Digital Audio Cables

geezer · « **on:** 30 Jan 2007, 12:30 am »

I'm about to upgrade from an old CD player to a DAC and I find there is a certain quirk of nomenclature whose purpose I don't understand. My DVD player has an (analog) audio output labeled, reasonably enough, "audio out", and a digital audio output labeled "digital out, coaxial". What I don't understand is the use of the word "coaxial" in one case but not the other, since the analog cables are also coaxial.

It seems to imply that the digital cable is somehow different from the usual audio cables, but I don't believe that. Will someone please explain?

Geezer

srb · « **Reply #1 on:** 30 Jan 2007, 01:56 am »

True, RCA connectors are "coaxial".

But the coaxial being refered to here, is coaxial cable, meaning there is a signal conductor in the exact center with outer shield as the return conductor.

Video and digitial audio coaxial connections are usually specified at a 75ohm impedance, and this type of construction allows for that.

Some people use 75ohm coaxial cables for analog audio, but it is not optimum. Many good analog audio cables use a twisted pair for the signal, and a third grounded shield, which serves only as shielding and does not carry the audio signal.

Actually, the correct 75ohm connector that should be used with video and digital audio coaxial cables is the BNC connector, as it is a true 75ohm connector (although several companies claim their RCAs are 75ohm!) This is what is used on professional equipment, but unfortunately, the very ancient RCA audio connections also became the standard connection for consumer video/digital coaxial.

geezer · « **Reply #2 on:** 30 Jan 2007, 03:23 am »

The digital audio output on the DVD is an RCA connector, which is why I'm assuming that ordinary audio cable is to be used. Also, at radio shack I have not seen any cables with RCA connectors labeled digital.

LightFire · « **Reply #3 on:** 30 Jan 2007, 03:25 am »

The RCA digital out coaxial cable is physically exactly the same as the RCA audio analog output pair. You have to use a digital output if you want real 5.1 playback. Only one RCA cable is necessary for your digital output (as you probably noticed). Any good quality RCA cable will work. Actually the digital signal is more resistant to interferences than the analog so your digital coax don't even need to be as good as your analog RCA.

NewBuyer · « **Reply #4 on:** 30 Jan 2007, 05:36 am »

Quote from: geezer on 30 Jan 2007, 03:23 am

The digital audio output on the DVD is an RCA connector, which is why I'm assuming that ordinary audio cable is to be used. Also, at radio shack I have not seen any cables with RCA connectors labeled digital.

Copied from the Blue Jeans Cable website:

"Turning back to the world of coaxial cable, we can now answer a few questions. Can analog cables be used in digital applications? Yes, up to a point; but the looser tolerances of older analog cable designs will limit their run lengths, at least when used in high-bandwidth applications like SDI video. Can digital cables be used in analog applications? Yes, absolutely; the same tight tolerances which make digital cables appropriate for digital applications make them superb for analog applications. One may not "need" the improvement, but it will never hurt, and can help. SDI coax, like Belden 1694A, costs very little more than (and in many cases less than) traditional analog coax designs, and will outperform it on every measure of analog cable performance. The advances in dielectric foaming result in a cable which is not only higher-performing, but more flexible and easy to work with, than its older counterparts. Contrast, for example, 1694A with a cable that was once the broadcast industry standard cable, Belden 8281. 8281 is heavier, thicker, and dramatically less flexible. And if the notion of heavier and thicker cable sounds better--bigger cable equals lower loss, right?--think again. Because of efficient dielectric foaming, 1694A is able to use a larger center conductor in a smaller cable, for lower loss."

srb · « **Reply #5 on:** 30 Jan 2007, 05:39 am »

Quote from: geezer on 30 Jan 2007, 03:23 am

Also, at radio shack I have not seen any cables with RCA connectors labeled digital.

Radio Shack lists 33 'digital coaxial cables':
http://www.radioshack.com/search/index.jsp?kw=rca+cable&f=Taxonomy%2FRSK%2F2032244&categoryId=2032244&kwCatId=2032058&cp=2032058.2032228

Quote from: LightFire on 30 Jan 2007, 03:25 am

The RCA digital out coaxial cable is physically exactly the same as the RCA audio analog output pair.

Well, actually it's not. All wire manufacturers - Belden, Canare, Mogami, etc. manufacture specific wire types for audio applications versus video/digital audio applications, as they have different impedance, capacitance and bandwith characteristics.

Audio cable can function as video/digital audio cable, but you can do better with coax.

26ga telephone wire can function as speaker cable, but you can do better with a heavier gauge wire.

I have, more than once, heard audible improvement of CD/DVD players, when the digital coaxial output that was previously wired with a standard audio cable, was replaced with an industry-standard 75ohm coax cable .

I have also seen the visual improvement of TV/DVD, when the component video connection that had been wired with 3 audio cables, was replaced with an industry-standard 75ohm component cable set.

geezer · « **Reply #6 on:** 30 Jan 2007, 07:10 pm »

Thanks for all your input. You've given me a lot to chew on.

I should have been more specific regarding my set-up: I am interested only in the (two channel) audio portion of the digital signal from the DVD. I presume this means high bandwidth is not as critical and I probably can use any decent audio cable without noticeable degradation.

Regarding Radio Shack: I did see "digital coaxial cables" there, but none with RCA connectors.

Geezer

enjoythemusic · « **Reply #7 on:** 30 Jan 2007, 08:30 pm »

Digital audio cables are spec'ed for 75 Ohm. Make your own and save $$$

http://www.enjoythemusic.com/etmcable.htm

PhilNYC · « **Reply #8 on:** 30 Jan 2007, 08:36 pm »

Quote from: geezer on 30 Jan 2007, 12:30 am

I'm about to upgrade from an old CD player to a DAC and I find there is a certain quirk of nomenclature whose purpose I don't understand. My DVD player has an (analog) audio output labeled, reasonably enough, "audio out", and a digital audio output labeled "digital out, coaxial". What I don't understand is the use of the word "coaxial" in one case but not the other, since the analog cables are also coaxial.

It seems to imply that the digital cable is somehow different from the usual audio cables, but I don't believe that. Will someone please explain?

Geezer

I think it's labeled "coaxial" primarily to differentiate it from "optical" or "toslink" for the average person who might go to a store and ask for a digital cable without knowing the difference...

geezer · « **Reply #9 on:** 30 Jan 2007, 08:53 pm »

Digital audio cables are spec'ed for 75 Ohm. Make your own and save $$$

I wonder, what would you guess a 3 foot length of an equivalent commercially available cable would cost?

Geezer

geezer · « **Reply #10 on:** 30 Jan 2007, 08:55 pm »

Thanks PhylNYC; that makes sense.

Geezer