Building a cable from sound system to "mic in"

Discussions around meetinghouse sound systems, microphones, assisted listening devices, and translation equipment
idjeeper2
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Re: Building a cable from sound system to "mic in"

#21

Post by idjeeper2 »

russellhltn wrote:
idjeeper2 wrote:My understanding is that I'm going from a line level TS connector to a mic level TRS connector. Does that sound correct?
That depends on the laptop. Is the icon by the plug of a microphone, or a headset(headphone with boom mic)?

TRS is correct for "mic", TRRS is needed for "headset".
This laptop actually has a mic-in along with two headphone-out plugs.

Thanks for your reply. I'll post my results when I have them.
idjeeper2
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Joined: Wed Jun 09, 2010 10:39 pm
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Re: Building a cable from sound system to "mic in"

#22

Post by idjeeper2 »

This project met a quick end. I went to the meetinghouse last evening and discovered we don’t have a record out plug. So I need to find a lapel mic to hang on the pulpit.
russellhltn
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Re: Building a cable from sound system to "mic in"

#23

Post by russellhltn »

idjeeper2 wrote:This project met a quick end. I went to the meetinghouse last evening and discovered we don’t have a record out plug.
Yeah, that doesn't go well.

The plug can be hidden. I've found them inside the podium, as well as inside a cabinet near the "clerk's desk" on the stand.
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johnv
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Re: Building a cable from sound system to "mic in"

#24

Post by johnv »

Would this work to send audio wirelessly from the 3.5mm record-out jack under the sacrament table to your cell phone via bluetooth?

https://www.amazon.com/gp/product/B01EH ... UTF8&psc=1

I ordered one of the above which will be here on Tuesday after 'hours' of reading/researching about how many different adapters and reasons that a simple 25' 3.5mm mono cable won't work getting plugged into a cell phone that needs TRRS mic input and what will work and what won't work. I'm a tech guy, but not necessarily an audio guy. I know something better is coming, but we are already into this 4 weeks now and I have heard some horror stories about broadcasts failing for many different reasons.
CalS201
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Re: Building a cable from sound system to "mic in"

#25

Post by CalS201 »

johnv wrote:Would this work to send audio wirelessly from the 3.5mm record-out jack under the sacrament table to your cell phone via bluetooth?
https://www.amazon.com/gp/product/B01EH ... UTF8&psc=1.
Yes, it will probably work as long as you stay within the distance limitation of Bluetooth technology. Most BT devices are limited to 10m/33ft. I have used several BT devices where you get intermittent dropouts when less than 20ft away and objects (people) get between the transmitter and receiver. Given the choice of "wired" or "wireless" in this application, "wired" may be more robust and reliable.
johnv wrote:....after 'hours' of reading/researching about how many different adapters and reasons that a simple 25' 3.5mm mono cable won't work getting plugged into a cell phone that needs TRRS mic input and what will work and what won't work.
There are a lot of forum posts discussing many audio issues and it can be difficult to extract the information you need. There are a few posts with concise information. See the attachments to these forum posts-
viewtopic.php?f=140&t=37590
viewtopic.php?f=140&t=37670
viewtopic.php?f=27&t=37528
russellhltn
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Re: Building a cable from sound system to "mic in"

#26

Post by russellhltn »

johnv wrote:Would this work to send audio wirelessly from the 3.5mm record-out jack under the sacrament table to your cell phone via bluetooth?

https://www.amazon.com/gp/product/B01EH ... UTF8&psc=1
If plugged in directly, you'd only get audio in the left channel. You'd need a mono to stereo adapter to fix that. I like the idea, but I'm concerned about the reviews on this device that report latency (about 1/4 second) as well as reports that it cuts off at low audio.
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johnv
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Re: Building a cable from sound system to "mic in"

#27

Post by johnv »

CalS201 wrote:
johnv wrote:Would this work to send audio wirelessly from the 3.5mm record-out jack under the sacrament table to your cell phone via bluetooth?
https://www.amazon.com/gp/product/B01EH ... UTF8&psc=1.
Yes, it will probably work as long as you stay within the distance limitation of Bluetooth technology. Most BT devices are limited to 10m/33ft. I have used several BT devices where you get intermittent dropouts when less than 20ft away and objects (people) get between the transmitter and receiver. Given the choice of "wired" or "wireless" in this application, "wired" may be more robust and reliable.
johnv wrote:....after 'hours' of reading/researching about how many different adapters and reasons that a simple 25' 3.5mm mono cable won't work getting plugged into a cell phone that needs TRRS mic input and what will work and what won't work.
There are a lot of forum posts discussing many audio issues and it can be difficult to extract the information you need. There are a few posts with concise information. See the attachments to these forum posts-
viewtopic.php?f=140&t=37590
viewtopic.php?f=140&t=37670
viewtopic.php?f=27&t=37528
I did actually see these posts today, thank you, and I did also order a 50' audio cable and mic-in adapter as well. I'll try them both out and see which one works the best (obviously the wired solution will be worry free, the wireless solution would be great if it works, much easier on the eyes). Would be nice to 'pin' those links up top to help everyone get at the information they need.

