[sdiy] S/PDIF cable.

[John L Marshall]

Gosh! You are a man of many word. I would hate to be in a cubicle next to
you at work. I hate cubicles anyway.

The point of the 75 ohms coax is to match the impedance. Matching impedance
eliminates reflected signal.

If the outputs are transformer coupled then the signal is isolated. It
probably doesn't matter if the transmission line is balanced or unbalanced.
But you would still have an impedance mismatch. Since AES/EBU impedance is
110 ohms, I would use RG-62 as the transmission line. RG-62 has an impedance
of 93 or 95 ohms depending flavor. This cable is a closer match to the 110
ohms.

RG-62 is plentiful in surplus stores. This stuff was used in older IBM
networks. It should be really cheap.

List of most popular coaxial cables:
50 ohms approximately:
RG-8 ~ 0.40 inch a (variant is used for thickwire ethernet)
RG-8x ~ 0.24 inch
RG-17 ~ 0.87 inch
RG-58 ~ 0.20 inch (thinwire ethernet)
RG-174 ~ 0.1 inch (popular when you want to put signal in one end and get
not much out the other end)

75 ohms approximately:
RG-6 ~ 0.27 inch (TV)
RG-11 ~ 0.40 inch
RG-59 ~ 0.24 inch

95 ohms approximately
RG-62 ~ 0.24 inch (IBM networks)

There is less signal attenuation in thicker cables. Use the thick cable then
you can tell the AH guys that you use fat cable for that PHAT sound.

Take care,
John


----- Original Message -----
From: Batz Goodfortune <batzman at all-electric.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, April 27, 2002 10:36 AM
Subject: Re: [sdiy] S/PDIF cable.


> Y-ellow All.
>          And thanks to everyone for that. Yeah. Sounds like 75ohm coax is
> the go here. I was kinda hoping the 50 would have been suitable simply
> because it's a narrower diameter but it really isn't overly traumatic. I'm
> not exactly spoilt for choice at the moment.
>
> At 09:01 AM 4/27/02 -0700, John L Marshall wrote:
> >Use the 75 ohm coaxial. Do you have RG-59 foam? That would be my first
> >choice.
>
> I always thought RG59 was thinner than this stuff but I don't have much
> choice. The sample I have here doesn't have anything printed on it so it's
> unclear what it is. But it IS 75ohm. It's actually designed as aerial
cable
> but it works OK for video. I have surveillance cameras running on 20
metres
> of the stuff with out any problems.
>
> >Transformers exist to convert from AES/EBU 110 ohm balanced to 75 ohm
> >unbalanced. The unbalanced lines can run long distances.
>
> The problem at that end is that the only option on a Yamaha O1V is a
> 4in/4out AES/EBU podule. I'm not even 100% sure it won't spit the dummy
but
> since S/PDIF is a superset of AES/EBU (logically) and none of the
> extraneous bits will be sent anyway, it should work. And Biphase mark
> doesn't require polarity concerns unless of course there's some
> non-isolated electrical imperative. Which of course, the transformers take
> care of.
>
> The podule has 8 tiny transformers on it, to, as you say, convert balanced
> to unbalanced. But these seem to be entirely isolated at the back end from
> everything so I should be able to connect the coax across the trannies
> ignoring the centre tap and that should work. I hope! The centre taps seem
> to be common grounded so it would be a good idea to leave those alone.
>
> Some of the sources have transformers but the RCA sockets are still
> grounded. Makes no sense to me but there you go. But one in particular has
> no isolation at all so I'll be relying entirely on the transformer in the
> back of the podule for that. The podule apparently takes care of
> synchronization so I don't have to worry about a central word clock
> generator. I have assurances from the supplier but Yamaha's documentation
> is less than clear. They'll hear me shouting abuse at them all the way
over
> in Japan if this is not the case.
>
> Believe it or not it's taken me 2 years to get to this stage. I remember
> discussing how to make a suitable transformer should I need to with Peter
> Ullrich just prior to having to leave the list. Hello Peter if you're out
> there. But it actually took them this long to get back to me with
assurance
> that it will handle asynchronous inputs.
>
> The only other problem I've just realized. The I/O on the back of this
> podule is actually a DB25. That's going to be a fiddley cow of a thing to
> hook coax to. Still, we have to do what we have to do. And I've had to do
> worse. And if it really is unworkable, I guess it wouldn't hurt to run a
> short length of some liter shielded cable out to an RCA socket. Or
> something along those lines.
>
> Anyway thanks all for that. Sounds like a plan.
>
> Be absolutely Icebox.
>
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