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Partyline buzz, line impedance, termination
Diogo Nunes Pereira:
Hi.
I've been chasing an intermittent buzz in a 2-wire comms line for 3 days now. Main station is a FreeSpeak 2 Base II. Comms are pretty clean most of the day: only a very tiny buzz in the background, but totally acceptable by my A1, imperceptible by the rest of the crew. Strangely enough, there's a timeframe where the buzz gets louder (around 8am to 9 am). Loud enough to bug me (and the A1) at least.
I've changed cables, Y's and packs, and haven't been able to fully understand the nature of the problem. I've also been checking all my Y's and cables for Pin1/Chassis continuity.
I'm attaching a comms diagram. I've measured the 3 lines coming out from the FS2 Base and read 4000 ohms on lines A and B; 2000 ohms in line C, symptom of a double terminated line, but there's no other power supplies in the system, just the FS2BaseII. Removing line A from the system cleans the channel, so I changed runs to this area a couple of times - I measured a lot of RF noise (with my RF Explorer) coming from the CCU rack - and it helped reduced the small noise to tiny noise. Still, at 8 am it gets louder...
FreeSpeak 2-wire Ports A and B are in RTS mode to feed ClearCom dual channel stations. 2-wire Port C is in ClearCom mode to feed single channel stations. Termination is enabled in all 3 ports.
I've been reading old posts here and elsewhere, but still decided to start a new topic on the subject of intercom buzz, line impedance and termination, to get ideas and suggestions from your brains.
Old topics and other links:
https://forums.prosoundweb.com/index.php?topic=75330.0
https://forums.prosoundweb.com/index.php/topic,156166.10.html
https://www.gravitymedia.com/assets/Uploads/product-downloads/A-Comprehensive-Guide-to-CC-Analog-Digital-Partyline-Systems.pdf
Andy Leviss has an good article on the topic; I found his website to be down, but still available in the internet archive:
https://web.archive.org/web/20080606004048/http://www.onefromtheroad.com/index.php?p=29
Brian Jojade:
If it's time specific, chances are there is something in the building that kicks in at a certain time that's generating significant noise. If you know when it's happening, maybe you can move stuff around during that time and see if you can pinpoint a direction it is coming from. Removing the source of noise, if possible, can often be more effective than trying to shield it from getting into your system.
As far as 'replacing cables', what kind of cable are you using? Different types of cables may make a difference in noise rejection, depending on where the noise is entering the system.
Mac Kerr:
--- Quote from: Brian Jojade on January 27, 2022, 02:10:01 PM ---If it's time specific, chances are there is something in the building that kicks in at a certain time that's generating significant noise. If you know when it's happening, maybe you can move stuff around during that time and see if you can pinpoint a direction it is coming from. Removing the source of noise, if possible, can often be more effective than trying to shield it from getting into your system.
As far as 'replacing cables', what kind of cable are you using? Different types of cables may make a difference in noise rejection, depending on where the noise is entering the system.
--- End quote ---
Yes, try to identify any equipment that cycles on when you are having the issue. While there is a small possibility that something like StrQuad might help, it is generally not recommended for use in 2W PL which is unbalanced by nature, and very impedance sensitive as well. StarQuad type cables have significantly higher capacitance than plain vanilla mic cables.
I assume you have removed everything from the system one item at a time to discover what area the noise may be coming from.
Mac
Pete Erskine:
--- Quote from: Diogo Nunes Pereira on January 27, 2022, 04:28:08 AM ---
I'm attaching a comms diagram. I've measured the 3 lines coming out from the FS2 Base and read 4000 ohms on lines A and B; 2000 ohms in line C, symptom of a double terminated line, but there's no other power supplies in the system, just the FS2BaseII. Removing line A from the system cleans the channel, so I changed runs to this area a couple of times - I measured a lot of RF noise (with my RF Explorer) coming from the CCU rack - and it helped reduced the small noise to tiny noise. Still, at 8 am it gets louder...
--- End quote ---
Definitely get yourself some "buzz busters" Commercially available as
http://steve1mac.blogspot.com/search/label/AudioMan%20ISO%20Box
this is guaranteed to fix the problem... Both sides of the line should have their own power but only one should be terminated.
Diogo Nunes Pereira:
The comms line that fed the CCU area was the one that introduced the noise. Removing that leg from the system indicated that. Those racks were also the ones closer to the Hotel kitchen and service corridor. All our equipment was up and running for a couple of hours before the noise ramped up, but something out of our control probability could be put to work at that specific time. Our mains were connected to the ballroom outlets, no generators on this one.
Cables… a variety of brands and models of mic cable were in the mix for this one. No starquad - not a common type of cable in my neck of the woods. I do want to take a better approach to comms cable for my next gig with this client. In this shop they have a lot of digital 3-pin XLR that is good quality and easy to identify (grey colored) - we usually use it for AES signals. I’ll request all comms cable for the next one to be of this kind. I think there shouldn’t be any problem with running 2-wire comms with this kind of cable, right? My understanding is that digital cable have better (lower) capacitance specs and from my readings and your comments that’s actually a good thing. Am I wrong in this assumption?
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