C66 is effectively from a DC bias slightly lower than Au_VCC33, coupled to ground via 10uF. It looks completely wrong for capturing audio.

Cheers,
Dana K6JQ

Thank you for looking into this. I also do not understand the purpose of C66 and expect it would also reduce the audio quality of the right input. I understand that R78 / R132 is for Aux Detect (detect when a connector is plugged-in to the jack), R107 is intended to load the right input the same as ...

The Aux input from the 3.5mm Aux input jack has a 0.1 uF capacitor from left channel to right channel (C62), as well as 0.1u caps shunting each of the left and right inputs to Au_ground (C64, C65). This seems really wrong for audio input processing. Anyone have any idea why? I'm about to look at the...

If I understand correctly, you're using the esp-idf example i2s_adc_dac, which uses the internal ADC and DAC and does nothing to interface with the ES8388 codec on the LyraT 4.3 board. What you need to do is configure the ES8388 codec via i2c, configure i2s 0 for the desired sample rate, and then us...

I am not an attorney, but, if you don't turn the radio on and don't attach an antenna to the chip, then it's not an intentional radiator. So I'd think it's like any other MCU in that case - you may still require some FCC certification for Part 15.

Cheers,
Dana