27 pm

Anyway, why are you not just using the quality video cable and an RCA phono with coupler?
I thought Id treat this as a learning opportunity :geek: but if I mess it up I will quietly follow your advice ;)
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