--- a/FC-handset-spec	Thu Jun 10 21:43:50 2021 +0000
+++ b/FC-handset-spec	Fri Jun 11 00:13:39 2021 +0000
@@ -537,3 +537,106 @@
 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.
+
+1.8. Ringtone generation
+
+In terms of the physical sound-emitting element, there are two principal ways
+in which cellphone ringing sounds can be produced:
+
+1) The oldest and most classic way is to use a magnetic buzzer controlled by
+   Calypso BU/PWT digital output.  The buzzer is driven with raw battery voltage
+   being switched with a "digital transistor" (BJT with bias resistors), and the
+   control input going to the base of the BJT is Calypso BU/PWT output.  This
+   method is standard in older phones that don't have hands-free loudspeakers:
+   since there is no loudspeaker for that other purpose, some loud noise-making
+   element needs to be implemented just for ringing, and old-style buzzers are
+   the traditional choice.  Motorola C1xx lower subfamilies (C11x/12x and
+   C139/140) use such buzzers for ringing.
+
+2) In phones that feature a loudspeaker for hands-free operation, the same
+   loudspeaker is also used for ringtone sounding, and the buzzer is eliminated.
+   Apparently hands-free loudspeakers weren't popular in the Calypso era, thus
+   Pirelli DP-L10 is the only known Calypso phone that features one.  There is
+   also a more bizarre possibility: some phones have replaced the ringing buzzer
+   with a loudspeaker, but that speaker is used *only* for playing ringtone
+   melodies - no hands-free call feature is offered.  This situation exists on
+   Motorola C155/156 phones.
+
+Furthermore, if the physical sound-emitting element is a loudspeaker and not an
+old-fashioned buzzer, and if the chipset is Calypso, as opposed to various newer
+chipsets, then a further distinction arises:
+
+2a) Most historical commercial phones that are based on Calypso and use a
+    loudspeaker for ringtone sounding, also contain a special ringtone generator
+    chip that drives this speaker when a ringtone is played - in other words,
+    their loudspeaker ringtones do NOT go through the same voice audio path that
+    is used for hands-free calls, if the latter feature is offered at all.  The
+    speciai ringtone generator chip is typically a combined MIDI player and
+    loudspeaker driver.
+
+2b) The much less popular approach is to not implement any extra hardware at all
+    that is specifically for ringtone generation, use only the same loudspeaker
+    audio path hardware that is already needed for hands-free calls (a simple
+    loudspeaker amplifier), generate ringtone melodies in the Calypso DSP
+    (Melody E1 or E2), and play them through the regular voice audio path,
+    routed to the loudspeaker.
+
+Approach 2b is the least popular one among historical commercial Calypso phones,
+but it is the one which I (Mother Mychaela) have selected for our FC Libre
+Dumbphone handset.  But let us nonetheless examine the pros and cons of
+different approaches, and see why I have settled on option 2b.
+
+Buzzer melodies (option 1) are monophonic (the signal carrying the melody to
+the buzzer is a digital square wave, and there is no way to create multiple
+overlapping notes in such a signal), but a decent range of tone frequencies is
+available: the magnetic buzzer hardware can easily produce frequencies all the
+way up to the limit of human hearing, while Calypso PWT produces musical notes
+from F4 through E8 in the scientific pitch notation, or 349 to 5274 Hz.  In
+contrast, any tones produced through the 8000 samples/s voice audio path have
+to be below the Nyquist frequency of 4 kHz - according to TI's document, Melody
+E1 notes go from E4 (330 Hz) up to G#7 or 3322 Hz.
+
+In terms of the richness of possible ringtone melodies that can be played,
+option 2a (external MIDI player chip that drives the loudspeaker directly,
+without going through Calypso+Iota voice audio path) is the absolute best.
+These specialized ringtone generator chips implement full MIDI (128 instruments,
+47 drums, up to 64 simultaneous notes, according to one datasheet), and the
+available ringtone melodies for these MIDI chips are very rich - just take a
+Pirelli DP-L10 phone and listen through various ringtone melodies it offers.
+All of Pirelli's melodies are stored as *.mid files in their FFS, thus we could
+very easily copy them if we were to adopt a MIDI player chip similar to theirs.
+
+However, the problem for us with adopting option 2a is that this option would
+introduce significant extra complexity and adversely affect our already long-
+overdue project schedule.  There is only one MIDI ringtone player chip for which
+we have enough documentation to attempt such a feat: this chip is Winbond
+W56964, a slightly more capable sibling of the W56940 used in the Pirelli phone.
+However, just because we have what at first glance appears to be reasonably
+complete documentation does not mean that it would be a slam dunk!  We would
+have to build a separate test board for this chip, connect it to a Caramel2
+motherboard (it needs to connect to Calypso MEMIF), and then spend significant
+time climbing the learning curve and getting this chip to actually work: getting
+it to play melodies, and just as important, getting it into and out of sleep
+modes.  In other words, a lot of extra work and time spent just for this part,
+not advancing any other project needs - whereas option 2b eliminates all of this
+extra work.
+
+Finally, a philosophical argument can be made that FreeCalypso should be all
+about simplicity, producing a phone that does its job: implement just what is
+needed for a functional phone, and omit unnecessary baggage.  An extra hardware
+circuit (a chip connected to Calypso memory bus, no less) and associated
+software complexity that serves absolutely no other purpose except to produce
+ringtones that are "richer" than what the Calypso can produce on its own, and
+does not assist in any way with hands-free loudspeaker operation for calls (it
+is more of a hindrance in that mode), can be seen as an unwelcome burden similar
+to other unwelcome burdens which we are already eliminating, like the camera.
+
+In contrast, with our chosen option 2b, we have exactly zero extra hardware
+that is just for ringing: our loudspeaker and its associated driver circuit
+(simple amplifier) is primarily for hands-free calls, and the ability to use
+the same audio path to play ringtone melodies comes literally "for free" with
+the Melody E1 feature built into our Calypso DSP.  We already have a selection
+of nice-sounding ringtone melodies in E1 format, lifted from the legendary TSM30
+source, and software complexity is minimal: the melody playing engine has
+already been implemented by TI, we only need to call RiViera Audio Service API
+functions.