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SDI Basics

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Nick Griffin
SDI Basics
on Feb 16, 2009 at 3:32:58 pm

Here's a few dumb questions from a non-techie:

Does HDSDI require anything other than standard 75 ohm BNC cables? For short runs is there any difference? And is there any need/benefit to using precision terminators versus standard ones?

Thanx, guys!


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Baz Leffler
Re: SDI Basics
on Feb 16, 2009 at 11:24:46 pm

"Does HDSDI require anything other than standard 75 ohm BNC cables?"

- 75ohm video cables will work but try and use double shielded (this is a broadcast forum so that should be taken as the norm...)

"For short runs is there any difference?"

- short runs would be fine with any 75ohm eg patch cables

"And is there any need/benefit to using precision terminators versus standard ones?"

- normal terminators would be fine.

Note - runs over 20 meters should use more precision cable. We use a cable type 75AY101 so google it.

Baz



What would I do without the 'UNDO' button!!!!


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Bob Zelin
Re: SDI Basics
on Feb 17, 2009 at 4:13:29 am

"Does HDSDI require anything other than standard 75 ohm BNC cables?"

- 75ohm video cables will work but try and use double shielded (this is a broadcast forum so that should be taken as the norm...)

REPLY - for a short run, anything will work. You are sending a hi frequency square wave, not an analog video signal. This SDI or HD-SDI signal will "distort" after a long distance. I have successfully used generic RG-59 up to 100' and "got away" with it, but since HD-SDI video cable is so common today, you might as well stick with the right stuff. The "thickness" of the cable means nothing - Baz is correct that true 75 ohm impediance cable that is double shielded is what you want to avoid trouble.


"For short runs is there any difference?"

- short runs would be fine with any 75ohm eg patch cables

REPLY - unfortunately, I see people use crap coax cable for SDI all the time. It works fine, and it's used all the time. 25' runs work without issue. With that said, I ALWAYS use broadcast video cable that is rated for HD-SDI use.


"And is there any need/benefit to using precision terminators versus standard ones?"

- normal terminators would be fine.

REPLY - again, Baz is correct. You are not trying to match a specific analog voltage here (like 700mV for analog video on an analog waveform monitor). Most SDI and HD-SDI devices auto terminate today, as well.


Note - runs over 20 meters should use more precision cable. We use a cable type 75AY101 so google it.

REPLY - I never heard of 75AY101, and I googled it, and I could not find it.

There are many fine brands of HD-SDI cable. These include Belden, Canare, West Penn, Clark, Gepco, and CommScope. HD-SDI cable is available in many sizes (thicknesses) - everything from super skinny (DT179) to the big fat stuff (RG-6 size, like your cable company uses).

I try to stick with normal size video cable (RG-59 style). This is Belden 1505A, Canare L-4CFB, Gepco VPM-2000. Most manufacturers and catalogs will cross reference the Belden #'s, as they are the largest manufacturer of this crap. The RG-6 thick stuff is Belden 1694A, but I never use it becuase it is too rigid, and difficult to deal with in larger installations. It only gives you an extra 60' of cable run beyond what Belden 1505A can do (which is about 300'). The skinnier cable is Belden 1855A, and this is widely used in HD trucks around the country. This is generically refered to as RG-174U style cable (but it's double shielded, and qualified for HD bandwitth). This thinner cable can be run over 200' without issue for HD-SDI.

The moral of the story is that cheap RG-59 coax cable will simply stop working with an HD-SDI signal at around 200'. I had a dramatic experience running cheap coax (belden 8241) about 100' to a patch panel, and then back to the room, about another 100'. I though I made a bad cable, but the HD-SDI signal simply could not travel that distance without being "reclocked" by something, and the patch bay was not going to do that, so the signal just failed. I put in the correct cable, and VOILA - all of a sudden, everyting was working.

So, if you are working with a few 20' runs, feel free to use your existing cheapo cable. It will work just fine.

Bob Zelin





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Nick Griffin
Re: SDI Basics - THANX!
on Feb 17, 2009 at 8:50:05 pm

Bob & Baz -
Thanks, guys! Truly comprehensive answers. You're the best!


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maurice jansen
Re: SDI Basics
on Feb 17, 2009 at 9:02:34 pm

hi there.

next to all the electrical things there is a other thing to keep in mind.
as you might have seen , on most product's the SDI output and input's are from a other type then the composite one's(for example sony). this has to do with cost's and influence of the connector type on the signal. the (HD)SDI connetor's do not have a silcon/plastic ring round the inner conductor. this is because this ring has a negative influence on the system impedance. the bad thing about this electrical better connector's is that they are more fragile. wrong connector's on cable's (inner conductor with a too large diameter) kill these connector's so good cable and connectors are also a must on short cableruns to physical protect your gear

great
Maurice



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