[antsinger]

I've been using Optical for years and always figured one cable is as good as any other cable.


I just got a Pioneer Elite 45 and it doesn't have enough optical inputs.


Since, watching DVD's is what I do most and it is the only component that has Coax out. I've been wondering if I can use I high quality Audio cable that I have or do I need to get a cable designed for Digital Coax?


I've read many threads about the differences or lack there of in sound quality between Optical and Coax. But, I cannot find anything on using Audio cable for digital purposes.


Thanks.

 

[smmcintyre]

That is a very good questions. I would like to know the answer as well.


I know I have been able to use a one standard RCA cable from an AC3 digital output to my creative live card. That worked, and I was hearing the digital output (not the analog)


No if this works on a fully enconded DVD I don't know. Might be a bandwidth issue of the cable?????


I am way over my head here.. Any experts care to help?

 

[Ratman]

After spending hundreds of $$ on equipment, get a quality digital coax cable (or make your own). RCA 'audio' cables are not engineered for digital applications. It will work, but it is not optimal due to the impedence mismatch.


Digital coax cable impedence is 75 Ohm and audio cable is 50 Ohm. Spend the $10-20 bucks and get a decent quality coax cable.

 

[antsinger]

Thanks for the advice.


Just picked up a 8 ft Monster version for $25.00.


As you said I've spent many thousands on the equipment ...

 

[jerndl]

A decent composite video cable will work also. It doesn't need to be "engineered for digital applications". It just needs to be a decent quality 75 ohm coax cable (which is exactly what a composite video cable is).


Jay

 

[nutt]

I need not be monster, but a 75 ohm cable will certainly work better than a 50 ohm cable. It is highly debatable whether the differences will be audible.


Tim

 

[spidey07]

but what is not debatable is reflections caused by an impedance mismatch. Keep it 75 ohms.

 

[antsinger]

I connected the 8 ft Monster and left the optical in place at the same time.


Doing a switch is imperceptible to me, they both sound the same on a DTS movie.


Thanks for all the help, $25 is not too bad for peace of mind.

 

[Larry Fine]

No, $25 isn't a lot, but there is no technical reason to use a specailty cable for this purpose. Impedance matters with video because of the high frequencies involved, but not a digital bitstream. You could use a wire coathanger if you wanted to. (it has been done: no error.) Just be sure to sand off the paint first.

 

[spidey07]

Quote:

but not a digital bitstream.

Larry,


Not trying to start any kind of back and forth but I must let you know that impedance is EVERYTHING in digital transmission. The transmitters and receivers expect 75 ohms, using 50 or 150 ohm cable is pretty far off spec and can cause reflections (the signal and bitstream is actually "reflected" back to the transmitter).


Most of this comes from experience as a data communications engineer where I've seen very slight to very massive corruption and timing errors cause by impedance mismatches.

 

[Spanc]

Not to hi-jack this thread but the conversation has gotten me a little worried. I'm about to connect a sub and instead of using a cable specifically sold as or meant for subs I thought I would use an extra one I had lying around. This one was meant for video hook up. It's a Monster cable that was sold with two others (it's labeled yellow, the audio cables are red and white). Essentially the video cable of the set of three that come with any VCR, DVD, etc... what is that, composite video? Anyway, the question is should I go out and actully buy a cable designated for subs or will the one I have work just fine?

 

[Ratman]

The yellow composite cable (for video which is 75 Ohm) can also be used for the subwoofer connection.

 

[Spanc]

Thanks Ratman. Saved some money.

 

[theSPOOLER]

How can I tell if a cable is 50ohms or 75ohms? I've got a BNC cable with RCA adapters on each end. I'd like to use that for my sub. My understanding is BNC can be either 50 or 75. And who's to say the RCA adapters aren't 50 while the cable is 75.


Yes, I've got a multimeter.

 

[HomeThtrLA]

Larry sez: wire coathanger if you wanted to. (it has been done: no error.) Just be sure to sand off the paint first.


Also be sure to get off the old marshmallow and hot dog detritus.


I have seen (yeah, visually) reflected signal; it was on a T connector on a CCTV circuit where a 200 foot lead was not terminated, and the ghost was horrible. Disconnected the cable, problem solved.


Spanc, this sounds ludicrous once you think about it, but a video cable will carry sync (essentially DC) to 6 mHz, so it will probably carry 20 Hz to 100 Hz just fine.


spidey07, have you seen mismatch problems with short single runs between equipment?


What the heck amount of delay and reflected signal will there be with a six foot cable?



There also is the concept that it is digital, so it will work or it won't, but that might not be exactly true. Any ideas about that?


SPOOLER, the multimeter won't help because this impedance is measured in the megaHertz range, and your multimeter just puts out DC.

BNC connectors are indeed made for 50 or 75 ohms, and with some practice you can see the difference. I don't have enough practice to clearly describe it.


Lastly, all this impedance stuff does not matter with unbalanced consumer audio ( = analog audio on RCA connectors). Video is made with a source of a particular impedance transferring energy to an input with the same impedance, and all the energy will be transferred if all connectors and the transmission line have exactly the same impedance. Audio stuff typically has about a 1000 ohm source impedance but input impedances on the order of 10,000 to 50,000 ohms, so we are not dealing with a transmission line, but a constant voltage system. This is more like worrying about the impedance of the wire in your wall to be sure your toaster gets the proper amount of power. The local 110-0-110 volt power transformer has a way lower impedance than the toaster, so the wire's impedance is pretty irrelevant. It just depends on its resistance, not on its inductance and capacitance.