FORUMS: list search recent posts

50ohm connectors for very Short run HD-SDI

COW Forums : Broadcasting

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
Gavin Newman
50ohm connectors for very Short run HD-SDI
on Mar 30, 2010 at 12:38:09 pm

Hi there,
Question for you digital video techies out there.
I'm currently fitting a full HD video camera with HD-SDI output into an underwater vehicle.
I know that the normal specification for HD-SDI video is 75ohm coax and connectors but the only readily available deep water subsea Coaxial connectors I can find are 50ohm. So my question is would I have any quality issues using 50ohm coax and connectors for the very short run from the camera pod to our electronics pod where the HD-SDI is converted to fiber. The distance would be approx 1 metre using 2 hi grade professionally terminated 50ohm coaxial connectors and cable.

The system is used for top end HD natural history filming so I cant afford to compromise quality.
Any thoughts on this would be gratefully received.
Thanks,

gavin


http://www.gavinnewman.com
http://www.nemo-rov.com


Return to posts index

Richard Crowley
Re: 50ohm connectors for very Short run HD-SDI
on Mar 30, 2010 at 3:07:28 pm

I suspect that over such a short distance, using 50-ohm connectors will not make a big enough difference to threaten the integrity of the signal. If you want to be sure about it, you would use an oscilloscope to probe the waveform going into the converter. See how distorted the "eye pattern" is. Compare to the same connection without the 50-ohm connectors.


Return to posts index

Bob Zelin
Re: 50ohm connectors for very Short run HD-SDI
on Mar 31, 2010 at 7:55:39 pm


I know that the normal specification for HD-SDI video is 75ohm coax and connectors but the only readily available deep water subsea Coaxial connectors I can find are 50ohm. So my question is would I have any quality issues using 50ohm coax and connectors for the very short run from the camera pod to our electronics pod where the HD-SDI is converted to fiber. The distance would be approx 1 metre using 2 hi grade professionally terminated 50ohm coaxial connectors and cable.


REPLY -
you can absolutely use 50 ohm connectors for a short run for your "hi end" application. But with that said, what kind of nonsense does this mean - " but the only readily available deep water subsea coaxial connectors I can find are 50ohm". THERE IS NO SUCH THING. What on earth is a deep water subsea coaxial connector? I never ever ever heard that. That is like saying that you need a special BNC for the space shuttle to work at odd gravitational force conditions. These do not exist, neither do "subsea coaxial connectors" exist. Talk about nonsense in marketing.

Coax cable comes in "indoor", outdoor (designed for submersion or dirt), and plenum (fireproof teflon coated cable). That's it. Connectors are connectors. No one is making a BNC that is immune to salt water exposure - certainly not any of the major brands that make HD connectors.

Bob Zelin




Return to posts index


Gavin Newman
Re: 50ohm connectors for very Short run HD-SDI
on Mar 31, 2010 at 8:46:17 pm

Hi Bob and Richard,

Thanks for the replies to my query and confirming my thinking.
Bob. The world of subsea cables and connectors is obviously a new world for you as indeed such things do exist. Maybe you misunderstood my application.
I have an underwater camera pod with HD camera in it and a seperate pod with fiber transmission systems etc. The two pods are fitted to a deep water robotic vehicle
http://www.nemo-rov.com
So the connectors and cable are exposed to the full pressure of the ocean at up to 500m depth. Normal BNC connectors would just flood and normal coax cable would crush. Hence the existence of subsea connectors. All the power and signal connectors on our ROV are capable of withstanding full ocean depth pressure of 6000m sea water.
For your interest details of the subsea coaxial connectors we're proposing to use are:
http://www.dgo.com/products_coax.cfm

The nonsense is not in the marketing but in the cost!

Thanks again,

gavin


Return to posts index

maurice jansen
Re: 50ohm connectors for very Short run HD-SDI
on Mar 31, 2010 at 10:28:56 pm

hi gavin

although the cow is a great source for info like this.
if i look on your site i see you already have a very good contact.
named BBC.

i understand it's one of your client's but they do know a lot!!!

wondering what cable you use. all "normal" HD cable's do have a foam
dielectric (based on the isolation power of the air in it) i guess this cannot be used in a nautic situation like your's. the air in the foam has a pressure so i guess the cable will implode in deep sea changing impedance.
what dielectric is used ???
how does it behave on normal height levels. does the performance increase while the rig is lowered???

interesting stuff really

grt
Maurice













People saying they don't make mistake's often make nothing at all!


Return to posts index

Bob Zelin
Re: 50ohm connectors for very Short run HD-SDI
on Mar 31, 2010 at 10:29:48 pm

These are DGO connectors, which have threaded ends. I have never seen these connectors, and I have no clue as to what professional video gear uses these - I have never seen a camera or monitor in my life that accepts these connectors.

So, if you have a camera, monitor, and VTR, or digital storage device that uses DGO connectors, then sure - it's a short run, so there would be no issue (you could use an RCA audio cable to run HD-SDI for one or two feet). I have worked with lots of guys that have underwater housings for their cameras, but I have never seen a DGO connector - nor have I ever seen a monitor, camera, or digital recording device that uses DGO.

Bob Zelin




Return to posts index


Richard Crowley
Re: 50ohm connectors for very Short run HD-SDI
on Apr 1, 2010 at 5:14:04 am

It would appear that Mr. Newman is talking about the BULKHEAD connectors that go through the outer shell of the containment enclosure, NOT about the connectors inside the enclosure that connect to the actual equipment. This is standard practice for extreme environmental system design. And the bottom of the ocean is probably more extreme than outer space.


Return to posts index

Gavin Newman
Re: 50ohm connectors for very Short run HD-SDI
on Apr 1, 2010 at 8:11:18 am

Hi there,

Yes Richard thats exactly the situation.
The DGO connectors are a proprietary design of connector designed to pass the Coax signal through the housing pressure bulkhead. Once inside the cabling reverts to the common BNC for connection to the video devices.

In response to your question Maurice about the subsea cables.
Yes its the air in the standard cable that makes it unsuitable for subsea use. The cable would get crushed at depth and the isolation gap lost.
The subsea coax we will use is a 6mm diameter cable RG59 Coax. The dielectric is made of Polyethylene and the cable is jacketed with Polyurethane.

All the best
gavin


http://www.gavinnewman.com
http://www.nemo-rov.com



Return to posts index

<< PREVIOUS   •   VIEW ALL   •   PRINT   •   NEXT >>
© 2017 CreativeCOW.net All Rights Reserved
[TOP]