Smartphone TRRS Headset cable Adapter?

Started by mbenigni, May 17, 2013, 11:16:49 AM

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mbenigni

Hello all,

I'm wondering if anyone can help me source a cable.  I don't even know what you'd call it, so I'm having trouble searching for it in Google, Amazon, etc. and no help that I'm not certain anyone is manufacturing them.

Basically I want a cable that breaks out the mic, left and right signals from an 1/8" three-conductor jack as you would typically find on a smartphone, ideally to three 1/4" male jacks.  Is anyone aware of a source for this part, or do I need to break out the wire-cutters and electrical tape?

Elantric


Elantric


Elantric


mbenigni

I had a feeling you'd come through, Steve.  :)  "4-pole to RCA" is the terminology that blows Amazon wide open with results.  I have dozens of those RCA to 1/4" adapters laying around, so mission accomplished.  Thank you!!!

gumtown

That 4 pole 3.5mm mini jack to 3 x RCA picture looks like a regular camcorder to TV cable,
Left (Black), Right (Red) audio and Composite Video (yellow).
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

mbenigni

Oh man... this is a weird one.  I guess I'll file it under "opportunity to learn something new", although last night I was pretty unhappy watching the hours tick by while I tried to make sense of this.

I started using this cable last night to get simple analog audio in and out of my laptop (a MS Surface Pro, Windows 8 64-bit.)  I'm using this to get clean guitar in from the FX send of my Kemper, convert to MIDI, and send synths back to the FX return.  Setting aside all of the weird little bugs I encountered with KPA output settings, here is my tale of impedance matching/ buffering woes.  Hoping someone can shed some light on this:

First of all, I found that the mic input was not on the yellow RCA cable, with L/R output on white and red as expected, but rather mic in was on red, with output on white and yellow... weird, but whatever.  It still appeared to be working as expected; I could monitor output, and touch the red input and see the mic meter jump onscreen.

Here's the really weird part:  whenever I plug that red cable (with RCA to 1/4" adaptor) into the KPA direct out (or monitor out, or main out left or right) Windows suddenly reports that the microphone is unplugged.  That's right - it's working fine, then I plug it into an audio source, then Windows indicates it's unplugged.

By sheer trial and error, I realized that I could plug the mic in into some sources without its disconnecting, but not others.  For instance, I could plug it into a Boss RC30, and even use the RC30 to buffer the signal from the KPA, i.e.

KPA direct -> RC30 -> laptop mic in

...and this also works with a cheap Behringer digital reverb pedal (DR600).  But it doesn't work with an MXR Dynacomp.  So there appears to be a sensitivity to some output impedance or something with each of these devices that's causing Windows to shut the input down.  This completely blows my mind.  I've never seen anything like it, and for software to be responding to the nature of input on a simple analog cable... it seems ridiculously over-engineered like so much of Windows 8.

So can anyone help me sort out the pattern here, so I can try to find the simplest set up that will work?  Leaving a digital reverb inline chewing on batteries just to buffer this finicky mic input seems like overkill.  Here's the rundown on what does and doesn't work:

KPA (all outputs) - NO (this one surprises me most, as there's plenty of A/D and D/A buffering happening in the KPA)
MXR Dynacomp - NO
Boss RC-30 - YES
Behringer DR600 - YES

Elantric

The wiring for these 4 pole 3.5mm cables is NOT consistent and its crucial to understand the implementation for your hardware 
Older 2007 era iPOD Classics used a 4 pole 3.5mm male to three RCA Output cable to provide LR Stereo Audio + NTSC Composite Video Output.




Here is a chart comparing other implementations of the 4 pole 3.5mm plug.
Observe the "Ground" and "Video" signals move around to different locations depending on the hardware device implementation



On most modern Smart Phones and Laptops, the vendors are using the former "video" signal for an electret condenser Mic Input


I understand the MS Surface Pro and most modern Laptops (like my Lenovo W520) follow the same "Headset" convention wiring - - i.e. the Audio Input is a Mono Mic level input with a 3.3V phantom power source for condenser headset mics  -

But these are not consistent

Here is iPhone Headset 4pole 3.5mm plug pinout



Compare to an Android Headset 4pole 3.5mm plug pinout - (see bottom ) and you will see that the Ground and Mic signals always get swapped  - depending on the hardware implementation.

(Rumor is the MS Surface Pro's 4 pole 3.5mm Headset Jack wiring is identical to the Nokia/HTC/Samsung on the bottom of this pic below


These are the mics that Microsoft lists as "compatible" with MS Surface Pro and Surface RT
http://www.microsoft.com/en-us/windows/compatibility/winrt/CompatCenter/ProductViewerWithUpdatedFilters?TempOsid=winrt&Type=Both&CurrentPage=0&TotalPages=2&SortCriteria=Relevance&ShowCriteria=1&Compatibility=All&TextSearch=Microphone&ProductName=Microphone&LastRequested=7&Locale=en-us

mbenigni

Thanks, Steve - you're the man! 

