android - How do volume control headphones work?

A recent question over on Android Enthusiasts got me to wondering how volume control headphones work.

I don't mean volume control which works by attenuating the incoming signal, but the kind of volume control that signals to the device to increase or decrease the signal output.

For instance, according to the Crossfade LP product page, the volume controls are only for apple devices:

Universal Compatibility and Microphone Communication LP comes with 2 cables for universal compatibility with all modern mobile and audio devices. The 3-Button remote microphone cable is designed for the latest Apple devices, including iPhone®,iPad®, iPod®, and Macbook series. The long audio-only cable and 1/4" adapter is universally compatible with all audio devices and professional equipment.

However this sort of device apparently works with some android phones and the question implies that it works with some computer sound cards on Windows machines too, but not being an Apple buyer (and only recently getting my first smartphone) I haven't seen this sort of thing before. I've done a quick google search, but can't find anything on standards for this technology.

• How does this work?
  
  
  
  • Is it something as simple as shorting left/right/mic channels?
  
  
  • If so, could using headphones of this type with equipment that doesn't support this technology, damage that equipment?
  
  
  • Is it a serial signal, for instance sent over the mic channel?
  
  
  
  • Does it just require a headphone TRS connector or a headphone +microphone TRRS connector?
  
  
  
  


• Does this technique have a name? _{Incidentally, is someone could answer this, I can probably look up the rest of the information myself. *8')}


• Is it as standard as it appears, or is everyone just following Apple?


• Is this something that is patented?
  
  
  
  • If so, who holds the patent and do they charge a license fee for people to use this technology?
  
  
  • If not, is there an open standard for it?
  
  
  
  
  

There is some excellent information in an answer to the question What protocol does a headphone jack use? but it doesn't answer the specifics of how this type of volume (etc.) control works, since it now seems to be rather more common and standard.

Answer

This article shows some good ways to make a volume control. Basically it is just some buttons switching in different resistors between a couple of pins of the 1/8" TRRS headphone connector. Normally these buttons would put the resistor across (in parallel with) the microphone.

A standard microphone needs two wires: The mic output signal and GND. But the mic also needs power, and this is provided on the same wire as the mic output signal. Inside the phone there will be a resistor between the mic signal and power. Our remote volume control will switch in a resistor between the mic signal and GND. These two resistors will form a simple resistive voltage divider, where the resulting voltage depends on which button is being pressed.

Inside the phone this mic signal goes into a simple analog-to-digital converter (ADC) that will measure the voltage and figure out from there what button is being pressed.

Doing it this way is a little tricky and creates some compromises. First thing is that you can't use the mic and the buttons at the same time. Most of the time this is not a problem, but occasionally you'll want to mess with stuff. The phone has to mute the mic whenever a button is pressed. Otherwise there will be a loud pop in the audio. Even if this wasn't an electrical problem, most remote volume controls also house the microphone and you would get a lot of mechanical handling noise in the mic just from moving your fingers around.