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Author Topic: 3.5mm TRS computer audio input wiring fiasco...  (Read 811 times)

Dan Keifer

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3.5mm TRS computer audio input wiring fiasco...
« on: July 25, 2018, 10:59:05 am »

Our IT dept and myself (veteran AV geek) spend days troubleshooting an audio issue with a newly installed PC based teleconference system at our hospital.  The problem was when we tried to get "Skype for Business" running we got no audio out to the far end.  The main teleconference software worked fine (Cisco Meeting App).  The Microsoft voice recorder app worked fine.  The IT people and the contractor who installed the system all thought it was software conflicts, outdated drivers etc etc.  We tried EVERYTHING for DAYS still no audio out with Skype FB.

It all turned out to be a simple but obscure wiring error on the 3.5mm TRS male connector going to the computer audio input.  The contractor wired a mono balanced output from the mixer to the TRS connector as follows:
+ audio to TRS tip
- audio to TRS ring
GND to TRS sleeve

The problem with wiring it that way is that if the software chooses to MIX the right and left audio inputs you will be mixing an audio signal and it's opposite polarity signal together resulting in SILENCE.  Some audio software may be using the right OR left so in those cases it would work.

I re-wired the connector:
+ audio to TRS tip and ring
- audio and GND to sleeve

This wiring puts the same signal on both L and R inputs so no matter what the software does with them you will get good output.

Skype for Business immediately worked fine after that fix.

I am posting this as a heads up for any of you AV installers or project managers out there that a simple issue like this can be very difficult to troubleshoot and waste a lot of time.

 
« Last Edit: July 25, 2018, 11:27:35 am by Dan Keifer »
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Lee Douglas

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #1 on: July 25, 2018, 04:28:22 pm »

Are you sure the 3.5mm input on the computer is a balanced input or for that matter even a TRS input to begin with?
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Mark Wilkinson

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #2 on: July 25, 2018, 06:32:27 pm »

Guess we gotta know how much trouble, how often, this problem comes up.... when even ebay has to offer a guide ...
https://www.ebay.com/gds/TRS-Cables-Balanced-or-Stereo-/10000000176789351/g.html
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Craig Hauber

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #3 on: July 25, 2018, 09:43:55 pm »

Are you sure the 3.5mm input on the computer is a balanced input or for that matter even a TRS input to begin with?

It's not.

But wiring mono balanced audio + to tip (L ch) and - to ring (R ch) results in very close to perfect cancellation if anything is summed to mono in the PC or anywhere after that point.

I see it the other way around way too often. 
L & R unbalanced audio from a PC feeding a single mono balanced input.  Result is the difference signal (L-R) in which most bass and any mono vocals are cancelled out leaving reverb and swishy highs.  (the root method behind many analog "Vocal Eliminators" used to generate karaoke tracks)

The quickest solution when you are in a rush is to just find a balance control somewhere in the PC and slide it to one side or the other.  Just then hope the music being played isn't some old Beatles or AC/DC tracks!
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Mike Caldwell

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #4 on: July 25, 2018, 11:10:29 pm »

It's not.

But wiring mono balanced audio + to tip (L ch) and - to ring (R ch) results in very close to perfect cancellation if anything is summed to mono in the PC or anywhere after that point.

I see it the other way around way too often. 
L & R unbalanced audio from a PC feeding a single mono balanced input.  Result is the difference signal (L-R) in which most bass and any mono vocals are cancelled out leaving reverb and swishy highs.  (the root method behind many analog "Vocal Eliminators" used to generate karaoke tracks)

The quickest solution when you are in a rush is to just find a balance control somewhere in the PC and slide it to one side or the other.  Just then hope the music being played isn't some old Beatles or AC/DC tracks!

Almost every new school install I have gone into to clean up after the low bid installer said it was finished and met spec had the 1/8 inputs wired that way back to a balanced mono input.

Even better is when they just drill hole in a metal plate and directly mount the 1/8 inch jack in the plate causing all kind of grounding noise in the system.

Pete Erskine

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Dan Keifer

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #6 on: July 26, 2018, 10:26:53 am »

Are you sure the 3.5mm input on the computer is a balanced input or for that matter even a TRS input to begin with?
Lee - No it is not a balanced TRS input it is the standard two channel unbalanced (stereo) 3.5mm TRS line input on the back of a normal Dell PC.  It is usually a blue connector.   The AV contractor wired it wrong leading to MANY hours of head scratching.  I saw the way it was wired and thought "that is wrong but it is working sometimes".  I finally broke down and wired it correctly and the problem disappeared.
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Tom Burgess

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #7 on: July 27, 2018, 12:47:16 pm »

I suppose one of these and an interconnect cable might've come in handy?

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David Allred

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #8 on: July 27, 2018, 01:40:21 pm »

I suppose one of these and an interconnect cable might've come in handy?

Isn't that a balanced mono to unbalanced dual mono?  Or is dual mono technically still stereo? 
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Tom Burgess

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Re: 3.5mm TRS computer audio input wiring fiasco...
« Reply #9 on: July 27, 2018, 03:30:19 pm »

Isn't that a balanced mono to unbalanced dual mono?  Or is dual mono technically still stereo?
I don't think you could accurately call the output from this device stereo.  I'd go with your first description.
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If the band sounds great, it's because the band IS great, if the band sound like crap, it's the soundman's fault.

Opinions expressed by me on this forum are my own and not necessarily that of the company for which I work.

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