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Author Topic: Partyline buzz, line impedance, termination  (Read 1139 times)

Diogo Nunes Pereira

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Partyline buzz, line impedance, termination
« on: January 27, 2022, 04:28:08 AM »

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
« Last Edit: January 27, 2022, 04:36:19 AM by Diogo Nunes Pereira »
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Brian Jojade

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Re: Partyline buzz, line impedance, termination
« Reply #1 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.
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Mac Kerr

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Re: Partyline buzz, line impedance, termination
« Reply #2 on: January 27, 2022, 02:34:25 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.

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
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Pete Erskine

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Re: Partyline buzz, line impedance, termination
« Reply #3 on: January 28, 2022, 05:10:41 PM »


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...

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.
« Last Edit: January 28, 2022, 05:17:50 PM by Pete Erskine »
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Diogo Nunes Pereira

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Re: Partyline buzz, line impedance, termination
« Reply #4 on: January 28, 2022, 05:13:26 PM »

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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Diogo Nunes Pereira

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Re: Partyline buzz, line impedance, termination
« Reply #5 on: January 28, 2022, 05:21:00 PM »

Definitely get yourself some "buzz busters" Commercially available as
http://steve1mac.blogspot.com/search/label/AudioMan%20ISO%20Box


Bought a couple of these 4 years ago and absolutely love them. Use them all the time when interfacing our 2-wire partylines with other crews (usually the ones on the director rack) main stations, but they dry the line and I did need power to feed those packs this time as mine was the only comms power supply in the system.
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Diogo Nunes Pereira
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Diogo Nunes Pereira

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Re: Partyline buzz, line impedance, termination
« Reply #6 on: January 28, 2022, 05:28:32 PM »

Both sides of the line should have their own power but only one should be terminated.

Termination is something i still need to fully understand… can’t grasp why my port-C reading (2000 ohm) was different from A and B (4000 ohm). Freespeak was the only power supply in the system.
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Diogo Nunes Pereira
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Mac Kerr

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Re: Partyline buzz, line impedance, termination
« Reply #7 on: January 28, 2022, 05:50:48 PM »

Termination is something i still need to fully understand… can’t grasp why my port-C reading (2000 ohm) was different from A and B (4000 ohm). Freespeak was the only power supply in the system.

One possibility is that Port C was set differently. Ports A & B were in RTS 2ch mode and C was in single channel CC mode. When you measured the DC resistance of each of the channels, was that at the FS Base with the 2W cables unplugged, or did you measure the circuit as it was for the show. You could have run everything in RTS mode and just assigned your single channel PL to ch B of the RTS line.

Mac
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Diogo Nunes Pereira

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Re: Partyline buzz, line impedance, termination
« Reply #8 on: January 29, 2022, 03:57:05 AM »

One possibility is that Port C was set differently. Ports A & B were in RTS 2ch mode and C was in single channel CC mode. When you measured the DC resistance of each of the channels, was that at the FS Base with the 2W cables unplugged, or did you measure the circuit as it was for the show. You could have run everything in RTS mode and just assigned your single channel PL to ch B of the RTS line.

Mac

Yes, good idea, I could have run everything in RTS mode… well, next time. Measurements were made at the end of  line, as things were for the show. Will check resistance in the FS base for the next one. I’ll try and make it an habit.
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Diogo Nunes Pereira
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ProSoundWeb Community

Re: Partyline buzz, line impedance, termination
« Reply #8 on: January 29, 2022, 03:57:05 AM »


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