School me on video cables

M

mikezip007

Active member
I do a lot of work as a 1st AC and I like to have a bin of spare 25' video cables that I use on jobs.

I have Component HD, HDMI, S-Video, DVI and VGA.

What about SDI/ HDSDI? We use these cables a lot but recently I have become confused by them. They may sound like basic questions but I would like to clear them up.

1. Does HDSDI have the ability to carry both an HD signal and an SD signal?

2. Does SDI only carry SD? No HD?

3. Does HDSDI and SDI both have BNC connectors?

4. Are there any cosmetic differences between the two cables?

Thank you!
 
1. Yes.
2. As far as I know, they are the same. Buy good quality, and you'll be fine.
3. Yes, typically.
4. Not that I am aware of.

You might consider visiting the page of a cable maker like Belden who offer wonderful guides for these things.
 
In the most basic terms, an SDI cable is simply a 75 Ohm Coaxial cable, terminated in BNC ends. Any standard composite video cord should have the same basic specs. In fact, one of the three of your component video cables, a composite video cable, an HDSDI, and an SDI cable could all be the same cable, in theory!

However, where HDSDI and SDI come into play is that instead of an analog signal being pushed down the cable, a very very large quantity of data is being sent (in some cases for 4:4:4 HD links, 3 Gigabits per second!). So, you need to get a good quality cable for SDI. Short runs matter less. I know I'm currently working with the cheapest BNC cables I could find for an SDI setup going from a Blackmagic card to a converter box. It works just fine for the 3 foot run that it needs to, but I don't know if I would use that same cable over 50 or 100 feet. A cheap cable on an analog signal could give you a less-than-optimal picture, but a cable that can't hold the data transmission for SDI or HDSDI will simply show no picture. That's certainly a situation you'd never want to be in.

In short, just make sure you've got good cables. 75Ohm and BNC are the biggest real requirements. Beyond that look for the cables to have HDSDI compatibility listed. If it'll cary HDSDI, it should carry SDI with no problem.
 
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Thank you for this. I forgot to mention that this will be cable running from camera to monitor up to 50' and maybe 100' away.

I had a situation last week where we needed picture up fast and I thought I had bought correct cable but when I plugged it in... No signal so I had to run back to the studio's cart to get their tried and true cable.

Thanks for the explanations
 
Simply put, SDI of any kind is simply "Serial Digital Interface". It uses the same 75-ohm cable that you can send baseband (composite) old-fashioned SD video through. There is generally no cosmetic/visible difference. For short runs (up to maybe 10ft, 3m), you can likely use any old 75-ohm coax cable with BNC connectors.

But SDI uses a much higher bandwidth than baseband video (SD or HD), and HDSDI uses an even higher bandwidth. So SDI is more fussy about the quality and length of the cable. You generally can't plan on sending SDI (and especially not HDSDI) over the same distances that we did with baseband/composite video.

You may have had a cable that would have been OK for baseband/composite video, but had too much high-frequency loss to adequately transmit the digital signal to the other end and keep it in good enough shape for the receiving end to decode it properly. Digital equipment tends to be "binary" (pun is semi-intentional). If it can adequately decode the digital stream you will see very good video. But if the data falls below its quality threshold, then it doesn't even try.
 
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