Genlock: What is it and Why is it important?

Genlock goes by many names: genlock, reference, black burst, master pedestal, master setup, sync, and more.  It is the behind the scenes hero in a video system and by far one of the most important aspects in a system.

So what is Genlock? Essentially, Genlock is just black video.  Yep, that is it.  It is black video, hence the name Black Burst.  SD systems use a composite signal and HD uses a mix of composite and digital. But before we can really understand why we need black video going to all the pieces of gear in our control rooms, we need to take a step back and understand a little of what I like to call theory.

You may think I am going on a tangent here, but stay with me.  NTSC standards state that we here in America capture video at 30 frames per second.  So where do we get that from?  Well, it is all based upon our power infrastructure.  Our power operates at 60 hertz per second.   So each cycle or hertz in our power lines up with a field of video, two interlaced fields make up a frame.  (for this same reason is why PAL standards capture frames at 25fps)

So why is having this knowledge important?  The answer is this:  All cameras need to capture their fields/ frames at the same time.  By sending a reference video signal tells all the devices when to capture their frames, and thus syncs everything up.  Naturally everything will be just a little different from one device to the next.

Think of genlock as a conductor of an orchestral.  The conductor sets the pace, the standard so that all the musicians are in sync.  They provide the rhythm, the beat, the backbone.  Let’s say this, when a musician is practicing, they set-up a metronome to play along with.  If you told every musician to start practicing at exactly noon, there is no way that they would all have their metronomes set to the exact same time, instead they would all be a little off.  This is why they all use the same conductor, so they all hit the beat at the same time.

Genlock is essentially the same thing for video.  It tells each device to capture frames at exactly the same time.  So Hertz is the metronome, the conductor uses a metronome so he knows the pace and then translates the information from the metronome to all the musicians.  A Test signal generator essentially takes in power, sets the timing and tells all other devices when to capture.

By having everything capture frames at exactly the same time gives us the ability to seamlessly dissolve from one source to the next, with out it, miss matched frames don’t work and instead you get a really nasty cut.  Find an old composite system, and pull gen-lock on a few devices and see what happens.  There will be lots of crazy things going on.

So what does this mean for systems? It means that in order to seamlessly transition from source to source EVERYTHING MUST BE TIMED TOGETHER!  If a source doesn’t accept gen-lock, it must go through a device such as a frame sync / Time Based Corrector, that will time a signal to your system.  Some switchers have this built in.  But here is what you need to know about using Frame syncs. They create Delay!  Really expensive ones can re-time a signal in under a frame, average is 2, and many HD switchers can take up to 6 frames to re-time a device so it can use it.

Some devices don’t accept gen-lock so you have to go through a frame sync, but if a device accepts gen-lock, then give it gen-lock.  Like any other video signal, you can use a distribution amplifier to distribute it to all devices, and you can loop from device to device.  I would say if you can home run from a DA, that is the best way to go, but looping works just fine, I just wouldn’t do the entire system that way.

I can’t stress enough how every single switcher has to deal with timing.  That is why many presentation switchers fade to black between devices, to cover the switch from source to source and adapt to that source’s timing.  Don’t overlook this very important part of your system.  By understanding what it is and how it works will allow you to better understand how to troubleshoot, add, modify and design your systems.

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About brhoda

I freelance doing a few different thing in the video and live production market. I worked for a church for 5 years directing services and designing control rooms.
This entry was posted in All, Misc. Bookmark the permalink.

9 Responses to Genlock: What is it and Why is it important?

  1. SkipKent says:

    Thanks for this write up, very informative. I worked with composite systems a loooong time ago and never fully understood the need for Genlock and/or TBC’s!

  2. Ayaz jabbar says:

    Thanks for defining the topic.

  3. sdfsdf says:

    Thank you for this detailed and easy to understand explanation!

  4. M Amir Ameen Khan says:

    Thank a lot i am really very impressed

  5. TC says:

    So what is the frequency of the GenLock signal? 59.94Hz? 29.97Hz? 10MHz?

    If the frequency is greater than the frame rate how does a device know which of the many transitions is the right one to sync to? In your musical analogy, if there are four beats in a measure, how does a device know which beat is the one that starts a new measure, i.e. a new video frame?

    Thanks.

    • brhoda says:

      Hi TC. The frequency in North america will always be 59.94 because that is what our power is operating on. Your master sync generator commonly called a Test signal Generator (TSG) converts these frequencies into fields. Fields are then turned into frame rates. Usually you set your TSG to the same frame rate standard as your system. However, Most gear will accept black burst (480i reference) and can use that with whatever frame rate you would need.

      As far as the music analogy, don’t think in terms of Bars but in beats. Reference tells it when it should start every frame, not when to start a group of frames as every frame is independent. so as a conductor would go 1-2-3-4, reference just goes 1-1-1-1-1. Because with music there is a start and an end, with live video there is no start and end, only syncing time.

      I hope this helps

    • Creative says:

      That’s a great question. Maybe the vertical blanking interval is still relevant here. As the author explained, the basic (SD) gunlock signal is just black video, so its frequency is the same as the rest of your video 29.97Hz. HD is a little different. It’s called tri-sync, which implies there are at least 3 frequencies, but I don’t know why exactly. Should be a quick wikipedia search.

  6. J says:

    Isn’t the 59.94 field rate a result of the NTSC standard attempting to be backwards compatible with the 60 fields per second of the original black-and-white specification and that slight drop is due to the information encoding of the color? The utility frequency actually fluctuates throughout the day anywhere from 59.95Hz to 60.05Hz.

    • brhoda says:

      My apologies, I was going off a vivid memory from a college lecture and I misinterpreted what my professor said. I have since adjusted my post. Thanks for spotting the error.

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