Multi-screen problem with Blackmagic converter

edited January 2018 in MAIN
Hello there,

I'm actually setting up a multi-screen retro-projection with 2 VP for a dance show (premiere is 3 weeks from now).

I'm using the macBook Pro 15" / Touch Bar / 2017 / 4 USB-C connections / 2,8 GHz Intel Core i7 / Intel HD Graphics 630 & Radeon Pro 560

Here is the line :

Macbook Pro > apple adapter USB-C/HDMI > HMDI cable > Black Magic converter HDMI/SDI > BNC cable > Black Magic converter SDI/HDMI > HDMI cable > VP

everything is x2 except the laptop.

Here is my problem : the first time I set this up, it was very stable. The second time, one of the two VP was unstable, image went black with VP mention 'no signal' and then came back. Yesterday, try it again, it was worst. Both of the VP went black, at least twice in 1 minute.

So I went behind the scree, and tried the macbook connected to the 2 VP only with hdmi cable and adapter, it was very stable. So I guess the problem is with black magic micro-converter or BNC-cables...

But this morning, a friend lend me a Mac Pro (2014), so connectors are now mini-display > hdmi and it instantly worked and was very stable.

Anyone could help me understand where is the problem with my first set up ?

For now I could keep the Mac Pro but not for the tour of the show, so I really would like to find how to run my original set up...

thank you in advance for your help !!


  • Hello @lemonloop,

    When dealing with extender, you have to take attention to the electric installation of the venue. 
    In some case, we've observed that the quality of the signal was closely related to the amount of harmonic rejection in the the electric installation. 

    That said, have you tried the first setup without the extenders so you can identify if this is the source of the problem ? 


  • edited January 2018
    Hello @antoineM,

    thank you for your answer !
    Yes I tried the macbook pro directly connected to the VPs, only with USB-C/HDMI adapters and HDMI cables, and signal was stable. So I guess my problem is with BMD converters and/or BNC cables... BUT it works perfectly with the Mac Pro.

  • Hello @lemonloop

    How do you power up the converters? 
    USB from the computer or external power supply? 

    In such cases, you can also contact BlackMagic about the issue, maybe they've encountered similar cases or can give you some technical inputs ? 



  • Hello @antoineM

    I use micro-converter, with micro-usb power supply. 

    Yes I post a similar question on black magic forum, still waiting for answers. 
    The fact that it work with one VP but start to bug when I plug the second VP, and that I use multi-screen projection... i thought maybe someone with a similar set up could help me on this forum...

    thanks ! 
  • @lemonloop, how long are the BNC runs and what kind of cable are they (Belden 1694a RG6, Canare L5-CFW RG6 etc...)?

    It could be that you are seeing signal degradation on the BNC side of things if the cable is excessively long for the cable type. The BMD stuff isn't the best and it can be very fussy, like you are seeing here. It may also be that your Mac Pro is outputting a slightly different resolution that is more comfortable than the MBP over that cable.

    For example, your Mac Pro may be putting out 1080i where the MacBook Pro is putting out 1080p. It may also be a frame-rate difference. Both resolution and frame rate contribute to the overall bandwidth required to transmit the signal and if you are exceeding the available bandwidth on that particular cable for that particular length, then you will see these issues.

    Digital video over BNC is transported via a square wave and as the signal degrades the square becomes "rounded" which can lead to interpolation errors at the receiver. If a signal is on the edge of having errors you will also sometimes see a sparkle or snow on the image. You can check this by transmitting black and looking for a handful of pixels that flicker white.

    In general, I would recommend staying away from the BMD HDMI->SDI and going with something more along the lines of a Decimator MD-HX or even a Grass Valley ADVC-G1
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