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What’s special about ‘digital coax’?

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#1
I ran into some connection problem between my streamer (Primare NP5) and my DAC (Hegel HD11) using a coax cable.
One of the questions I got asked in the process was if I was using “a dedicated digital 75 Ohm cable”.
I solved the problem by replacing the Madrigal MDC-2 digital coax with a standard A/V RCA interconnect.

Which leads me to the questions if there is actually such a thing as ‘digital cable’ and if it offers any improvement over a good quality RCA cable.

I’m interested to read some experiences on this issue.
 

DonH56

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#3
Yes, a "digital" audio (or video) cable will have 75-ohm nominal impedance by design. In practice, it is far cheaper for companies to buy 75-ohm cable in bulk and use it for audio, video, and digital audio cables. And most coaxial audio interconnects are of such a size that they are about 75 ohms anyway. Thus, the differences among interconnect cables tend to be more about shielding and wire gauge (larger wire, lower gauge, means lower loss) than impedance (which does not matter for audio signals).

As for "improvement", who knows. A large-gauge interconnect with foil or 100% braid that is 75 ohms but marketed for "audio" signals may work better than a cheap, small, poorly-shielded video RF/digital cable. And vice-versa. Look around, ask questions, and buy a decent cable from a decent manufacturer (which usually means one that does a good job mounting the connectors). Such cables often cost <$10 for a 1-m pair.
 

MZKM

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#4
Yes, a "digital" audio (or video) cable will have 75-ohm nominal impedance by design. In practice, it is far cheaper for companies to buy 75-ohm cable in bulk and use it for audio, video, and digital audio cables. And most coaxial audio interconnects are of such a size that they are about 75 ohms anyway. Thus, the differences among interconnect cables tend to be more about shielding and wire gauge (larger wire, lower gauge, means lower loss) than impedance (which does not matter for audio signals).

As for "improvement", who knows. A large-gauge interconnect with foil or 100% braid that is 75 ohms but marketed for "audio" signals may work better than a cheap, small, poorly-shielded video RF/digital cable. And vice-versa. Look around, ask questions, and buy a decent cable from a decent manufacturer (which usually means one that does a good job mounting the connectors). Such cables often cost <$10 for a 1-m pair.
Mediabridge on Amazon sells RCA cables designated as subwoofer cables ($19/8ft), RCA cables ($28 for 8ft y-adaptor), or digital coax ($12/8ft). I would love for all 3 to be measured to see how they differ, and/or a teardown to see differences in shielding and gauge.
 
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#5
The BJC articles do point out a salient but sometimes overlooked aspect of cabling: both the quality and means of affixing end connectors to the cable. I always look for manufactured cabling that specify soldered or welded connectors, name brand, and assured tight fit to the device. For a few dollars difference, one can purchase quality cabling without seeking out ridiculously priced “audiophile” kit.
 

Dialectic

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#7

sergeauckland

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#8
Is that 75ohm rating for a km of wire?
It's the characteristic impedance, not resistance.
At Radio Frequencies, cables act as Transmission Lines, where the Capacitance and Inductance combine to create an impedance, which again at radio frequencies, should be correctly terminated both at the sending and receiving ends. When that is done, signal is transmitted cleanly, with no reflections. It then doesn't matter whether the cable is 1m long or 100m long, although there is attenuation with increasing distance.

At Audio frequencies, a cable, regardless of impedance, presents an open circuit to the source, and passes through whatever the terminating impedance is, so it is normal to send from a very low impedance and receive into a high impedance as this provides the best voltage transfer. Reflections only become an issue at radio frequencies, which can be typically around 1MHz and higher.
S.
 

Vini darko

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#9
It's the characteristic impedance, not resistance.
At Radio Frequencies, cables act as Transmission Lines, where the Capacitance and Inductance combine to create an impedance, which again at radio frequencies, should be correctly terminated both at the sending and receiving ends. When that is done, signal is transmitted cleanly, with no reflections. It then doesn't matter whether the cable is 1m long or 100m long, although there is attenuation with increasing distance.

At Audio frequencies, a cable, regardless of impedance, presents an open circuit to the source, and passes through whatever the terminating impedance is, so it is normal to send from a very low impedance and receive into a high impedance as this provides the best voltage transfer. Reflections only become an issue at radio frequencies, which can be typically around 1MHz and higher.
S.
Thanks for the clarification.
 

