Newsgroups: rec.audio.tech
Subject: Re: Convert rca to coaxial - audio only
References: <[email protected]> <[email protected]> <[email protected]> <[email protected]>
"Geoff Wood"  writes:

>  wrote in message 
> news:[email protected]...
> > By coaxial, I mean the kind that runs through almost every home for TV
> > signal.
> >
> > By RCA, I am talking about the input jacks on a stereo receiver.
> The idea is totally flawed.  Such outputs and cables are only designed to 
> go a short distance.

It is not toattly flawed. Generally the RCA jacks on the equipment 
are designed to drive short cables, all work there well. 
Some equipment perform well even with long cables, say 10-20 meters 
without considerable problems (and longer distances with some 
low frequency loss). 

The output impedance on the soudn source has the greatest effect 
on how long cable you can run. The output impedance on different
equipment can have vatiotion of more than 1:100 (usually in 30 ohms 
to 5 kilo-ohms range). The lower the output impedance, the less 
effect the cable capacitance has and less sensitve the cable 
is for noise pickup (at least for capacitively coupling noise). 

> Also TV coax is a totally different concept to screeened unablance audio 
> cable.

The actual cable used for TV coax and the cable used for 
unablance audio cable are not entirely different concepts. 
The cable construction used is the same. There is a center 
wire, insulator, outer shield and insulator over it. 
This same construction applies for for unbalanced audio 
cables and TV coax. Those both cables have coaxial construction. 

The unablanced audio cable is a cable optimized for audio applications. 
It is designed to perform well on audio frequency (reasonable 
cable capacitance, reasonable inaulation material, copper conductors) and 
usually designed to be convient to use (flexible and not to thick). 

The TV coax is a cable optimized to carry RF signals of several 
hundred MHz with low loss and have right characteristic impedance 
for the application (75 ohms). To make right impedance 
and low loss, the cable geometry (right ratio of center connector 
and outer shield innter diameter throughout the cable) and suitable 
insulation material (suitable plastic or foamed plastic typically). 
The cable hss good outer shield, nowadays typically consisting of 
foil + braid (doubly shielded cable). 

You can use a TV coax cable material to make audio cables, 
jus solder the RCA cables on the end. If you have a good quality 
doulby shielded cable that uses copper conductors, it 
will perform well as audio interconnection cable. 
You don't need to worry about the "75 ohms" label on the coax, 
this impedance has nothign to do the cable properties on the 
audio frequencies (unless your cable is many kilometers long!).

> Forget the whole idea, or look into an audio-over-cat5 LAN cable product.

Those audio-over-cat5 LAN cable products are a good option if you have 
CAT5 wiring installed.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/