It works... Why?

[Hi Ho]

I have always been told that a coax digital signal can only be transferred through about 15 feet of cable before it doesn't work. I was bored this evening and started messing around with my system. My PC has been connected via two 50ft coax cables to my theater system. I took one cable and connected it to the digital input on the receiver and the digital output on the sound card. It worked! I can now get digital quality sound! Why? The cables are home made. I used 12cent/ft braided coax with twist on F-type connectors and RCA adapters.

My second question is about getting 5.1 channel sound to the receiver. As of now I can only get 2.0 channels from my sound card even though it is set to 5.1. During the speaker test only the front left/right make any sound. Is there a cheap sound card that will send a Dolby Digital 5.1 stream to my receiver via a coax digital output?

[juppy]

I've personally never heard that about anything over 15 ft. of coax not working. Are you sure that's not just the length where you start losing a little bit of the signal? Kinda like how IDE cables need to be kept under 36". 15 feet of coax wouldn't even make the run for most houses with mini-dish satellite systems. I mean, ours is right outside the wall from where the tv sits and we're probably pushing that 15 ft limit, so its obviously going to take more than that for some of these two-story houses you see around with the dish clear up on the roof. So I would think that's just the length at which the signal starts to fade maybe.

Dunno about your second question. Sorry.

[Hi Ho]

I'm talking about a digital signal. You don't lose just a little bit. It's all or nothing. I have been told by many, including SonicVangaurd, that you can't run a coax digital signal more than 15 feet without a repeater.

[juppy]

I'm talking about a digital signal.
So am I. Satellite tv signals ARE digital, so like in the example I gave above, I'd have to differ on that 15 foot limit.

And yes, I know what a digital signal is. I have to disagree about there only being on or off with it though. In a perfect circuit, yeah, that'd be right. But if you've ever used a digital circuit probe then you know that's not always the case. On the probes I've always used, for example, they would call a signal between say 0 and 2.5 volts a low (off) and one from 3.5 to 5 volts would be a high (on), but when the test falls between the 2.5 to 3.5 volt range, both lights on the probe come on, so therefore the signal is neither a definite high or low. Theoretically its not supposed to happen, but it does. :)

[Hi Ho]

I know there isn't just an "all or nothing" rule. It's more complicated. However, with digital, if you are getting a steady signal you are getting exactly the same signal that came from the source. If it was corrupt you would get artifacts in the picture or pops/clicks in the audio. You could also get dropouts. As long as the picture/sound is steady and clean then it is basically perfect and almost immune to interference.

Appearently what I have always been told about the maximum cable lengths for digital audio signals is not correct. I'm not disputing that because I am listening to it right now.

[juppy]

I got to wondering about this myself, so I did a Google search. There was one site that said, for audio, lengths up to 300 feet would usually provide good sound quality without having to use amplifiers or equalization, and up to 1,000 feet have been done WITH using the amplifiers. They also said that RG-6 coax, being better shielded than RG-59, would also help the signal travel farther. Maybe that's why your 50 footer is working.

[David M]

Right...different coax can carry a radio frequency signal different distances....just as 14 guage wire can carry a given voltage a greater distance than 16 guage wire.

[Hi Ho]

I use RG-6 for everything. Maybe that is what made the difference.

[Hi Ho]

In case anyone is interested. I mentioned above that I was looking for a card that would encode everything into Dolby Digital 5.1. I found it. The BlueGears X-Mystique 7.1 Gold (http://www.newegg.com/Product/Product.asp?Item=N82E16829127001). It is appearently the first and maybe the only card that will do this. I have been reading a lot about it. The nForce2 Soundstorm onboard audio did the same thing but this is the first add in card.

[SonicVanguard]

There are a lot of variables to running coax over 15 feet - RF interference, magnetic interference and signal degridation. Using RG-6 you covered the first two issues pretty well. As far as signal degridation, it will happen after the first 12 feet in a measurable way - it just didn't happen enough for you to notice it audibly.

[Hi Ho]

How can there be audible signal degration with a digital signal other than clicks/pops and dropouts?

[SonicVanguard]

It's all in the headroom and S/N ratio my young padawan. There is more to digital than one would first believe.