Thanks again. Today was my 3rd broadcast using OBS and Sony HDMI camera and mixer. I have it down pat and it's 100% reliable, now I just want to reduce the setup and complexity of it for the next guy.
russellhltn
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Re: Building a cable from sound system to "mic in"

#28

Post by russellhltn »

Coming back to this, I'm finding that building the cable is not so simple. I had thought that if I just drop line-level down to the same voltage as standard electret microphone, everyone would be happy. Boy, was I wrong!

It turns out my collection of Android cell phones wanted 40dB attenuation. But apparently, all indications are that iOS wants only 20dB.

Things got even stranger when I tested my laptop and found it wanted -10dB on it's TRS input. And digging into the microphone settings, that was with 10dB boost! Turn off all boost, and it's happy with full line level input.

So, using CalS201 excellent ASCII art as a starting point, here is my suggestions with the values for different levels of attenuation.

Code: Select all

          RECORD            DIY             3.5mm to Device       
          OUT JACK      ATTENTUATOR                  
                                             TRRS       TRS          
                                          
316mv-)|(---< [T]RS <-----[R1]-------------> TRR[S]  or [T]RS
      )|(                          |   
  600 )|(                         [R2]
  ohm )|(                          |
 -----)|(---< TR[S] <----------------------> TR[R]S  or TR[S]


For TRRS (headset) Connection:

          R1       R2
-20dB     12K     1.2K
-30dB     39K     1.2K
-40dB    120K     1.2K


For TRS (mic in) Connection:

          R1       R2
-10dB     10K     4.7K
-20dB     47K     4.7K
I'm in the middle of designing a cable that will work with either TRS or TRRS and will have 3 levels of attention to choose from. That way, I should be able to deal with most anything.
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CalS201
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Re: Building a cable from sound system to "mic in"

#29

Post by CalS201 »

russellhltn wrote:Coming back to this, I'm finding that building the cable is not so simple. I had thought that if I just drop line-level down to the same voltage as standard electret microphone, everyone would be happy. Boy, was I wrong!

It turns out my collection of Android cell phones wanted 40dB attenuation. But apparently, all indications are that iOS wants only 20dB.

Things got even stranger when I tested my laptop and found it wanted -10dB on it's TRS input. And digging into the microphone settings, that was with 10dB boost! Turn off all boost, and it's happy with full line level input.
Things get even stranger on the iPhone/iPad because the Apple lightning audio adapter presents a low impedance (~2K) to your external attenuator. An Android phone "may" also be the same way. For example, if you use a 20db HeadSetBuddy attenuator (which has a 2.2K output impedance), you end up with 26db of attenuation when you plug it into the Lightning audio adapter. You expected 1/10 (20db) of the signal level, but ended up with 1/20 !!

Also, something to consider in your choice of 1.2K for your R2 in the TRRS configuration. We know the R2 value determines the DC bias level on the Mic activation circuit. If the bias level is too low or too high the mic circuit is not turned on. All the commercial adapters seem to be using ~2K. If 1K, 2K, or 3K resistors all activate the mic circuit, it might be better to pick the middle value (for best reliable activation) so you don't get close to the de-activation points.
russellhltn
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Re: Building a cable from sound system to "mic in"

#30

Post by russellhltn »

CalS201 wrote:Also, something to consider in your choice of 1.2K for your R2 in the TRRS configuration. We know the R2 value determines the DC bias level on the Mic activation circuit. If the bias level is too low or too high the mic circuit is not turned on. All the commercial adapters seem to be using ~2K. If 1K, 2K, or 3K resistors all activate the mic circuit, it might be better to pick the middle value (for best reliable activation) so you don't get close to the de-activation points.
I was going by the Android specification of "1000 ohms or higher". My brief search for iOS specs suggested the lower switching point was "at least 800 ohms". Even that still seems to be well above the value of the resistors used to issue commands via the mic jack.

At no point was I able to find a upper threshold for either system. That really messes with trying to find a "safe zone".

My choice of 1.2K was driven by making sure I stayed above 1K even considering the "record out" jack to be a DC short due to being a transformer.

I'll note that by making R2 as low as possible, I minimize the effect of the input impedance on the attenuation.

Edit: If I was designing for a mixing board input, I'd probably make R2 about 150 ohms, since that's the impedance most mic preamps expect to see.
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