The differing implementations (camcorder vs. iPod/Zune) would explain the fact that mic in fell on the red cable instead of the yellow as expected.  As for ground, looks like I had a 75% chance of success, verified by the fact that audio I/O is now working (provided I have sufficient buffering.)

So that leaves the question: why is this buffering necessary, and how does e.g. the Dynacomp differ from the RC-30 in this regard?  Not sure how that 3.3V phantom power is implemented but perhaps this is related.  Is it just a DC offset output with the mic signal summed on top as input?  And is it possible that the Dynacomp is draining this 3V off the top and killing the port?

Elantric

Thread with details on the MS Surface Pro "Headset 3.5mm jack" specs:
http://www.surfaceforums.net/forum/microsoft-surface-pro/4179-surface-pro-headphone-jack.html
QuoteMy Windows Phone headset works great with my Surface Pro. It does use the headset microphone.

If you have a recent Nokia Windows Phone Headset - this is supposed to also work with MS Surface Pro
One would think these specs would be widely available - but that is not the case.


Reference for computer electret condenser mics from old Soundblaster type "Mic Input" - this is useful as these same electret condenser mics are now used in most low budget "Headsets" for modern smart phones.
http://www.epanorama.net/circuits/microphone_powering.html


Too bad Most new Computers got rid of a "Stereo Line Input"  - my old 2008 HP HDX18t 18" laptop employed "smart 3.5mm jacks" and only last year did I discover that the "mono mic" input jack also works as a Stereo Line Level Input jack.


(Rumor is the MS Surface Pro's 4 pole 3.5mm Headset Jack wiring is identical to the Nokia/HTC/Samsung on the bottom of this pic below

mbenigni

QuoteThread with details on the MS Surface Pro "Headset 3.5mm jack" specs:
http://www.surfaceforums.net/forum/microsoft-surface-pro/4179-surface-pro-headphone-jack.html

Ha, small (virtual) world - I turned up the same thread while I was searching for answers last night.

All of my testing is consistent with the wiring charts you've provided, and it's all working for the most part.  I just can't explain why some devices cause the mic input to go into an "unplugged" state when they're connected.  It's got to be some kind of an impedance issue, but I can't sort out the specifics.

Elantric

QuoteI just can't explain why some devices cause the mic input to go into an "unplugged" state when they're connected. 

This is typically due to the "call answer" momentary switch which is on the Mic Signal line. Look at most recent "smartphone stereo headsets"  -see the "call answer" switch inline on the cable. Some hardware device implementations use these "inline momentary mic switches for Track advance for the music player.



So the scenario is that when you start unplugging and then re-plugging in any external audio input, inevitably the "Surface Pro" responds  - perhaps in a negative manner.

But my 2 cents - the headset mic input on most laptops / PCs is really sub par audio spec wise, they typically have circuits designed to emphasize voice frequency response, and subtract all frequencies outside that range . So to use a Surface Pro Headset Mic Input as a mono recording audio input for a Kemper Profiling amp seems like a path that opens you up to a wild goose chase. 

mbenigni

QuoteSo the scenario is that when you start unplugging and then re-plugging in any external audio input, inevitably the "Surface Pro" responds  - perhaps in a negative manner.

Makes sense.  It's just weird that it responds differently to seemingly equivalent 1/4" outputs on various products.

QuoteBut my 2 cents - the headset mic input on most laptops / PCs is really sub par audio spec wise, they typically have circuits designed to emphasize voice frequency response, and subtract all frequencies outside that range . So to use a Surface Pro Headset Mic Input as a mono recording audio input for a Kemper Profiling amp seems like a path that opens you up to a wild goose chase. 

I would agree if I were using it to monitor an audio path, but it's actually sufficient for this very specific application:

     KPA direct out -> laptop (JamOrigin MIDI Guitar to VST instruments) -> KPA returns

The guitar signal is run straight through the Kemper, the mic in component is only "heard" by JamOrigin, and synths are routed through the laptop audio out (sub-optimal, but adequate).

BackDAWman

"It's just weird that it responds differently to seemingly equivalent 1/4" outputs on various products."

Could be the way the different devices are coupled. Unlikely but if one device is transformer coupled and another capacitive then the laptop could be looking for a DC short to ground (or no short to ground) to know if something is connected or not. Alternatively it could be the opposite and looking for a DC voltage for detection.

...just a theory...

mbenigni

QuoteCould be the way the different devices are coupled. Unlikely but if one device is transformer coupled and another capacitive then the laptop could be looking for a DC short to ground (or no short to ground) to know if something is connected or not. Alternatively it could be the opposite and looking for a DC voltage for detection.

