I have just came up with an interesting idea. If we can use ADF7021 instead of Si4463, we could possibly implement analog FM too.
Transmitting could be done by modulating the VCO externally (via the “VCO IN” pin) and sending the same symbol constantly (to get the carrier).
Receiving: enable internal AFC (to track the FM carrier) and take voltage input of the VCO (again - the “VCO IN” pin).
What do you think? Si4463 can only allow analog TX, because there’s no VCO control voltage anywhere, but you can still modulate the VCTCXO.
I haven’t looked at the spec sheet for any of these. How much control do we have over the frequency? How fast can you update (read: modulate) it? Can you just update the VFO frequency digitally to modulate FM?
I was considering this with GMFSK: It’s only two bits per symbol, but those two bits specify what the next frequency the transmitter will put out. Can you basically send 2-bit PCM audio over FM? How horrible would that sound? Can you do 8FSK, and get 3-bit PCM? How high can these go?
Getting back to reality: I like the idea of hardware that’s more capable rather than less. But I’m also a fan of making something that is actually likely to succeed. So if it complicates the design too much, leave it for a v2 to be done later. If it’s a 6-of-one, half-dozen-of-the-other choice, then sure! Leave the options open!
Might have to look at how much processing power it would take, but you already need an ADC to record voice, why not get a combination ADC+DAC and use the DAC to feed an analog FM modulator, with an analog multiplexer switching the ADC between an FM demodulator and the microphone input.
In analog voice mode you can do some simple filtering in software and add CTCSS/DCS squelch tones before sending the audio out to the DAC. In digital mode you can essentially bit-bang the codec.
It is NOT enough to simply send 2-bit audio to an FM modulator and call it a day.
To meet bandwidth requirements and minimize intersymbol interference you need to filter it with a root-raised-cosine filter before pushing it out.