what is the difference between dig coax and audio cable

[peterS]

i see "rca" tips for same but never digital coax
i was always under the assumption that digital coax used 75ohm tips
hmmm

[NonSense]

Coax is a single conductor surrounded at a uniform distance by a second conductor usually a braid often combined with foil to enhance shielding. The shielding and common return path are shared.

Most audio cable consist of at least two conductors, the signal and the return. If the cable is shielded, you usually have an additional drain wire that is most often connected to the common at the source. The characteristic impedence is most often not stated.

Several companies make RCA connectors to terminate Coaxial cable for either video or digital. But most are not true 75 Ohm terminations. BNC connectors are a good example of true 75 Ohm termination.

[Audiophiliac]

Not all BNCs are 75 ohms. Some are 50 ohms. Not sure what the uses would be.....maybe some CCTV, or "pro" applications?

Either type of cable will work for analog audio. How they differ is a good question. I haver asked other places and never really got a good answer.

We always run 22/2PS (2 pair shielded) for analog audio and/or subwoofer applications. A lot of "other" outfits simply run RG6.

[wolfgang]

Digital audio connections vary between digital interface selected. The cable that connects to the coaxial interface is designed to be 75 ohm coaxial cable. It is the same cable type as used in video industry to transfer video signal. For example the video industry standard RG-59 coaxial cable is very ideal to carry coaxial S/PDIF signal for long distances. For shorter runs practically any 75 ohm coaxial cable will do nicely. For very short runs (up to 2 meters) the needs of coaxial cable are not so critical, and a normal "RCA audio cable" usually works well.

Reference taken from eParanoma.net

http://www.epanorama.net/links/audiodigital.html#digitalinterface

[NonSense]

The big difference is whether you need an impedence matched cable or not.

If your cable is behaving as a transmission line, you need to keep the source, cable and termination matched. Coax is used because it offers many benefits due to its construction. Uniform dielectric equidistant from the ground plane with shielding to reduce radiated emisions. The impedence is calcuated using the dielectric constant of the insulating material and the distance of the transmission line to the ground plane. The cable television industry uses 75 Ohm. RG-6 and RG-59 are both 75 Ohm, but the material construction used in the RG-6 offers reduced attenuation at higher frequencies.

At every connection point, a mismatch will cause reflections. Distortions and a loss of transmitted power as some of the transmitted signal has been reflected back to the source.

With basic digital systems (not employing pulse shaping) the fast edges of the time domain digital pulse can contain strong high frequency harmonics which begin to exhibit transmission line characteristics depending on the data rate.

Impedence matching becomes a bigger issue when the cable length and the signal wavelength have greater interaction. RF frequencies.

[peterS]

how can a cable be fixed resistance? it will increase with distance

[chrispy35]

characteristic impedance != resistance

[NonSense]

This is a transmission line characteristic impedence. The signal loss increases, but not the characteristic matching impedence. In an ideal situation, the goal is to have load as seen by the source to appear the same whether it is 1 foot away or 100 feet away. The cable would then have an associated loss/distance. Such as 0.3db/100ft etc.

This way, the transmission equipment can be designed to drive a known load. Adhering to a standard, so to speak. (for example: DVD manufacturers design their CH3 outputs independently, but all expect to see a similar load regardless of the tv used or the length of cable in between.) Matching the impedence, maximizes transmitted power vs. reflected power.

[whoaru99]

how can a cable be fixed resistance? it will increase with distance

The characteristic impedance of a coaxial cable is a completely different phenomena than the DC resistance of the cable. Characteristic impedance has mostly to do with the distributed capacitance and inductance of the cable, whereas the DC resistance is mostly due to the thickness, composition, and length of the conductors themselves.

Here http://www.allaboutcircuits.com/vol_2/chpt_13/3.html is a little lite reading on the topic.

[peterS]

The characteristic impedance of a coaxial cable is a completely different phenomena than the DC resistance of the cable. Characteristic impedance has mostly to do with the distributed capacitance and inductance of the cable, whereas the DC resistance is mostly due to the thickness, composition, and length of the conductors themselves.

Here http://www.allaboutcircuits.com/vol_2/chpt_13/3.html is a little lite reading on the topic.

thanks!

[peterS]

edit

[fauzigarib]

So the above discussion was just a tad technical for myself.

