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Unbalanced/Balanced Audio confusion

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Kris McMullin
Unbalanced/Balanced Audio confusion
on Oct 5, 2012 at 11:57:21 pm

We're trying to fit a Blackmagic SDI-Analog Converter to receive fiber through and optical receiver. Outputs on the Blackmagic are 1/4", and our old Knox Server is looking for RCA inputs. We have it all together, but the output is giving us an annoying "crackling"; which I've heard to be a possible unbalanced/balanced issue. Any feedback on converters for audio to solve this issue?


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Bob Zelin
Re: Unbalanced/Balanced Audio confusion
on Oct 6, 2012 at 7:16:40 pm

I believe that I have responded to you on the Blackmagic forum about this. You are taking a +4dBu analog audio signal, and sending it into a -10dBu unbalanced input on the Knox router. The crackling you are hearing is peak distortion. You must drop down the level to a -10dBu level.

there are countless manufacturers that make balancing amplifiers for this. Henry Engineering is one of the most famous, but if you want to spend very little money, you can get a TecNec balanced (XLR) to unbalanced (RCA) converter box from http://www.markertek.com
As I told you before, you currently have 1/4" to RCA cables. You will now have to also get 1/4" to XLR to go into the TecNec (or Henry) box, and then RCA-RCA cables to go from the converter into the Knox switcher.

I am sure that you realize that it's time to retire that old composite video switcher. The entire world is HD video these days, and Blackmagic certainly makes very inexpensive HD routers, and video switchers.
The cheapest cameras out there (like the GoPro for 300 bucks) will outperform your current composite video cameras. Think about doing an upgrade at your company, and retire that Knox.

Bob Zelin



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Kris McMullin
Re: Unbalanced/Balanced Audio confusion
on Oct 6, 2012 at 8:49:48 pm

Thanks! We certainly will be taking the Knox out of the mix as we move towards a better configuration. Unfortunately, our Cable providers will not support an HD-SD SDI signal, so we are stuck broadcasting Public Access as SD 4:3. In terms of the Blackmagic Box and Converter; thank you again for helping us with this issue. I just wanted to be a bit more specific as to the nature of the distortion- so as to get a better confirmation of the +4db and -10db issue. We've since made the move to purchase the audio converter, and are hopeful that this will solve things. Much thanks again!


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Kris McMullin
Re: Unbalanced/Balanced Audio confusion
on Oct 12, 2012 at 9:14:16 pm

So we got a Rolls +4 to -10 audio converter. We also purchased brand new 1/4" to XLR balanced cables. No luck unfortunately, we're still stuck with a crackling/static signal. We've emailed Blackmagic to see if they suspect that it may be the box. Wish we knew for sure, however, where the distortion was coming from. We tried to run RCA audio directly from our JVC HM750U camera into our Knox- with no distortion at all. Then we tried putting the Blackmagic back in the mix and it was partially distorted again. We know we're using the right cables, but we're just not able to trace where this distortion is coming from.


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Mike Squires
Re: Unbalanced/Balanced Audio confusion
on Oct 16, 2012 at 2:47:04 am

Two questions:

1. Are you using any kind of splitters for the audio?

2. Have you tried removing the ground from the audio sources and destinations? (Try removing the ground from the XLR cable)


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Kris McMullin
Re: Unbalanced/Balanced Audio confusion
on Oct 16, 2012 at 1:18:17 pm

We're not using any type of splitter for audio. We're coming out of our Blackmagic to a Rolls audio converter (+4 to -10) and directly into our broadcast router (Knox). Interesting thing we noticed last evening while testing for a live broadcast; when we switch our on location JVC HM750U to output HD-SDI to the Blackmagic (over a fiber network in town)- the audio is crystal clear through the Blackmagic path. Unfortunately, the HD-SDI signal distorts every 15 seconds or so (bit rate issue maybe?). We found we couldn't keep a stable signal with it, but we did have great audio. Dip settings on the box are correct, but we were forced to use SD-SDI into the Blackmagic; which then goes back to giving us that sort of "crackling/static" audio. Wondering if the audio being generated in HD-SDI mode is that ASI that the box refers to (as opposed to Analog), and if it only wants to accept that. This is a tough one to troubleshoot.


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Mike Squires
Re: Unbalanced/Balanced Audio confusion
on Oct 16, 2012 at 5:42:36 pm

We use a JVC GY-HM790U, mic into the camera, video and audio over HD-SDI (no converters) into an ATEM TVS. We haven't noticed any major problems with the audio.


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john mahoney
Re: Unbalanced/Balanced Audio confusion
on Jan 22, 2016 at 5:54:47 pm

Please explain why splitter for audio can be an issue,I recently had an issue with my black magic camera. the problem was a sudden lack of level when using direct mic in. The camera had been working great, I was feeding the BMC and an FS 100 using an XLR splitter. worked fine. after a second set up, the bmc audio was super low. and stayed problematic on my next shoot using the BMC alone, no splitter involved. Was my original issue related to the splitter? I think the camera was suffering from some phase canceling issue which resolved itself. while testing I sent 1K line level tone to the camera set at line, and it worked fine, mic level has been fine since then also. so problem fixed for now.


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