Newsgroups: comp.dcom.cabling
Subject: Re: Coaxial vs CAT 5
References: <[email protected]> 
[email protected] (Dmitri(Cabling-Design.com)) writes:

> Ro Cathain wrote:
> 
> 
> > I have Cat 5 and Coaxial cabling installed in my house.
> 
> > I want to feed (a) an audio signal, (b) video signal, from a wirelessly
> > controlled sub-floor PC to wall connections.
> 
> > So, I anticipate using 4 of the Cat 5 wires to feed an audio signal to
> > hifi
> > equipment. (ok quality?)
> 
> > Then, should I use the other cables of the CAT 5 to make up a scart
> > connection to the TV... or should I use the co-axial cable for best
> > quality???
> 
> > Cheers.
> It is best to use CAT5 for both audio and video because the coax cable is
> designed for broadband video (TV). 

All TV video signals are originally designed to be transfered through 
75 ohm coax cable. That's the what the video professionals, 
TV broadcasters, majority of CCTV installations etc. use. 
There are various kinds of 75 ohm coaxial cables available (like 
there are varioust types of 50 ohm ciaxila cables as well), 
thicker, thinner, higher loss, lower loss, optimized for 
high frequency use, designed for lower frequency use in mind, 
cables that do well all frequencies well from DC to highest frequencies. 
Propably most common type of cable used to carry video signals is 
RG-59 B/U type 75 ohm coaxial cable. Many other 75 ohm cioaxial cables 
work very all also. Generally if you take practically any 75 ohm coaxial 
cable, evenones designed for antenna installations only in mind, 
those carry baseband video signal at least somehow acceptably..
There are some data of some vieo coaxial cables at 
http://www.epanorama.net/documents/video/videocoax.html

Nowadays it has also been possible to tranfer vidoe over 
other kind of wire types, for example though CAT5 twisted pair 
wiring with pretty good quality and quite cheaply. 
Because the video is originally designed for 75 ohm coaxial 
cable, you need to use a balun adapter between your video equipment 
and the twisted pair wiring. This balun adapter adapts the unbalanced 
video signal to balaced signal that can travel through tw�sted 
pair wiring well and maybe also adapt the 75 ohm impedance to 100 ohm 
wiring (there are many adapters that do not adapt the impedance, 
they just rely that the 75 ohm - 100 ohm mismatch does not 
cause too severe problems to picture quality at normal uses, 
and usually this is true). 

> One thing you should look for in terms
> of quality of the signal is a balun (balanced-to-unbalanced transformer)
> on both ends. 

You need to use baluns for both audio and video signal if we are 
talking about the audio and video signals that ae used in consumer 
audio equipment, because both of those signal interfaces normally unbalanced 
on those equipment. 

Video and audio signals need different kind of balun transformer designs 
to work well. There are commercial adapters available that have 
baluns for both audio and vidoe signals built inside on compact unit. 
It is also possible to build such adapters yourself. I have done that. 
For audio signals the balun is notnign else than just 1:1 audio 
isolation transformer. For video signals either a current mode balun 
(common mode coil) or 75ohm-100ohm video isolation + impedance adaptation 
transformer (possibly with DC blocking capacitor in input) is used. 
Components are available for those tasks. 

> I have never seen one for SCART though. If your equipment
> has either S-Video or RCA outputs, use those instead. 

I have never seen blauns with SCART interface either. 
Generally with SCART interface you need to take an adapter that 
for example adapts the SCART connector signals to one video connector 
(S-video connector or composite video RCA) and stereo audio signals 
(RCA connectors). Then you just wire those to your adapter 
that adapts those signals to CAT5 wiring. 

> For short distances you could even go with unbalanced outputs. 
> That is, use simple direct - connection jacks. 

Very short distances whee cables are away from noise sources 
direct connection to jacks might work somehow.. 
With shielded CAT5 wiring usually somewhat better than with more 
common unshielded CAT5 wiring. In most practical 
real world systems this kind of direct connection leads to a system 
that picks lots of humming noise.. The unshielded CAT5 cabling resistance 
against outside noise lies with the combination of use of twisted pair 
wires and use of balanced signal sources+receivers. The twists and 
balanced signal source make the systme so that the noise picked 
by the wires is picked in such way that it gets canceled in the 
balanced receiver. If you connect this wires to unbalanced signal source 
and unbalanced signal receiver, those properties do not work, and the 
noise that gets picked up the wiring gets added to the noise 
(the twisted pair signal carrying wire picks less noise compared 
to untwisted wire, but considerably more noise than a properly 
shielded signal wire would pick in the same environment). 

> Besides, check with your equipment manuals - some of the
> outputs may already be balanced, so you can save some money by not buying
> one of the two baluns that are required to complete one circuit. one
> circuit represents one channel of audio or one baseband video signal.

That's one option if you have equipment that use this kind of interface. 
balanced audio signals are seen on many professional audio equipment 
and some expensive home hifi equipment. 
I have seen balanced video signal interfaces built in only on some 
CCTV cameras and related CCTV equipment... Practically other video 
equipment I have seen have used "normal" unbalanced video interface 
(connectors being usually RCA, BNC, SCART or S-video minidin). 


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