After verifying my I2C communication primitives, programming the audio chip to convert an AUX signal into the stereo’s internal format was a matter of plug & chugging through the data sheet. That means I’ve finished stage two of Project RAUX Box!

The image above is logic analyzer output I captured last night showing noisy I2S signals from the chip. I’m pretty sure the noise is because I had to leave them “floating”, meaning they’re unconnected and there’s nothing pulling them to ground or 3.3 volts; when they go into the I2S receiver in the stereo’s DAP I’m confident completing the circuit will remove the noise. From what I’ve read, you don’t put pull-up resistors or anything on an I2S connection like you do with the I2C bus. I tried anyways and it only swamped the signal: the lines were either always at ground or almost entirely at 3.3V.

Getting this done last night means I’m on schedule so far. Next I’m moving from the development phase to prepping for installation. Today I hope to turn this breadboard circuit into something slightly less janky on protoboards with real solder joins for the connections, and add two relays for the I2S switching. It’s probably around a couple hours of diagramming and soldering, which I should be able to finish today with any luck.

I think installation will be pretty straightforward, in part because I’ve popped the stereo out before. After it’s out I just need to take some of the shielding off, locate a huge ribbon cable, cut the leftmost three wires, make six or nine solder joins, then test & debug.

I might actually finish this in time for the cruise! Fingers crossed. I think it will mostly depend on how much debugging I need to do post-installation.