Topic: diy SPDIF cables

[Ed.]

+t

[Chuck]

Hmm... I thought it was good practice to connect the shield at one end...

[Ed.]

probably connect the shield to the (-) on one side, but if it isn't connected on the other side, i don't see where it would matter that much.

[sanaka]

Umm... for a coax cable, such as the ones mentioned in this thread, the braided shield is also the - return path. So you need to connect center conductor>pin, shield>ring. If the shield is unconnected anywhere, you'll get no signal.

Mic cable has two (or four in the case of starquad cable like Canare L-4E series) jacketed conductors inside a braided shield. These make nice unbalanced, analog RCA cables by connecting one conductor>pin and the other one>ring. The shield also connects to ring, either at one or both ends. The lifting one end ("single ended") idea endorsed by many audiophiles is that you can decide where any noise that the shield is picking up is directed. Usually ppl direct it the same direction as the signal, i.e. between preamp and MT for instance, the shield is connected only at the MT end. Some pro audio folks eschew this idea, and say that lifting one end of a shield is not better, but just a sometimes solution for ground loops. In a portable, non-mains connected setup, I don't think there's much argument for lifting either end. If it's left unconnected at both ends, it won't really shield anything. Rane is a great reference for analog hookup.

The physics for a binary stream such as SPDIF is totally different. Shielding is not the same issue. The thing I've gleaned as important for a SPDIF cable is its impedance. Somewhere in the ancient origins of the now ubiquitous RCA type connector scheme is a spec that the signal should see an impedance of 75 ohms along its path through the connectors and cables. It turns out this isn't that important for audio, so most of your RCA type cables and connectors are not very close to 75 ohm. But it is more important for the MHz frequencies of video and digital. So, for a SPDIF cable, this is what I'd look for. Any pro-grade video cable should work for SPDIF. Canare coax terminated with Canare connectors will give you the true 75 ohm goods.   Which, again, is one point I'm cheezed w/ MAudio about the MT - I have Canare tooling, cabling and connectors, and they won't fit 

If you want to go truly nuts about this, check out the Cable Asylum.

Peace,
Sanaka

[Ed.]

ok, sorry to keep asking stupid questions, i just want to get this right.

so then I'll have my cable, stripped and ready to go.  When I'm looking at the ends, should I see three inner cables (neg, pos, shield) or should I see two inner cables (center conducter, shield) - or is the center conducter the neg and pos inner cables?  (stupid questions, but i don't have the cable in my hand yet, and i've only looked at xlr cable in the past)

second question: so instead of doing...
Center pin - (+)
Outer ring - (-)
Shield - not used
...i'll be doing center conducter > center pin and shield > outer ring?  and should i connect the shield on both sides or only on the mt side?

again, sorry for all of the questions and thanks for the help thus far
-ed

[sanaka]

Depends on the cable. Signal needs two wire paths, + and -

So if your cable only has two wires, a center one and the shield, then center>RCA pin/+, and shield>RCA ring/- . This is called "coax" cable.

Standard microphone cable has two separate jacketed conductors and a shield. In this case, one conductor>RCA pin/+, the other conductor>RCA ring/-, and you can do pretty much what you want with the shield. The simplest and most accepted practice is just connect shield at both ends.

I've made analog RCA interconnects with mic cable that I Like. But again, I'm not sure that's the best way to go for SPDIF.

Learning is often harder than it seems like it should be, finding good info even harder. So don't feel sorry about your questions, they are fine!

Peace,
Sanaka

[Ed.]

thanks for the help.  If i had the cable in front of me, this would probably be a lot easier.  I'm pretty sure I have coax cable coming so it shouldn't be too difficult.  still, this is just for fun, so no big deal if i can't get it to work right.  but its fun to make your own cables so i thought i'd try.

hopefully I'll get the cable tomorrow, so by tomorrow night, all will be in working order.

thanks again.

[sanaka]

If i had the cable in front of me, this would probably be a lot easier.

It will be. Meantime, here's a stripped back picure of your typical coaxial constructed cable:



And the same for a typical mic cable:



Peace,
Sanaka

[Ed.]

hey, thanks for all of your help sanaka, i just made the cable and it seems to be working great.  i guess i'll find out for sure come saturday night.