L = Left audio channel
R = Right audio channel
Mic = External Microphone input/button presses
Gnd = Ground (common) connection
The photo below shows the ends of an adapter cable from a Creative Technology 7.1 sound card used for testing the functionality of the 4 pole headphone jack under various conditions.
If a 3 pole plug (e.g. a typical simpler headphone cable with no microphone capability) is connected to this type of 4 pole jack, the longer ground part of the 3 pole plug will short out the microphone
conection in the 4 pole jack to ground. On the S4, shorting out the microphone connection in the jack enables use of the internal microphone of the S4 for recording with apps
like the native "Voice Recorder". (In addition, temporary shorting of the mic/ground connection is recognized as a button-click command; see below).
With the microphone connection completely open (NOT connected and not shorted to ground) there is a voltage of ~ 2.3 VDC (measured) between that connection
and ground and the internal S4 microphone is disabled. If a resistance of 1.4kΩ or greater corresponding to the impedance of an external microphone (or a microphone and a preamplifier) exists between the microphone connection and ground, the internal microphone will
remain disabled and the external microphone circuit is available for recording. Measurements on one S4 shows that the 1.4kΩ value corresponds to the DC voltage on the microphone jack pin
dropping to 1.0 VDC. If the resistance between the microphone pin and ground is changed from above to below the 1.4kΩ value, the microphone enable/disable function will not activate unless the headphone
plug is removed and reconnected. Similar results, but with a lower resistance threshold of 1.0kΩ have been reported for
