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join:2003-11-01
Juana Diaz, PR

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S/PDIF - does coaxial refer to the cable used or the connector?

I have been experimenting connecting the coaxial digital audio output from my computer to my home theater receiver. Not going optical because the receiver already has its only optical input taken. I have been using a regular RCA audio cable for the test and it works perfectly. This will not be the cable I will use permanently because there is already an existing stereo mini plug to RCA cable routed between the two devices through a cable raceway that I will repurpose to be used for the digital thingy as soon as I get an adapter to go from miniplug to RCA in the computer end. That particular cable consists of two RG59 cables terminated with RCA plugs in the receiver end which I got from Monoprice years ago. I also noticed that the cables sold by Monoprice for digital audio are RG59.

My question is, does the "coaxial" part in "Coaxial Digital Audio" refers to the type of connector or does it refer to a real coaxial cable as in RG59? I ask because the standard RCA cable worked perfectly.

Tursiops_G
Technoid
MVM
join:2002-02-06
Brooksville, FL
ARRIS TM1602

Tursiops_G

MVM

"Coaxial" refers to the type of cable construction, regardless of it's actual impedance.

S/PDIF (Sony/Philips Digital Interface) is a coaxial digital signaling standard specified at 75 Ohms characteristic impedance. RG59 is 75 Ohm impedance cable, and is typically used for S/PDIF.

If your "standard" RCA cable also has a 75 Ohm characteristic impedance, it should work as well... but if it's impedance is different, I'd expect some degradation of the signal due to reflections (SWR)... but that may or may not be audible (Excessive "Jitter" only observable with Test Equipment), depending on the amount of mismatch.

-Tursiops_G.

EliteData
EliteData
Premium Member
join:2003-07-06
Philippines

EliteData

Premium Member

that being said, if you are using regular RCA type cables and they're short length, you should not have any issues at all unless you notice audio breakups/distortion.

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Juana Diaz, PR

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By "standard" I meant regular analog audio cables with white/red RCA plugs at either end. I don't even know what impedance they may have but I guessed that for the distance involved it didn't matter. It was a regular 6ft cable I was able to put between the two devices in straight line and relocating the computer as close as I could. The permanent cable is a 25ft cable that runs through a raceway attached to the wall behind the computer desk, around a corner and behind the entertainment center. About 16 feet in length. That cable is RG59 as I said before but used for analog audio at present and has a stereo mini plug attached to a switch that selects sending audio to the PC speakers or to the receiver. A cheap mini plug to RCA adapter will let me use one channel from that cable on the digital audio output of the computer.

I didn't notice any problems using that cable. No dropouts, just pure joy. Even got 5.1 Dolby Digital from the computer, something I was never able to do before due to using analog audio.
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I noticed one thing, unrelated to the question above. The computer is also connected to the HDTV via HDMI (also a 25ft cable) as secondary monitor and it provides yet another route for PC audio to the home theater via the TV Audio input in the receiver. The receiver has audio return via HDMI but the TV does not so I am using and optical cable from the TV to the receiver for that.

When comparing the PC audio via the TV vs direct path on the digital coax output in the computer the difference was day and night. Audio passing through the TV sounds a bit muted and with what I will call artifacts as if it had lots of compression. In contrast, the digital audio direct from the computer was crystal clear. I don't know if it is a difference in the quality of the adapter (audio from the ATI card via HDMI vs onboard Realtek HD adapter. Or may be the TV itself. Can only find out by plugging the PC directly to one of the HDMI inputs of the receiver.
BoulderHill1
join:2004-07-15
Montgomery, IL

BoulderHill1

Member

The reason for your audio difference is that your TV does not pass digital audio through it.

Digital audio received into it via the HDMi input does not pass out the optical port.
Sound does come out the optical but it is a mixed down 2 channel audio PCM signal. NO straight through digital hand off.

This is not a fault of your TV rather it is a legal stipulation of the DMCA. Basically it is a DRM thing. This is why the audio that is passing through the TV is inferior.

You can feed your TV digital audio via the HDMI input connection but since your TV has only the two speakers there is no benefit to a dolby digital 5.1 track.

The digital audio needs to go straight from the source (DVD, Bluray, PC) to the audio receiver capable of decoding the digital signal and playing it on the surround speakers connected to it.

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Juana Diaz, PR

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Since my prior post I tried plugging the HDMI cable from the computer directly into the receiver and again the difference is day and night. Sound was just as good as using the digital output from the PC. So your explanation about the TV is likely the reason.
ke4pym
Premium Member
join:2004-07-24
Charlotte, NC

ke4pym to Tursiops_G

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to Tursiops_G
said by Tursiops_G:

S/PDIF (Sony/Philips Digital Interface) is a coaxial digital signaling standard specified at 75 Ohms characteristic impedance. RG59 is 75 Ohm impedance cable, and is typically used for S/PDIF.

Yep. And if you switch the interconnect media to fiber, it is the exact same interface. The exact same bits are being sent over light instead of electrons. Pretty nifty.

And a standard old cheap, crappy run of the mill RCA cable will work just fine in addition to RG-59 or RG-6 even.

You may notice, if your cable is near a high current device such as a fridge, you may get drop outs when the fridge comes on if you're using an RCA cable, though. The coaxial cables do a better job of keeping the noise out.

Hard Harry7
join:2010-10-19
Narragansett, RI

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Wow, you did a whole post on this and it was right there in front of me. I feel like a dufus. Great thread!