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comparison FC-handset-spec @ 47:f0419ac0a815

FC-handset-spec: ringtone generation documented

author	Mychaela Falconia <falcon@freecalypso.org>
date	Fri, 11 Jun 2021 00:13:39 +0000
parents	1fec0a3f09cc
children	cb8d43a5023c

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 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.
+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.

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comparison FC-handset-spec @ 47:f0419ac0a815