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75 ohm vs 50 ohm cables
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paw



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


PostLink    Posted: Mon Jan 07, 2008 10:07 pm    Post subject: 75 ohm vs 50 ohm cables Reply with quote


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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.

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Aubrey
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zaphod



Joined: 16 Jun 2006
Posts: 2002
Location: Cloverdale


PostLink    Posted: Mon Jan 07, 2008 10:13 pm    Post subject: Reply with quote

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

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Person99



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


PostLink    Posted: Mon Jan 07, 2008 11:45 pm    Post subject: Reply with quote

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!
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Dave

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....
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Joust



Joined: 05 May 2006
Posts: 2430
Location: Almonte, Ontario, Canada

TV/Projector: Marquee 8501LC


PostLink    Posted: Mon Jan 07, 2008 11:53 pm    Post subject: Reply with quote

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
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David_Web



Joined: 02 May 2007
Posts: 418
Location: Sweden


PostLink    Posted: Mon Jan 07, 2008 11:59 pm    Post subject: Reply with quote

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.
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lyd



Joined: 15 Sep 2007
Posts: 390
Location: Pending: Deerfield, WI


PostLink    Posted: Tue Jan 08, 2008 12:19 am    Post subject: Reply with quote

Joust wrote:
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


It's not that complex. We used to tweak our CB antennas for.... Oh hell nevermind, this is just making me feel old.

lyd
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Bucketfoot



Joined: 17 Mar 2006
Posts: 696
Location: Centennial, CO


PostLink    Posted: Tue Jan 08, 2008 12:50 am    Post subject: Reply with quote

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



Joined: 05 May 2006
Posts: 2430
Location: Almonte, Ontario, Canada

TV/Projector: Marquee 8501LC


PostLink    Posted: Tue Jan 08, 2008 1:48 am    Post subject: Reply with quote

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.
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Bucketfoot



Joined: 17 Mar 2006
Posts: 696
Location: Centennial, CO


PostLink    Posted: Tue Jan 08, 2008 2:41 am    Post subject: Reply with quote

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.
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Joust



Joined: 05 May 2006
Posts: 2430
Location: Almonte, Ontario, Canada

TV/Projector: Marquee 8501LC


PostLink    Posted: Tue Jan 08, 2008 2:51 am    Post subject: Reply with quote

Bucketfoot wrote:
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.


agreed...
you are a rare breed of practical engineer then Smile
I know lots of professional engineers who wave books and talk your ears off in meetings while scribbling on whiteboards. meanwhile, I fix things in the lab... Rolling Eyes
I use 75ohm cable with 50ohm BNC connectors. Embarassed but I DON'T wear socks and sandals Mr. Green
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Bucketfoot



Joined: 17 Mar 2006
Posts: 696
Location: Centennial, CO


PostLink    Posted: Tue Jan 08, 2008 2:58 am    Post subject: Reply with quote

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
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AnalogRocks
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Joined: 08 Mar 2006
Posts: 25503
Location: Toronto, Ontario, Canada

TV/Projector: Sony 1252Q, AMPRO 4000G


PostLink    Posted: Tue Jan 08, 2008 3:01 am    Post subject: Reply with quote

lyd wrote:


It's not that complex. We used to tweak our CB antennas for.... Oh hell nevermind, this is just making me feel old.

lyd


Breaker ONE-NINE for Lyd. You got yer ears on boy?

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Joust



Joined: 05 May 2006
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Location: Almonte, Ontario, Canada

TV/Projector: Marquee 8501LC


PostLink    Posted: Tue Jan 08, 2008 3:01 am    Post subject: Reply with quote

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
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Bucketfoot



Joined: 17 Mar 2006
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PostLink    Posted: Tue Jan 08, 2008 3:09 am    Post subject: Reply with quote

Joust wrote:
Ok then, we can be friends. I'll forgive you your degree.


Glad to hear it Smile

Heck, I've got several people who work for me that I've promoted to engineer positions despite the fact that they do NOT have engineering degrees. So real world experience and knowledge definitely mean a lot to me Thumbs Up

Two of them actually still refer to themselves (proudly I might add) as techs more often than as engineers.
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Joust



Joined: 05 May 2006
Posts: 2430
Location: Almonte, Ontario, Canada

TV/Projector: Marquee 8501LC


PostLink    Posted: Tue Jan 08, 2008 3:20 am    Post subject: Reply with quote

Bucketfoot wrote:

Glad to hear it Smile

Heck, I've got several people who work for me that I've promoted to engineer positions despite the fact that they do NOT have engineering degrees. So real world experience and knowledge definitely mean a lot to me Thumbs Up

Two of them actually still refer to themselves (proudly I might add) as techs more often than as engineers.


I don't even come close to qualifying for my own job.
Hey, maybe you could give me a job. Is it warm where you live? This Canadian weather sucks. I hate winter, although its above freezing now.
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Bucketfoot



Joined: 17 Mar 2006
Posts: 696
Location: Centennial, CO


PostLink    Posted: Tue Jan 08, 2008 3:40 am    Post subject: Reply with quote

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...
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Person99



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


PostLink    Posted: Tue Jan 08, 2008 3:22 pm    Post subject: Reply with quote

Joust wrote:
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.


I will bet you any amount of money that you want that he will not have horrible ghosting with those cables. In fact, I'd be willing to bet alot of money that if you took 6' 75 Ohm cables of your choice over there, you could not correctly identify which cable was which reliably in a 10-20 run single blind test.

Pete posted about a year ago a nice objective test that was done with equipment 50 Ohm vs. 75 Ohm. The conclusion? Didn't matter.

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Dave

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Joust



Joined: 05 May 2006
Posts: 2430
Location: Almonte, Ontario, Canada

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PostLink    Posted: Tue Jan 08, 2008 3:41 pm    Post subject: Reply with quote

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.
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9kids



Joined: 25 Jun 2007
Posts: 138
Location: Manassas, VA


PostLink    Posted: Tue Jan 08, 2008 6:22 pm    Post subject: Reply with quote

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.
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paw



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


PostLink    Posted: Fri Jan 11, 2008 3:58 am    Post subject: Reply with quote

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.
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