FM Transmitter Mod

Very possible, I recommend starting with the Audio Personality Card example.  This uses i2s to transfer digital audio from the Moto Z, and plays out a tiny mono speaker.  It should be extremely easy to rip this apart and stick on an FM transmitter.  Control of the transmitter, such as tuning, may be a different story.  If you use the Audio Personality Card, i2c/SPI/UART are more difficult to access.

https://developer.motorola.com/build/examples/audio

Once you've gained understanding, you may choose to build your own circuit on the perforated board (sounds like you're capable).  Or, take it to the next level and Partner with Moto to build and ship your Moto Mod!

Please continue asking any questions you have along the way.

Very possible, I recommend starting with the Audio Personality Card example.  This uses i2s to transfer digital audio from the Moto Z, and plays out a tiny mono speaker.  It should be extremely easy to rip this apart and stick on an FM transmitter.  Control of the transmitter, such as tuning, may be a different story.  If you use the Audio Personality Card, i2c/SPI/UART are more difficult to access.

https://developer.motorola.com/build/examples/audio

Once you've gained understanding, you may choose to build your own circuit on the perforated board (sounds like you're capable).  Or, take it to the next level and Partner with Moto to build and ship your Moto Mod!

Please continue asking any questions you have along the way.

I'm going to read through the schematics. I'm considering doing this for my Capstone class. I would need to have it done by April. Sounds like a nice challenge. No?

Reading through the Internal schematics, I can see first that there is a TFA9890AUK. This is an audio amplifier. So If I can find a way to channel this, I might be able to utilize the software to amplify the signal.

There are two metal knobs that connect to the speaker. I'm guessing this is some form of Audio output that is Analog. I would love for this to be so, since I would have a left and right output. Either or, I would be able to implement a radio transmitter.

Still investigating the schematics. Hope to have it done by the end of the week.

So a little background information of what I interpreted from circuit diagrams, and such.

The audio system seems to draw it's power from a TFA9890A module on the board. Drawing power from the system, in different forms there are power supply mechanisms it uses to do this. In the main schematics, Moto was kind enough to provide us with a BTB connector diagram, as well as a schematics for this power supply chain. I've attached both below.

Just a side note, I am documenting everything for the purpose of better serving those in the Moto Mods community.

The Loudspeaker system contains the pin diagram fro this module. Just by researching it. I identified the two outpins that lead to the SPK_OUTA and SPK_OUTB. Not knowing what these were, I researched the technical paper on it and read it. Seems that there is an inverted output that is given off by this system. Normal being A and Inverted being B. I don't know if this will lead to anything, but I know that it's a start.

Notice the bottom right hand corner. The outputs line up with what is on the board itself. You can also identify power being drawn from the EE_PWR pin on the board and on the diagram. The correlation is easily identifiable.

Just another update. I've successfully read through the specification, and the hardware side of things. I have to look over the software, more specfiically the Audio MDK. If I can tweak this, with using an alternative Class-D Audio Amplifier, such as the TFA9890AUK, I can have an audio output that can be tweaked using another ADAFruit FM Transmitter. I've ordered both, but I still have to investigate the interface. I'll post on here once I find out more about these three things.

Some Sources I'm using in relation to this.

https://www.infineon.com/dgdl/an-1071.pdf?fileId=5546d462533600a40153559538eb0ff1

https://cdn-learn.adafruit.com/downloads/pdf/adafruit-si4713-fm-radio-transmitter-with-rds-rdbs-support.pdf

https://github.com/MotorolaMobilityLLC/mdkaudio