Tag Archives: sdi

There Is No Such Thing As A “12G” SDI Cable.

The way the coax cables used for SDI works is very different to the way an HDMI cable works. HDMI cables are indeed constructed quite differently between early HDMI 1.0 – 1.4 classes and the more recent 2.0+ classes. So with HDMI you will find that an old, early version HDMI cable won’t work with the latest standards.

SDI cables are nothing fancy.

SDI uses nothing more sophisticated than a single core coax cable that is no different in it’s basic design, construction and mode of operation to an ordinary TV aerial down lead. It is a very simple type of cable and really nothing fancy.

Frequency matters.

The SDI signal is very high frequency; in effect it is a radio signal. From a cabling point of view the ONLY difference between the original SDI standard and the latest standards is the frequency. The way the cable works is no different between the original SDI standard and the latest and a camera or monitor has no way of telling or knowing what type of cable you are using.

Frequency is important because the higher the frequency, the more lossy ANY coax cable will become (leaky kind of describes what’s going on). Low quality cable – more signal leaks out, high quality cable less leaks out so the signal will go further.

But even the very earliest SDI cables were normally made using good quality very low loss coax. These original SDI cables are perfectly capable of carrying the higher frequencies used by 12G SDI. BUT over very long lengths there will be more loss at 12G than at 1.5G.  

It’s not the “G” that counts, it’s the quality.

So really when looking for SDI cables, the question isn’t – “is it 12G” the question should be “what are the cable losses” or more simply “is it a good quality cable”. There are plenty of original SDI cables that can be used at 50m at 12G without issue. At the same time I have also seen cables marketed as “12G” that are nowhere near as well screened, with much higher losses, that barely work at 10m.

Just as important as the cable losses is the construction. Have the connectors been fitted correctly? Are the connectors correctly sized for the cable that’s being used, has the crimping or soldering been done well? Most coax cable failures are due to poor connector assembly or the use of low-quality connectors.

Impedance Matter.

One other thing to watch for is the cable impedance. SDI cables should be made using 75 ohm impedance cable and connectors. Radio cables for radio communications normally use 50 ohm cables and connectors and the two are not really compatible. But often cheaper cables sold for SDI and video applications may be made using 50 ohm parts as often these are cheaper. These cables will fit and more often than not they appear to work. BUT the pins in the BNC plugs are a different size and this can result in intermittent connections and over time can even damage the connectors on cameras and monitors etc. So do make sure your cables really are 75 ohm.

In the real world:

For most shorter cables, up to 5m cable losses are rarely an issue unless the cable is of particularly low quality or badly made. For between 5m and 10m you should avoid the very thin coax cables as the losses become more significant. Above 10m use only low loss cables with good quality screening. A cable sold as a “12G” cable should indicate good quality low loss cable, but it is not a guarantee. And the vast majority of well-constructed normal SDI cables will work just as well unless you want extremely long runs in which case you need ultra-low loss cable.

Why you need to sort out your post production monitoring!

One of THE most common complaints I hear, day in, day out, is: There is banding in my footage.

Before you start complaining about banding or other image artefacts ask yourself one very simply, but very important question: Do I know EXACTLY what is happening to my footage within my computer or playback system? As an example, editing on a computer your footage will be starting of at it’s native bit depth. It might then be converted to a different bit depth by the edit or grading software for manipulation. Then that new bit depth signal is passed to the computers graphic card to be displayed. At this point it will possibly be converted to another bit depth as it passes through the GPU and then it will be converted to the bit depth of the computers desktop display. From there you might be passing it down an HDMI cable where another bit depth change might be needed before it finally arrives at your monitor at goodness knows what bit depth.

The two images below are very telling. The first is a photo of a high end TV connected to my MacBook ProRetina via HDMI playing back a 10 bit ProRes file in HD. The bottom picture is exactly the same file being played back out of an Atomos Shogun via HDMI to exactly the same TV. The difference is striking to say the least. Same file, same TV, same resolution. The only difference is the top one is playing back off the computer, the lower from a proper video player. I also know from experience that if I plug in a proper video output device such as a Blackmagic Mini-monitor to the laptops Thunderbolt port I will not see the same artefacts as I do when using the computers built in HDMI.

And this is a not just a quirk of my laptop, my grading suite is exactly the same. If I use the PC’s built in HDMI the pictures suck. Lots of banding and other unwanted artefacts. Play back the same clip via a dedicated, made for video, internal PCI card such as a Decklink card and almost always all of the problems go away. If you use SDI rather than HDMI things tend to be even better.

So don’t skimp on your monitoring path if you really want to know what your footage looks like. Get a proper video card, don’t rely on the computers GPU. Get a decent monitor with an SDI input and try to avoid HDMI for any critical monitoring.

20170620_091235-1024x576 Why you need to sort out your post production monitoring!
Shot viewed on a good quality TV via HDMI from the computers built in graphics card. Notice all the banding.

20170620_091347-1024x576 Why you need to sort out your post production monitoring!
Exactly the same shot/clip as above. But this time played back over HDMI from an Atomos Shogun Flame onto the very same TV. Not how all the banding has gone.