FORUMS: list search recent posts

SDI cables

COW Forums : Apple Final Cut Pro Legacy

<< PREVIOUS   •   FAQ   •   VIEW ALL   •   PRINT   •   NEXT >>
Scott Davis
SDI cables
on Aug 15, 2010 at 2:42:28 pm

Quick question. What is the difference between an "SDI" cable and a high quality "regular" BNC cable?


Return to posts index

Rafael Amador
Re: SDI cables
on Aug 15, 2010 at 3:00:41 pm

Hi Scott,
Basically is the same.
BNC is the connector. The cable is call coaxial.
For SDI streaming they need to meet certain specs.
Rafael

http://www.nagavideo.com


Return to posts index

Mark Raudonis
Re: SDI cables
on Aug 15, 2010 at 3:31:31 pm

Over a short run (less than 20 feet) you will probably NOT notice a difference. Over a longer run,
the HD-SDI cable will have less signal attenuation, and therefore be the correct choice.

There is of course a price difference, with the SDI cable costing more.


Unless you know how to analyze a signal on a scope, you'll never see the difference.

Mark



Return to posts index


mark jenner
Re: SDI cables
on Aug 15, 2010 at 3:51:05 pm

none they are the same thats what we use BNC to BNC

regards

mark j


Return to posts index

Chris Wiggles
Re: SDI cables
on Aug 15, 2010 at 9:24:43 pm

Really, there is no such thing as an SDI cable. It's just a marketing term applied to a particular cable construction.

You need to use high-quality 75ohm solid-copper coaxial cabling for SDI, with high-quality BNC connectors.

There are zillions of types of coaxial cables out there in various sizes and various qualities and various impedances.

What you require quality-wise and gauge-wise will depend on how far you're going, and the bandwidth of the signals you're pushing.

Probably the most common RG6-type 75ohm coaxial cable is the Belden 1694a. You can use this for all kinds of tasks that require 75ohm cabling such as analog video, SDI, digital audio, analog audio(which doesn't require 75ohm, but this is still an excellent cable, overkill really). Raw cable stock is not very expensive, frankly. It adds up if you're pulling miles of it, but given your question it doesn't sound like that's the case here.

And as stated before, BNC is a connector type, not a cable type.

You can purchase raw cable stock and ends and tools to make your own cables. Crimping tools are not particularly cheap, so unless you have several cables to make, it's more expensive than buying a finished cable. You can buy terminated broadcast-grade 75ohm coax cables such as Belden 1694a or comparable Canare and other excellent cables from places like BlueJeansCable.com or Ram electronics. I'm sure there are other stores that assemble finished cables using these cable stocks as well.

Stick to tried-and-true brands and cable stocks with thorough specifications for critical tasks.

Regards,
Chris


Return to posts index

Richard Boyd
Re: SDI cables
on Aug 15, 2010 at 10:42:01 pm

Actually there is a difference between regular 75 ohm video cable and cable rated for HD-SDI. HD rated cable has a lower inherent capacitance which improves its bandwidth substantially, especially over long distances - say 25 feet or more. Also, BNC connectors rated for HD-SDI are manufactured to higher tolerances and have improved bandwidth specifications.

Belden 1694 is a fine choice if you are going to stay analog or Standard Def, at least for video, but it would be completely un-useable for professional balanced audio signals since it has only a single inner conductor.


Return to posts index


Chris Wiggles
Re: SDI cables
on Aug 15, 2010 at 11:15:23 pm

[Richard Boyd] "
Actually there is a difference between regular 75 ohm video cable and cable rated for HD-SDI. HD rated cable has a lower inherent capacitance which improves its bandwidth substantially, especially over long distances - say 25 feet or more. Also, BNC connectors rated for HD-SDI are manufactured to higher tolerances and have improved bandwidth specifications.


Huh? Low capacitance is actually more important for digital signals than analog video. And it's basically inherent to the impedance of the cable, so you can't really change it anyway. Pretty much all 75ohm coax will have very similar capacitance because of the physical design because Impedance and Capacitance in a cable are inherently linked. You can't make a 75ohm cables with wildly different capacitances. The only thing that can have an impact on the capacitance outside of the physical structure of the cable which is basically fixed already by the need to have 75ohm cable, is the dielectric. And that has minor impact.

If you look through Belden's catalog for instance, all of their 75ohm coax are all listed for digital and analog video, because you can use them no problem for both. And nearly all of them have capacitance right around 16pf/F as one expects inherent to a 75ohm cable. Some are a little higher depending on the dielectric, but not wildly so.


There are all kinds of different BNC connectors. High-bandwidth BNCs can be just as important for analog video. Just depends what kind of bandwidth you require. But fundamentally, 75ohm coax is 75ohm coax. There's nothing different in any fundamental way between video and SDI in that sense. There can be application differences, for instance if you're running very high-bandwidth HD-SDI or whatnot, you'll need really good coax, particularly over distance. If you're running analog NTSC or something, then your bandwidth is very low and you can get away with inferior cable. But if you have the best coax possible for your most critical and highest-bandwidth application, with appropriate connectors, etc, you can use that for anything else too that requires 75ohm coax, and some things like unabalanced audio as well that don't.

There's nothing magically special about SDI. It just goes over coax. Good coax.


Belden 1694 is a fine choice if you are going to stay analog or Standard Def, at least for video, but it would be completely un-useable for professional balanced audio signals since it has only a single inner conductor."


Well obviously it couldn't be used for balanced audio. It's great for unbalanced analog audio though.

And I don't know if you are implying here that 1694a is somehow not preferred for digital when you say "stay analog or standard def" since it is the dominant video coax for digital and analog video in professional applications within the confines of planet: earth. It's the standard. It's used everywhere for SDI and for analog video. That's because it's an exceptional coax and is entirely appropriate for both of these tasks, because they're really the same task. I don't understand where you get the idea that different coax is required???

Regards,
Chris


Return to posts index

Michael Gissing
Re: SDI cables
on Aug 15, 2010 at 11:38:45 pm

I should point out that running digital audio over unbalanced coax is perfectly fine, but the spec for AES/EBU digital audio is for 50 ohm cable and connectors. Over short runs, this difference is not an issue.


Return to posts index

Chris Wiggles
Re: SDI cables
on Aug 16, 2010 at 1:16:39 am

[Michael Gissing] "
I should point out that running digital audio over unbalanced coax is perfectly fine, but the spec for AES/EBU digital audio is for 50 ohm cable and connectors. Over short runs, this difference is not an issue."


Hmm? Which AES/EBU standard is this? AES3, which is the norm, is 110ohm twisted pair for balanced runs, and 75ohm coax for unbalanced runs. And then there is an optical standard as well. I am not aware of any 50ohm AES/EBU standard. I have never heard of such a thing.

These are some decent overviews:

http://broadcastengineering.com/mag/broadcasting_aesebu_digital_audio/

And:

http://en.wikipedia.org/wiki/AES/EBU

50ohm connections/cabling are not common(I'm not aware of any at all actually, so perhaps that's an understatement) for audio or video standards. They're usually relegated to antennas and radio transmitters and things like that, and some looong defunct coaxial ethernet standards.

Regards,
Chris


Return to posts index


Michael Gissing
Re: SDI cables
on Aug 16, 2010 at 1:20:59 am

Apologies. Yes digital audio is 110 ohm. I was getting confused with microwave gear which my father used which was 50 ohm coax


Return to posts index

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