In short, when you have a digital coax out from, say your dvd player. I've usually used just a regular RCA cable out of that to my receiver for example.

Is that incorrect? So there is such a thing as a coaxial cable that offers benefits over the RCA ones?

Keep it simple please! :E

Regards,

Fauzi

[whoaru99]

So there is such a thing as a coaxial cable that offers benefits over the RCA ones?

Fauzi

Well, now you are on the verge of a cable debate.

Some cables have attributes that differentiate them from others in terms of shielding, exotic metals, construction methodology, etc. However, whether or not YOU can hear any difference in the resultant sound is an entirely different matter.

Personally, I have not been able to perceive any differences.

[fauzigarib]

Well, now you are on the verge of a cable debate.

LOL! No that's not what I meant. I was at a dealer's place a couple days ago negotiating a new projector, and he told me that I was stupid to have plugged in just a regular RCA cable into the coaxial, because that's like listening to music with ear plugs in. You just don't get the entire picture.

The reason was that a the tip of a coaxial cable is completely different, and does different things, even though the plug looks just like a regular RCA. Now I've never seen "the tip" of a coaxial cable, so I thought I'd run it by you guys here.

Thanks for the "tip!!"

Later,

Fauzi

[NonSense]

Fauzi

Your dealer may have been exagerating just a little. (I would have agreed if he was talking about regular cables being used for component video connections)

IMO there is no definitive answer. There are many factors to consider, some of which may be specific to your setup. What is the data rate being used, 44.1kHz, 96kHz or 192kHz? What is the length of the cable? Does the receiver reclock the data? Are you using any other components with analog interconnect? How well are they shielded? What is the level of your gear and would you see a greater benefit from spending those resources elsewhere.

IMO, coax is a better choice for the digital interconnect, but after spending the money to change out the cable, you still may not hear any difference.

The best advise is to borrow a cable from your dealer and give it a try. If you like the results, buy it.

[whoaru99]

Many "RCA" cables are of coaxial construction and therefore will carry digital audio signals without causing any obvious problems.

However, it is generally advised to use cables having a characteristic impedance of 75 Ohms for best results. Since video cables are (or should be 75 Ohm cables) they double quite well as digital audio interconnects.

[mjb]

Digital and video cables should ideally be from 75 Ohm coax and connectors (like BNC's), not RCA audio cables. RCA's are not 75 ohm (the dimentions are all wrong), so I suppose an impedance missmatch will still occur, strange as 75 Ohm cables with RCA's are used for HD Component connections. I guess the thing is not to get too hung up on all this stuff!

[Lex]

LOL! No that's not what I meant. I was at a dealer's place a couple days ago negotiating a new projector, and he told me that I was stupid to have plugged in just a regular RCA cable into the coaxial, because that's like listening to music with ear plugs in. You just don't get the entire picture

The reason was that a the tip of a coaxial cable is completely different, and does different things, even though the plug looks just like a regular RCA. Now I've never seen "the tip" of a coaxial cable, so I thought I'd run it by you guys here.

Thanks for the "tip!!"

This seems confusing how you said this. I'm not even sure I know what you are talking about, audio? video? You mention a projector and a picture, but then you are talking audio.

I think you are just saying you plugged a standard 50 ohm audio cable in as a digital coax. It will mostly carry the signal, but without getting technical, things are not optimal here. As to the connectors, well, an RCA connector itself cannot be true 75 ohm, yes, even the Canare. They can be optimized for 75 ohm, but the connector itself will not measure as 75 ohm. So, his argument about the connectors themselves is pretty much unfounded, however, the argument about the wire vs coax is of significance. As someone menetioned earlier, typical audio wire is at least a twisted pair with a wire shielding, if shielded. The drain wire is of no real significance, it is merely for strength when running long runs, perhaps hanging cable even and for our purposes in audio can just be snipped away if you use wire with a drain wire. But the difference in ohms of a true coax designed for composite video (typically a yellow connection) and a 50 ohm audio cable is of significance, and you should for a fact only use 75 ohm video/digital cable for these connections.

Doug
Owner
CATCables.com

Doug

[Karma]

HI All,
Can we confirm that the source and destination electronics impedances are 75 ohms? Or are some manufacturers conforming and some not? I'm not sure at all.

Sparky