I think you're on the right track.  No short to ground as a requirement is most likely, since the input is fine if the cable is actually disconnected, and acts up only if connected to certain devices.

The devil is in the details, though.  I'd need to know how these circuits differ in order to find the best fix - preferably a small, passive fix as opposed to running a battery-draining pedal inline which serves no other purpose.

It doesn't surprise me that a little analog design from the 70's (MXR Dynacomp) would differ from the digital pedals (RC30, DR600), but it does surprise me that the Kemper outputs behave similarly to the Dynacomp.

Elantric

Its probably the 50 - 100 ohm output impedance of the Opamp in the Dynacomp or Kemper not playing well with the 3V Phantom mic power sourced by the Surface Pro.

Install a large .47uf cap or 1uF or 10uF non polarized electrolytic cap in series with your audio, to AC couple the external audio device.

mbenigni

Thanks - I'll try the capacitor.  I assume this runs between signal and ground, as in:


red >-----+-----> red
          |
          |
          = (cap)
          |
          |
blk >-----+------> blk


A direct box was the first thing that came to mind last night, but of course I couldn't find mine... yet another wild goose chase.  I've also got an inline transformer on a stereo RCA cable (typically used to eliminate ground loops) tucked away somewhere.  I'll test that, too.

Elantric

QuoteI assume this runs between signal and ground, as in:

Actually like this - install the cap in Series with the +Mic signal only.


                  (1uF cap)
Signal - red >----- ||-----> red
           
       
GND     - blk >------------> blk



The Cap will block the +3VDC phantom Mic power (from Surface Pro Headset Jack) getting into the output section of the connected device. It also prevents the external connected audio device's low Output impedance from loading down the whole MS Surface Pro's +3V phantom Mic power circuit.



Elantric

thinking more - I bet this would be easy to hack to make it work for your Surface Pro Audio Input needs



$27 at Amazon

http://www.amazon.com/TASCAM-iXZ-Instrument-input-iPhone/dp/B005NZ05J8

It may be plug and play right out of the box

at worst case just open it up and swap the signals on the "D" and "C" signals described here


mbenigni

Quote from: Elantric on June 28, 2013, 12:31:00 PM
Actually like this - install the cap in Series with the +Mic signal only...
The Cap will block the +3VDC phantom Mic power (from Surface Pro Headset Jack) getting into the output section of the connected device. It also prevents the external connected audio device's low Output impedance from loading down the whole MS Surface Pro's +3V phantom Mic power circuit.

This makes sense.  Glad I asked!  Embarrassed to say I earned a BSEE 25 years ago but I've been writing software for a living ever since and don't remember a lick of the analog h/w stuff.

That Tascam unit is a good comprimise if I can't get the job done without - nice and small.

Thanks again!

mbenigni

Quote from: Elantric on June 28, 2013, 08:12:09 AM
Its probably the 50 - 100 ohm output impedance of the Opamp in the Dynacomp or Kemper not playing well with the 3V Phantom mic power sourced by the Surface Pro.

Install a large .47uf cap or 1uF or 10uF non polarized electrolytic cap in series with your audio, to AC couple the external audio device.

This worked!  I put a 4.7uF electrolytic capacitor inline and everything is working like a charm now.  You guys are amazing - thanks so much!!!

And just a couple of days ago Kemper released a new firmware that improved aux in mixing flexibility, so my whole rig sort of fell in line over coffee this morning.  Awesome!

Thanks again!

mbenigni

The 4-pole madness continues!   :o

I'm all set on the Windows side now, but I've circled back to do some testing with a 3rd gen iPad (for Samplr and GarageBand, mainly.)  When I plug in this same cable (with or without the 4.7pF mod), the iPad continues to "listen" to its built-in microphone.  It does not automatically sense the presence of a 4-pole cable and enable the external mic input.  Is that something that has to be tweaked in settings, or something?

Elantric


Elantric


Elantric

QuoteBasically I want a cable that breaks out the mic, left and right signals from an 1/8" three-conductor jack as you would typically find on a smartphone, ideally to three 1/4" male jacks.  Is anyone aware of a source for this part, or do I need to break out the wire-cutters and electrical tape?
Rode SC6 Breakout Box for Smartphones and Tablets with Dual TRSS Input and Headphone output

https://www.amazon.com/Rode-Breakout-Smartphones-Tablets-Headphone/dp/B00IEM2NM4/ref=pd_rhf_dp_s_cp_6?ie=UTF8&dpID=41-hH7sE1nL&dpSrc=sims&preST=_SL500_SR126%2C135_&refRID=WP1P6GKXZFQ5RCXMCN5Z