I have video output (from my receiver) in one room and a TV in
another. Right now I take the coax output and run a 50ft cord from one
room to the other. Unfortunately, that's the worst quality possible
and mono sound to boot (I might be mistaken about that, but I think
coax sound is mono). There's also the problem with electronic
interference. I've had to replace to 50' coax twice because the cord
seems to get damaged and suddenly I get video with interference lines
in it. (I don't know what the damage is because the cord looks fine.
But, replacing the cord has worked.)

I don't want to keep buying $50 coax cables twice a year. I also want
the higher quality video.

So, I would like to transmit RCA composite video and sound from one
room to the other over ethernet. I'll just run a dedicated ethernet
cable between the rooms, so it can use the cable in a dedicated way.

I haven't even bothered to try using three 50' rca cables because
they're hard to find, ridiculously expensive, and they're not shielded
anyway so I would almost certainly get lots of interference.

As a bonus, I would like to be able to transmit video from the RCA
output on my ATI Radeon 9700 in its hi-res form. I'm assuming that'll
work if I get something that can transmit composite video, but I just
wanted to throw that in there just in case that assumption is
mistaken. The output from that card is currently going into my
receiver and from there to the composite input on my vcr and from the
coax output on that to the other room. (My receiver doesn't have a
coax output, so that's why I use the vcr.)

I have found a source for all of "AV over CAT 5 (ethernet) cable" you
need... whether you want to send RCA Composite, S-Video, RGB
Component, or VGA from your PC Video Card, this site has the balun
connectors for you.  (The Balun connectors match the impedence so that
you don't introduce impedence mismatch issues.)  My goodness, they
even have video web servers if you are trying to cover a longer
distance via the Internet.

I also found an informative thread at the AVSForum that strongly urges
the use of Shielded CAT 5 (or at the very least CAT 5E cable). 
Finally, I have included an additional link to a source for Shielded
CAT 5 cable.

svideo.com:  List of AV to Ethernet Balun Connectors
http://www.svideo.com/videobalun1.html

svideo.com:  RCA Composite to Ethernet Balun Connectors
http://www.svideo.com/avbalun12.html

AVS Forum:  RGBHV over shielded Cat 5 success
http://www.avsforum.com/avs-vb/showthread.php?threadid=23850

L-COM.com:  Shielded CAT 5 cables at lengths up to 300 feet (Suggest 50 feet)
http://l-com.com/jump.jsp?lGen=category&itemID=59&itemType=CATEGORY&iMainCat=102&iSubCat=55&iSubSubCat=59

I hope that you find this information useful and easy to understand. 
If you have any questions about the information provided, please do
not hesitate to post a clarification request prior to rating the
answer.

Hi mxnmatch-ga,

   Actually coaxial cable is more reliable than other types of cable(this
being the reason, cable companies depend on it). The issue seems not to be the
cable, but your tranmission signal. You could use a RG-6 coax(used for
satellite reception) instead of the RG-59U if you need better shielding and
lower distribution loss. Actually most vcr's are equipped with
a "cheap" rf modulator, which should not be used directly for distribution.
If you really want to use it, use with an inline vhf/uhf amplifier.
I would sincerely advise you to invest on a quality RF modulator(such as 
one from channel vision, and use the UHF, instead of the default 3-4 channel 
VHF output).  The lower cost option is definitely the use of a 10-15dB 
inline/distribution amplifier fed by your vcr rf output.
You could also try using S-Video output(with a suiable S-Video signal 
amplifier-should be available from Rshack) since it could give more vivid
colors, less color bleeding(esp. if your target TV does not have a good Y/C
comb filter-3D/digital comb. filter preferable).  If you have component outputs
available from your DVD/VCR/PC, you could think about using an ethernet cable
with appropriate impedence matching.  But this requires your target TV to 
have a component video input too.

Let me know, if you have any questions.

Regards
ldavinci-ga

mxnmatch-ga:

Sorry to hear about your problems with the coax; ldavinci's comment is
right on the money.

One other option you can consider is using a wireless video
sender/receiver combination that operates in the 2.4 GHz range. The
problem with this, of course, is that there are a lot of other devices
that you may use that also operate in this range (such as cordless
phones, microwave ovens, WiFi networks). If, however, you do not have
any of these other devices that operate in the vicinity of your source
and your TV, and as long as you are able to provide S-Video and/or RCA
outputs/inputs at both ends, then this is one more option to consider.

Regards,

aht-ga
Google Answers Researcher