Jimbob54

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#10
Mediabridge on Amazon sells RCA cables designated as subwoofer cables ($19/8ft), RCA cables ($28 for 8ft y-adaptor), or digital coax ($12/8ft). I would love for all 3 to be measured to see how they differ, and/or a teardown to see differences in shielding and gauge.
More importantly, do they have direction arrows ? Nothing sounds worse than sound travelling the wrong way down a wire. Needs arrows to help it navigate.
 

MZKM

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#11
More importantly, do they have direction arrows ? Nothing sounds worse than sound travelling the wrong way down a wire. Needs arrows to help it navigate.
I have a pair of nice RCA cables that are directional (they were $8 due to closeout, forgot which member told me about them), it just means the shield is “floating“ (aka not connected at one end), I unscrewed the terminals to verify it was true.

It is supposed to help with external noise (not sure if electrical or radio). I guess you could test if these do anything by inducing external noise (either running it along a power supply/transformer or having some radio transmitter).
 

Jimbob54

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#12
I have a pair of nice RCA cables that are directional (they were $8 due to closeout, forgot which member told me about them), it just means the shield is “floating“ (aka not connected at one end), I unscrewed the terminals to verify it was true.

It is supposed to help with external noise (not sure if electrical or radio). I guess you could test if these do anything by inducing external noise (either running it along a power supply/transformer or having some radio transmitter).
Joking aside, I had heard that there were some cables built like this. Obviously the arrows serve a valid and necessary purpose in such cases.
Im also sure some of the ones I've been sold with directivity in the *bad* (pre ASR) days and also guidance in some of the hifi press to "follow the writing" were not based on such a design or need.
 

FrantzM

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#13
Hi @Richardk3000

Please re-read @DonH56 post above. Digital connection between the streamer and the DAC is not at audio frequencies. The digital stream on which the audio signal "rides". the transmission, is at Radio Frequencies between 1.5 and 4 MHz ( don't hold me to the exact frequency, this is from the top of my head ) ... Again the transmission, NOT what is coded inside of it which are audio signals between 0 Hz to 22.1 KHz for SPDI/F, take the transmission as the envelope and the audio signal as the letter. Transmission is at Radio Frequencies and at those, cable behave like something called transmission lines. At such frequencies, there is a notion called "Characteristic Impedance"... I will not delve into it, but it is good in RF transmission that the characteristic impedance from Transmitter (Your streamer or CD transport or SPDI/F/Coaxial Output of your player) to the receiver (the DAC) be closely matched . In SPDI/F 75 Ohms of characteristic impedance is the standard. It also happens that in the old days TV signals were carried on a cable with such characteristic impedance and most interconnect cables have a configuration (the coaxial sort) that, if they were to be measured, would present a characteristic impedance of 75 ohms... Most coaxial cables on the market are close enough... even if they are labelled Audio cables.

Long story short any coaxial cable you see with RCA at both ends is likely to connect your streamer to your DAC with little to no issues. If at all in doubt get the cheapest "digital" cable you can get from reputed cable companies such as Blue Jeans... I make do with anything cheap that Amazon sells ;)
 
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JeffS7444

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#14
I solved the problem by replacing the Madrigal MDC-2 digital coax with a standard A/V RCA interconnect.
Which may be the best move. I like those thick premium analog A/V cables one can sometimes buy for pennies at a thrift store: I don't know about the "audio" portion, but video cable(s) are generally an RG59-type coaxial cable with full foil shielding, and very low capacitance: Excellent for SPDIF use I think! I suspect subwoofer cables are going to be the same, at least the better ones. What audiophiles won't like is that the center conductor may be copper-plated steel, copperweld or some other "bad" alloy said to cause hysteria "hysteresis", but I've never seen such claims quantified.
 

Speedskater

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#18
What audiophiles won't like is that the center conductor may be copper-plated steel, copperweld or some other "bad" alloy said to cause hysteria "hysteresis", but I've never seen such claims quantified.
Cable TV installers often use that type of coax. TV channel signals up to almost a Gigahertz don't seem to have any problems with "hysteresis".
 
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