Support for CTIA / OMTP control in playerd

[sandreas]

Hello,

on mobile devices media playback can be controlled via buttons. Some headphone remotes have an integrated remote to do this. I use a Samsung EG920 EO-EG920BW headset with three buttons (for reference, let's call them vol+, vol-, toggle):

  |
┌───┐
│   │
│ + │    -> vol+
│   │ 
│   │ 
│ - │    -> vol-
│   │
│   │
│   │
│ > │    -> toggle
│   │
└───┘
  |

Most devices support click-codes like this:

• play / pause: single tap toggle
• next: double tap toggle
• previous: triple tap toggle (alternative: single tap and hold toggle)
• fast forward: double tap and hold toggle
• rewind: triple tap and hold toggle

After doing some research I found out the reason for this:

Basically there are two (three) different standards: CTIA and OMTP (and a proprietary CTIA for apple devices).

These use an audio jack with 4 connectors (instead of the normal 3), also called TRRS. Apple uses proprietary CTIA standard, Android devices use OMTP standard where the pinouts switched the aux channel.

In Both cases the control buttons on the remote are just shorting the auxiilliary line (mic) with a specific resistance:

Play/Pause  <70 Ω   
Vol+  210–290 Ω   
Vol−  360–680 Ω  
(reserved for voice control)  110–180 Ω   

More about this here: https://thecablesland.com/ctia-vs-omtp/

I own a ThinkPad T480s, supporting a Headset with microphone via audio jack.

product: HDA Intel PCH Headphone
     physical id: 1
     logical name: input11
     logical name: /dev/input/event9  

Now to my question:

   Would it be possible to support the same controls like apple does via some kind of driver?  

Since the buttons are only shorting the mic channel, this could be possible with a generic driver / hook, but the only commit with the topic CTIA / OMTP in the kernel I found is this: https://patchwork.kernel.org/project/alsa-devel/patch/[email protected]/