CurtPalme.com Home Theater Forum

paw · Joined: 08 Mar 2006 Posts: 1176 Location: Arvada, CO

I just purchased some 6' RGBHV cables from Monoprice. After closer inspection, they are 50 ohm cables. What kind and how much impact will this have on my PQ? I'll be running 720p 60KHz, 1080i 60KHz and 960p 60KHz.

zaphod · Joined: 16 Jun 2006 Posts: 2002 Location: Cloverdale

http://sosnick.uchicago.edu/BNC_50_75.html
http://www.l-com.com/content/Article.aspx?Type=N&ID=97

Person99 · Joined: 09 Mar 2006 Posts: 4901 Location: Flower Mound, TX

If anyone can tell the difference of a 50 Ohm and 75 Ohm on a 6' run in a double blind test, I want to see that!

**Joust** · Posted: Mon Jan 07, 2008 11:53 pm Post subject:

Impedance mismatch will cause VSWR problems.
In extreme cases you can see it.
You can definitely measure it with equipment.

The weird thing about VSWR is that if you cut the cable at the appropriate length, you can eliminate any evidence of the mismatch. Determining that length involves a lot of complex math OR a lot of trial and error. Smile

David_Web · Joined: 02 May 2007 Posts: 418 Location: Sweden

Anywhere from none to nothing. Should not matter that much unles the cables are very long. And in that case unless I remember wrong it will only benefit you. The only thing can be if your line driver is weak but then that would be a far bigger problem.

I am not sure if you can get some artifacts but I don't think so. Is far as I can read it should undetectably softer due to insignificantly longer rise time but should in the meantime be less sensitive to HF noise.

But don't quote me on that. You would have been scared by the way I hooked up my Barco the other day.

*edit* VSWR?
Interesting topic and very useful for the future.

lyd · Joined: 15 Sep 2007 Posts: 390 Location: Lake Mills, Wi

Bucketfoot · Joined: 17 Mar 2006 Posts: 698 Location: Centennial, CO

You will not see anything at these frequencies on a 6' cable.

**Joust** · Posted: Tue Jan 08, 2008 1:48 am Post subject:

VSWR = Voltage Standing Wave Ratio. basic transmission line theory.
and....It has nothing to do with the finite length. it is entirely possible that at 6feet you could have a horrible signal with lots of ghosting. then cut off 6inches of cable and have it work perfectly.

my recomendation is to try it and if it works ok for you, don't worry about theory. If you have something that doesn't look right, suspect the cable.

Bucketfoot · Joined: 17 Mar 2006 Posts: 698 Location: Centennial, CO

As someone with nearly 20 years experience as an RF/EMI & Audio/Video Engineer, I understand VSWR quite well.

The bottom line is that unless it is an absolutely piss poor cable, he will not see anything at these frequencies. And if he does, it will be because of the piss poor cable or connectors, not VSWR.

**Joust** · Posted: Tue Jan 08, 2008 2:51 am Post subject:

Bucketfoot · Joined: 17 Mar 2006 Posts: 698 Location: Centennial, CO

I used to do a lot of EMI compliance work and learned that theory has very little practical use in that world. Tidal patterns, the phase of the moon and sunspots seemed to have more impact than anything Mr. Green

I worked on many projects were the results were the exact opposite of what theory said Smile

I think my practicality comes from the fact that I worked as a technician first. I have found that the majority of the good engineers I know were either techs or electronic geeks first Thumbs Up

**AnalogRocks** · Posted: Tue Jan 08, 2008 3:01 am Post subject:

**Joust** · Posted: Tue Jan 08, 2008 3:01 am Post subject:

Ok then, we can be friends. I'll forgive you your degree.
I'm still working on Kal but he is buying me with beer so he has that going for him at least Smile

Bucketfoot · Joined: 17 Mar 2006 Posts: 698 Location: Centennial, CO

**Joust** · Posted: Tue Jan 08, 2008 3:20 am Post subject:

Bucketfoot · Joined: 17 Mar 2006 Posts: 698 Location: Centennial, CO

I've been to Toronto several times (including in the winter) and haven't found it to really be any worse than Colorado. It looks like you are closer to Ottawa though, so I don't know if it is quite a bit colder there or not.

The Denver area isn't too bad. For the most part the snow doesn't stick around for more than a couple of days before melting.

Sorry, getting way off topic now...

Person99 · Joined: 09 Mar 2006 Posts: 4901 Location: Flower Mound, TX

**Joust** · Posted: Tue Jan 08, 2008 3:41 pm Post subject:

Dave, I said it is theoretically possible. Mostly poking fun at book smart type engineers. Practically speaking, I agree that 10 chances to one that no effect will be noticed.
I was bored last night and looking for a debate.

in reality, Coax is mainly designed for radio frequencies and the effects I described really only come into play up in the Megahertz range. We use them down below 250Khz. To that freq, its just a piece of wire.

9kids · Joined: 25 Jun 2007 Posts: 138 Location: Manassas, VA

Well if I were you I would demand that Monoprice send me the correct cables. If you order RGBHV cables they should be 75 Ohm and they know that. If you don't want to have to wait to send them back and then wait until you get the correct ones, then tell them they should ship the correct ones first and then you will ship the wrong ones back. That way you can use them until you get the new ones. Everyone is probable right you will never see the difference, but if you order one thing and they send another, they should make it right.

paw · Joined: 08 Mar 2006 Posts: 1176 Location: Arvada, CO

OK! The cables arrived. They are 75 ohm. The picture of the connector on their website was wrong and nothing was mentioned of the impedence in the product description. Thanks for the info and discussion. It was helpful.