Tag Archives: sdi

SDI Failures and what YOU can do to stop it happening to you.

Sadly this is not an uncommon problem. Suddenly and seemingly for no apparent reason the SDI output on your camera stops working. And this isn’t a new problem either, SDI ports have been failing ever since they were first introduced. This issue affect all types of SDI ports. But it is more likely with higher speed SDI ports such as 6G or 12G as they operate at higher frequencies and as a result the components used are more easily damaged as it is harder to protect them without degrading the high frequency performance.

Probably the most common cause of an SDI port failure is the use of the now near ubiquitous D-Tap cable to power accessories connected to the camera. The D-Tap connector is sadly shockingly crudely designed. Not only is it possible to plug in many of the cheaper ones the wrong way around but with a standard D-Tap plug there is no mechanism to ensure that the negative or “ground” connection of the D-Tap cable makes or breaks before the live connection. There is a however a special but much more expensive D-Tap connector available that includes electronic protection against this very issue – see: https://lentequip.com/products/safetap

Imagine for a moment you are using a monitor that’s connected to your cameras SDI port. You are powering the monitor via the D-Tap on the cameras battery as you always do and everything is working just fine. Then the battery has to be changed. To change the battery you have to unplug the D-Tap cable and as you pull the D-Tap out, the ground connection disconnects fractionally before the live connection. During that moment there is still positive power going to the monitor but because the ground on the D-Tap is now disconnected the only ground route back to the battery becomes via the SDI cable through the camera. For a fraction of a second the SDI cable becomes the power cable and that power surge blows the SDI driver chip.

After you have completed the battery swap, you turn everything back on and at first all appears good, but now you can’t get the SDI output to work. There’s no smoke, no burning smells, no obvious damage as it all happened in a tiny fraction of a second. The only symptom is a dead SDI.

And it’s not only D-Tap cables that can cause problems. A lot of the cheap DC barrel connectors have a center positive terminal that can connect before the outer barrel makes a good connection. There are many connectors where the positive can make before the negative.

It can also happen when powering the camera and monitor (or other SDI connected devices like a video transmitter) via separate mains adapters. The power outputs of most of the small, modern, generally plastic bodied switch mode type power adapters and chargers are not connected to ground. They have a positive and negative terminal that “floats” above ground at some unknown voltage. Each power supplies negative rail may be at a completely different voltage compared to ground.  So again an SDI cable connected between two devices, powered by different power supplies will act as the ground between them and power may briefly flow down the SDI cable as the SDI cables ground brings both power supply negative rails to the same common voltage. Failures this way are less common, but do still occur. 

For these reasons you should always connect all your power supplies, power cables and especially D-Tap or other DC power cables first. Then while everything remains switched off connect the SDI cables. Only when everything is connected should you turn anything on. If unplugging or re-plugging a monitor (or anything else for that matter) turn everything off first. Do not connect or disconnect anything while any of the equipment is on.  Although to be honest the greatest risk is at the time you connect or disconnect any power cables such as when swapping a battery where you are using the D-Tap to power any accessories. So if changing batteries, switch EVERYTHING off first, then disconnect your SDI cables before disconnecting the D-Tap or other power cables next.

(NOTE: It’s been brought to my attention that Red recommend that after connecting the power, but before connecting any SDI cables you should turn on any monitors etc. If the monitor comes on OK, this is evidence that the power is correctly connected. There is certainly some merit to this. However this only indicates that there is some power to the monitor, it does not ensure that the ground connection is 100% OK or that the ground voltages at the camera and monitor are the same. By all means power the monitor up to check it has power, then I still recommend that you turn it off again before connecting the SDI).
 
The reason Arri talk about shielded power cables is because most shielded power cables use connectors such as Lemo or Hirose where the body of the connector is grounded to the cable shield. This helps ensure that when plugging the power cable in it is the ground connection that is made first and the power connection after. Then when unplugging the power breaks first and ground after. When using properly constructed shielded power cables with Lemo or Hirose connectors it is much less likely that these issues will occur (but not impossible).

Is this an SD fault? No, not really. The fault lies in the choice of power cables that allow the power to make before the ground or the ground to break before the power breaks.  Or the fault is with power supplies that have poor or no ground connection. Additionally you can put it down to user error. I know I’m guilty of rushing to change a battery and pulling a D-Tap connector without first disconnecting the SDI on many occasions, but so far I’ve mostly gotten away with it (I have blown an SDI on one of my Convergent Design Odysseys).

If you are working with an assistant or as part of a larger crew do make sure that everyone on set knows not to plug or unplug power cables or SDI cables without checking that it’s OK to do so. How many of us have set up a camera, powered it up, got a picture in the viewfinder and then plugged an SDI cable between the camera and a monitor that doesn’t have a power connection yet or already on and plugged in to some other power supply? Don’t do it! Plug and unplug in the right order – ALL power cables and power supplies first, check power is going to the camera, check power is going to the monitor, then turn it all off first, finally plug in the 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.