I realize this thread seems to be a bit dusty now, but it is still relevant as I've seen others out in the web refer to it. Since I'm new to the FaberAcoustical products I too found it useful, but have collected additional information which may make interfacing to an iOS device less fuzzy.
Apple Headset Input / Line Output Jack (3.5mm 4 pole TRRS)
None of this is verified other than seeing a schematic for my MacBook Pro and by empirical measurements of both my iPhone5C and MacBook Pro.
Various headset jacks comply with 1 of 2 standards, CTIA or OMTP.
1. The Cellular Telephone Industries Association (CTIA) or American Headset Jack (AHJ) standard. This is used by products from HTC, recent products from Sony and Nokia and Apple devices. Although, Apple accessories use a patented signaling protocol which is likely not compatible with accessories from other manufacturers.
2. The Open Mobile Telecommunication Products (OMTP) standard. This is used by most mobile phones, most Samsung SmartPhones and older products from Sony/Ericsson and Nokia.
However, the OMTP ceased operations and was eventually absorbed by the Groupe Speciale Mobile Association (GSMA) in 2012.
As technology moves forward, it is unknown how long this information will continue to be accurate, or even relevant.
Standard “CTIA” “OMTP”
Contact Signal Signal
T Tip Left Output / HP Left Output / HP
R Ring1 Right Output / HP Right Output / HP
R Ring2 Ground / Common Microphone
S Sleeve Microphone Ground / Common
Clearly, differing accessories are not compatible. Also, Apple products further differentiate themselves by using Apple’s patented signaling.
A simple resistor connected between R2 and S seems to trick my iPhone 5C and my MacBook Pro into thinking an External Mic is attached. A resistance between 300 and 10,000 ohms seems to do the “trick.” When this resistance is somewhere above 10K or below 300 ohms, the device will turn off the Mic Bias voltage. (My iPhone 5C still powered the mic bias with a 12K resistor, and my MacBook Pro turned off the mic bias somewhere around 275 ohms. I didn’t bother checking further.)
For my particular MacBook Pro (MBP);
There appears to be isolation inductors in the signal paths at the jack. The series resistance of the one in the Mic signal input path seems to have a nominal 1K ohm resistance and the ones in series with each of the output signal paths appear to have a nominal 220 ohm resistance. Measurements I made of the output level capabilities of the output driver (open circuit vs a known load) confirm this 220 ohm series resistance.
The “T” tip connector of the device has a switch contact and the “S” sleeve has a separate sense contact. The sleeve has separate DC and AC circuit paths / functions.
From quickly reading one of many patents and observing the action of my MBP;
The Mic Bias voltage seems to be applied only after a valid accessory has been detected. The apple device determines when a plug is inserted into the jack by sensing the tip switch. The apple device also determines if the plug is only a 3 pole (TRS headphone only), or when it is a compatible 4 pole TRRS accessory.
After a plug has been detected, the device applies the Mic Bias voltage which flows through a series 2,200 ohm isolation resistor and the afore mentioned inductor, and then monitors the state of the applied Mic Bias DC voltage. If the voltage is outside of the design limits, it turns off the Mic Bias voltage.
The device senses the Mic Bias voltage right after the 2,200 ohm series resistor, and this sense path does not include the isolation inductor mentioned above.
The mic audio signal (AC) has 2 or 3 different paths. One is the load of the series bias voltage components (2,200 resistor, plus the afore mentioned inductor) and a 4.7uF bypass capacitor to ground (at the DC bias source). Another consists of 3 components in parallel to ground (1nF, 15pF, 100,000 ohms). The remaining path is not completely known, but is the AC coupled input to the ADC.
The Mic Bias voltage is controlled by an IC which is powered by a 3.3V supply and has an I2C bus connection. Depending upon the DC load of the Mic, or in the case of measuring several different resistances, the expected output of this IC won't seem to be higher than about 2.8 volts or lower than about 2 volts. This is calculated from actual measurements across external resistances and then applied to internal components as indicated by the schematic of my MBP. These values may not be entirely precise, but are hopefully less speculative than some of the information I've seen out on the web (not from FaberAcoustical though).