The following should be valid for any iPhone or later smartphone (including all Androids). The information was obtained from a major carrier, but “minor” device requirements (like these) are regularly waived for business reasons…so Your Mileage May Vary.
3.5mm 4-pole jack (aka “3-ring” or “TRRS”) pinout:
Tip = Left Audio output
Ring = Right Audio output
Ring2 = Ground
Sleeve = Microphone input
(Audio OUTPUT is available on the USB port, but not audio INPUT).
Bias voltage on Sleeve is between 1.65 and 2.1 volts.
Internal resistance is 2.2 kohm. A device connected to the jack should be detected if the
bias current drawn exceeds 100 uA.
** BUT a connected device of <100 ohms will perform other functions. So there is a minimum resistance for detection as well. Other functions may be available anywhere between 100 uA and full output current.
Max. input amplitude -45 dBV to -62 dBV
AF output -20 dBV into 32 ohms at mid-volume level
IMPORTANT: pinout standards for the 3.5mm 4-pole jack are different than the 2.5mm 4-pole jack. But this should not apply to Android devices.
I have tested a new, simpler interface for the Android, based on the information above.
Basically both TX (audio output) and RX (microphone input) are isolated with 600 ohm 1:1 transformers. The microphone input is shunted with 1.6 kohms (for the mic-detect) and then a series 0.1uF capacitor to block the phone’s DC bias from hitting the transformer (this particular transformer has windings of 235 mH, so 0.1uF is series-resonant at 1kHz).
It’s not intrinsically adjustable, but you do have MIC GAIN and AF OUT on the radio.
Schematic available upon request from nx7u at cox dot net.
Some measured audio data from a Motorola Bionic:
RX audio (into the mic):
-84 dBV…first faint blue line shows up on waterfall
-65 dBV…first green bar on RSSI indicator
-50 dBV…first yellow bar
-42 dBV…first red bar
-35 dBV…full scale RSSI indication
TX audio (out of the headphones, into a 2.2k resistor):
Media volume setting = 1/4…-48 dBV
Media volume setting = 1/2…-38 dBV
Media volume setting = 3/4…-28 dBV
Media volume setting = 1/1…-15 dBV
So what’s the best method for setting up audio? I got a Wolphi interface today and it seems I have the receive audio working well enough, but I’m not able to get the transmit audio set correctly. It was basically an all or nothing situation while adjusting the POT on the interface and watching the ALC. Listening to my audio on a nearby receiver seemed to sound OK, but not really sure if it’s right or not. My TX output power seems lower than it should be as well.
I’m using a Galaxy tab 10.1, with the output volume 2 clicks down from full.
I am not 100% if this is the correct way and maybe someone else can give a better advise.
This is the way how I adjust the tx level:
- adjust level on phone to maximum
- set radio to view ALC level
- turn tx level on wolphi-link to minimum (all the way to the right)
- start transmit in DroidPSK
- turn tx level on wolphi-link slowly left and watch the ALC level on the radio
- as soon as the ALC gets activated (first level bar on the FT817) stop tuning the tx level of wolphi-link.
Like I said I hope this is the right way and of course this procedure should be done on a dummy load. If someone has a better way or any suggestion please post it here.
Thanks, that’s basically what I did although I didn’t go full volume on the tablet. I went back and tried again, this time with a plastic tweaker instead of a metal screwdriver. That helped keep the level where I set it, but it is still a very touchy control. Not complaining, mind you (it’s not a very complex circuit after all), but just pointing it out to folks who may also have some trouble setting it up.
Yes the TX gain is a little difficult to adjust. I replaced the resistors around the RX gain pot last week to different values. RX gain is much easier to adjust now. I tried different values on the TX level as well but I couldn’t improve it if I want to be compatible with radios other than the FT817 as well.
I have been trying to get the interface to work with an Asus transformer Infinity without much success. When I connect the interface to the tablet it is detected (message comes up showing headphones are connected) and it will key up the transmitter once using the “start TX” button in Droid PSK. After that first time it will not key the transmitter again. I put a voltmeter on pins 3 and 4 and detected 3.8VDC as soon as I plugged in the cable, but only until I hit the “stop TX” button. It then drops like a rock. If I play music into the interface (transmitter on a dummy load) it always keys up the transmitter as does any system sounds, and the voltage doesn’t drop. However, if I transmit then stop with DroidPSK the voltage drops and no other sounds/apps will bring it back up, I have to unplug/plug in the audio cable to get the voltage back again. The tablet continues to indicate that a headset is attached even after the voltage drop. It doesn’t seem to happen with DroidSSTV, but there’s something else going on with that.
I wonder if it is going into some sort of sleep mode? I’ve not been able to duplicate on my SGS2, also running ICS.
This reply was modified 254 days ago by K0JEG. Reason: clarification
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