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In Reply to: RE: what make coaxial cable impedances of 50 ohms and 75 ohms ? posted by lovetube on January 16, 2017 at 18:51:44
It's the relationship of the center conductor with the braided shield, meaning the plastic dielectric that separates the two, and how well a perfectly round shape is kept along the entire length of the 75 ohm cable. It can be a bit confusing, since there does not seem to be a clearly identifiable relationship that meets the eye. There are 75 ohm mini-coax cables that look very compact vs. an RG-59 or RG-6 for that matter that can also seem to look a bit arbitrary in nature. However, if proper measurements are followed during the design and manufacturing process, a proper 75 ohm characteristic impedance can be determined and achieved.
Good question. I'd like to know why it should be so bad to use an analogue interconnect to carry a digital signal.
I'm using a balanced analogue interconnect between my Mark Levinson player and my DAC. Sounds good to me.
Can someone explain why it should be better to buy a new cable labelled "digital".
It depends of what make/model analog XLR interconnect cable it is. The established specification of a balanced AES/EBU XLR digital cable has a wide-range characteristic impedance, so perhaps the specs of your specific analog XLR interconnect cable falls somewhere between those extremes. The major difference between a typical analog XLR cable and an XLR digital cable is an AES/EBU digital cable tends to feature filler a.k.a spacer rods that act as a former for a twisted pair, in order for the conductors to have a more consistent and precise geometry from end to end. BTW, what's the length of the cable, and what make/model is it?
The analogue cable I'm using in place of a "proper" digital XLR AES/EBU is a basic Mogami Neglex 2534 microphone cable 2 m long. I have a parallel RCA S/PDIF digital cable from Signal Cables USA of 3 ft length. Can I tell the difference? Frankly there's so little in it.
I use the CDP so rarely now, I won't get a bespoke XLR digital cable right now. However I'll probably soon get a NAD Master Series M50.2 when they become available (cdp / ripper / server / streamer) which I will connect to my DAC (NAD M12) with a proper digital balanced cable.
The impedance of the cable is not the same as the resistance. And I think it's a reflection of it's characteristics at RF, not AF use. No?
Yes, the term is "characteristic impedance", which is based on a sophisticated determination based on the relationship of the dielectric with the conductors, rather than an actual measurement of the conductors resistance.
what i understand is if you take off braided shield then it seem the cable will be the same impedance wise if else where the same
Well, it won't because it's the precision structure of the dielectric in relationship placed between the center conductor and outer braided shield is what matters for a 75 ohm interface. It's a measured relationship based on a specification. Otherwise, the wire itself is only a signal conductor, without any relationship with the return conductor or its dielectric energy storage and quick-release tendencies, for example.
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