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Discussion Starter · #1 ·
I'm looking for breakout cable for my LT150 and came across avcable.com which uses Gepco cables. Here's their description from an email sent to me:

The cable used in this product is produced by Gepco International. It is considered a miniature cable (necessary to fit into the HD15), but it specs out better than RG59. It has a stranded center conductor for flexibility and a double shield (braid + foil). The connectors used are Canare impedance matched 75 ohm type video RCAs.

I checked the Gepco website and here are more detailed specs: http://www.gepco.com/products/cable/...nirgbsnake.htm

Will these work out fine for a 45-foot run?? How will they compare to something from Canare??
 

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You might look into RG174/U. 50 ohm, 0.1" OD.

Pretty cheap at the surplus places, especially with BNC connectors attached. Don't worry about the apparent impedance mismatches- the manufacturers don't.


apg
 

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"A mismatch of .4 inches long (the connector for example) will not matter. A mismatch of 5 feet long (a cable) will DESTROY your image."


(Some real-life examples follow- long and a bit technical...)

Not necessarily true. Not even probably true. Try it some time- I've been running detector, video and RF signals on everything from RG11/U to 125 ohm PM cable for years now, terminated and unterminated, into the GHz region. On large signals, as in grid drive for an RCA 4648, yes, you want matched connectors and cables into a properly terminated load. The SWR should be low to minimize peak voltages at nodes, and to efficiently transfer power to the load.

With small signals, especially unterminated ones, on long cable runs (+100'), the variation in signal level at the load end is more dependant on source impedance. Load impedance typically is variable with frequency, and better monitors actually have a switchable Hi-Z/75 ohm load. Scopes may have a switchable Hi-Z/50 ohm load. Professional video uses what are known as line drivers for very long (~1,000') cable runs- thase are amps designed with a very low source impedance.

On very long runs, especially with VHF and above frequencies, you _will_ get attenuation with coaxial cable, and typically, the smaller the cable, the greater the attenuation. Again, much more important with large signals...


So what does this all mean for a breakout cable? Not a lot. Unless you are using _really_ cruddy cable, (The brown/gray phono cable from old, cheap turntables spring to mind...), for the distance of a few feet, cable impedance shouldn't matter. Neither should connector impedance. _But_, if you actually play around with different lengths of Beldon cable with different kinds of quality connectors, and actually do see a difference- something else is going wrong, and I would immediately suspect that either the source or load impedances aren't anywhere near what they are spec'd to be.

apg


BTW, it might be interesting, with a 45' run, to stick with BNC from projector to the component out, with a BNC T's at source and load end, followed by a BNC to phono component adapter. One could try various terminators to see if there is any difference, other than in signal level, with the image. Since I've got drawerfuls of this kind of stuff here, I volunteer to give it a try.

 

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the impedance switch that you see on a monitor is for whether or not the circuit is considered terminated at that point. It is for a termination of the loop through on the monitor, not for a determination of what kind of cable you are using. The switchable impedance on the input side of a scope is so that you can measure something and not intrude upon the signal at the same time. FOr example, measure a video signal while watching it on screen at the same time. As far as your technical description, i am assuming that you are a smartie pants interlekchewal as you are in berkley... YOu are correct in all elements.


But in the practical elements of a video system if you do not match impedances you get ghosting. PERIOD. i have visited many system whre even a bad termination threw off the impedance enough to cause a mismatch and the end result is a ghost. When putting video systems together, care should be taken to always use 75 ohm cable. If i can find some 50 ohm stuff i will do some tests and show it on screen, and take pictures and post them.
 

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Discussion Starter · #5 ·
So what does it mean to me?? I can get a similar setup using Canare V3_3CFB cable for a couple bucks cheaper. Which one should I go with - Canare or Gepco??
 

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"So what does it mean to me?? I can get a similar setup using Canare V3_3CFB cable for a couple bucks cheaper. Which

one should I go with - Canare or Gepco??"


Either will work fine- take your choice. What I was trying to point out in my original reply is that you can get equivalent performance, for this application, for a _lot_ less money. And beware of snake oil...

BTW, you might take a shot at making up the breakout cable assembly yourself. It's pretty easy, doesn't take long, and fits in with the growing LT150 DIY mystique.

apg
 

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Discussion Starter · #7 ·
Thanks for the information......

Another quick question: what termination do I need for the VGA end of the cable - male or female??
 

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If i might... i would disagree with that statement about mismatches. Manufacturers don't care (and neither should you, i can prove it if you like) about 50 ohm CONNECTORS... they do care very much about 50 ohm cable. A mismatch of .4 inches long (the connector for example) will not matter. A mismatch of 5 feet long (a cable) will DESTROY your image.
 
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