--- a/FC-handset-spec	Thu Jun 10 19:08:05 2021 +0000
+++ b/FC-handset-spec	Thu Jun 10 21:43:50 2021 +0000
@@ -27,7 +27,7 @@
 * hands-free loudspeaker
 * vibrator
 * USB port that combines charging and computer interface
-* wired headset jack
+* wired analog headset jack
 * single SIM slot
 
 The following features which are commonly found in mainstream proprietary
@@ -452,3 +452,88 @@
   of the right side button invokes no other action besides switching on full
   display brightness and the keypad backlight, this right side button can be
   found by touch, and thus solve this particular problem case.
+
+1.7. Audio routing
+
+3 different audio routing modes will be supported on our FC Libre Dumbphone
+handset:
+
+* Default mode: there will be a 32 ohm earpiece speaker physically mounted in
+  the usual place, on the front bezel above the display, to match up with the
+  user's ear in handheld operation.  There will also be a microphone toward the
+  front bottom of the phone, again in the usual place.
+
+* Hands-free mode: there will be an 8 ohm loudspeaker physically separate from
+  the 32 ohm earpiece speaker, physical location in the handset TBD.  In the
+  hands-free mode, the downlink audio will be switched from the earpiece speaker
+  to the loudspeaker, while the microphone input for the uplink will remain the
+  same.
+
+* There will be a wired analog headset jack with plug insertion detection; when
+  a headset is inserted, both audio input and output will be redirected to this
+  headset interface.
+
+1.7.1. Earpiece and loudspeaker separation
+
+Most current mainstream phones (in fact, all that I am familiar with) have
+physically separate speaker transducers for the earpiece function (hold up to
+ear to talk) and the loudspeaker+ringer function.  The earpiece speaker is a 32
+ohm load, and the loudspeaker is an 8 ohm load.  In a Calypso+Iota design, an
+external amplifier chip is needed to drive the 8 ohm loudspeaker, whereas the
+little 32 ohm earpiece speaker can be driven directly by Iota EAR output.
+
+However, a very different design was implemented by TI on their D-Sample and
+Leonardo boards.  They have only one speaker, one of 8 ohm kind, that is
+physically mounted in the position where the earpiece speaker would normally go.
+In order to not overwhelm the user's ear in handheld operation, they have
+peculiar circuit wiring where the analog signal from Iota to the loudspeaker
+amplifier goes through different resistor values depending on whether EAR or AUX
+output from Iota is used, and when the EAR output is selected, the high resistor
+values produce attenuation, such that the sound pressure level produced by the
+pressed-to-ear loudspeaker becomes comparable to that produced by a more
+traditional 32 ohm earpiece speaker.
+
+Furthermore, TI's single speaker design was not limited to their development
+boards.  Some years ago I found schematics for some very old LG phone (called
+A316 or B1200, not sure of the correct designation), this phone is from early
+TI era (pre-Calypso, using Ulysse/Nausica/Clara chipset), and it has the same
+arrangement as D-Sample and Leonardo.
+
+For our own FC Libre Dumbphone, I am going with the separate speakers
+architecture, using physically separate earpiece and loudspeaker transducers.
+This architecture feels more native to me, and it will allow for independent
+tuning of the two audio paths.  In my defense, all current mainstream phones
+seem to use the same architecture - the other approach with a single loudspeaker
+in the earpiece physical position seems very uncommon.
+
+1.7.2. Loudspeaker implementation
+
+The external amplifier chip for driving the 8 ohm loudspeaker will be TI
+TPA6203A1, copied from Leonardo schematics and proven good on FCDEV3B.  On
+FCDEV3B this amplifier is fed with signal from Iota EAR output, but on the final
+handset and on the Venus development board this amplifier will be fed with
+signal from Iota AUX output instead.
+
+The loudspeaker amplifier has an on/off control by way of a Calypso GPIO; in
+order to save battery, this amplifier needs to be off normally, and only turn on
+when a loudspeaker call is in progress or when a ringtone melody is played.
+
+1.7.3. Wired analog headset jack
+
+The analog headset jack on our FC Libre Dumbphone handset will be of 2.5 mm
+TRRS type, using pinout copied from iWOW DSK.  The headset needs to be wired as
+follows:
+
+* 32 ohm earpiece speaker connected between Tip and Ring2;
+* electret condenser microphone, positive connected to Ring1;
+* Sleeve is ground, should be needed only for the microphone.
+
+The advantage of this TRRS headset specification, as opposed to the simpler kind
+with a TRS plug and a common ground for the earpiece and the mic, is that our
+TRRS headset can be driven with either single-ended or differential earpiece
+driver outputs.  On the final handset, the wired headset interface will be
+connected to the Iota headset channel (HSMICBIAS, HSMICP, HSO) and thus the
+headset earpiece driver will be single-ended (HSO and GND), but the same headset
+can also be plugged into other FreeCalypso devices in which the jack is wired
+to the main Iota audio channel, with Iota EARP & EARN driving Tip and Ring2 on
+the TRRS headset jack.