Tone Generator :

  https://www.szynalski.com/tone-generator/


Specs

  https://source.android.com/devices/accessories/headset/plug-headset-spec#electrical


  Tip = Left
  next = right
  next = gnd
  next(if 4 conductor) = mic

  Ear speaker impedance	16 ohms or higher	Recommend 32 - 300 ohms
  Mic DC resistance	1000 ohms or higher
  Control Function Equivalent

      0 ohm	[Function A] Play/Pause/Hook
    240 ohm +/- 1% resistance	[Function B]
    470 ohm +/- 1% resistance	[Function C]
    135 ohm +/- 1% resistance	[Function D]


  need about 220uF AC coupling caps for headphones

References :

  https://www.onsemi.com/pub/Collateral/NCP2809-D.PDF

  https://www.ti.com/lit/an/sloa043/sloa043.pdf






Less Useful
-----------
http://www.ti.com/lit/ug/tiduag7/tiduag7.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/AN-1429.pdf

https://www.nutsvolts.com/magazine/article/precision_stereo_headphone_amplifier