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what make coaxial cable impedances of 50 ohms and 75 ohms ?

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Posted on January 16, 2017 at 18:51:44
lovetube
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Location: Melbourne Australia
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i assume it is the braid shield that make the cable having different impedances ?

 

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RE: what make coaxial cable impedances of 50 ohms and 75 ohms ?, posted on January 16, 2017 at 20:01:35
Duster
Audiophile

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Location: Pacific Northwest
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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.

 

RE: what make coaxial cable impedances of 50 ohms and 75 ohms ?, posted on January 16, 2017 at 20:37:57
lovetube
Dealer

Posts: 3008
Location: Melbourne Australia
Joined: June 8, 2003
cool.
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

 

RE: what make coaxial cable impedances of 50 ohms and 75 ohms ?, posted on January 16, 2017 at 22:50:25
Duster
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Posts: 12806
Location: Pacific Northwest
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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.

 

RE: what make coaxial cable impedances of 50 ohms and 75 ohms ?, posted on January 18, 2017 at 08:41:23
SteveBrown
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Location: Portland, OR
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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?

 

RE: what make coaxial cable impedances of 50 ohms and 75 ohms ?, posted on January 18, 2017 at 13:54:35
Duster
Audiophile

Posts: 12806
Location: Pacific Northwest
Joined: August 25, 2002
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.

 

Good Question, posted on February 3, 2017 at 13:14:40
cawson@onetel.com
Audiophile

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Joined: September 27, 2004
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".

Thanks

 

RE: Good Question, posted on February 3, 2017 at 14:45:39
Duster
Audiophile

Posts: 12806
Location: Pacific Northwest
Joined: August 25, 2002
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?

 

RE: Good Question, posted on February 4, 2017 at 06:20:35
cawson@onetel.com
Audiophile

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Joined: September 27, 2004
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.

 

That's simple, posted on February 4, 2017 at 10:04:45
jedrider
Audiophile

Posts: 11729
Location: No. California
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Missing permeability mu-zero, so what the hell could that be??

Better explanation in wikipedia, however:

https://en.wikipedia.org/wiki/Coaxial_cable

I think mu-zero is fortunately '1' most of the time.

 

Characteristic Impedance...., posted on February 9, 2017 at 09:13:33
Todd Krieger
Audiophile

Posts: 32946
Location: SW United States
Joined: November 2, 2000
This applies to RF and digital transmission, not audio signals....

In order to minimize "signal reflection" on transmission, the transmitter and receiver impedance must be matched at 50 or 75 ohms. If the cable's characteristic impedance matches the transmitter/receiver impedance, the impedance "seen" by the transmitter and receiver remains at the proper impedance, regardless of cable length. (This does not account for potential signal losses.) This also keeps signal reflections to a minimum, preserving the integrity of the signal at the receiver. In digital transmission, this minimizes jitter induced by signal reflections.

This is why SPDIF digital cables should be rated at 75 ohms. Consumer digital transmitters and receivers are spec'ed to 75 ohm impedance.
   

 

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