I'm currently running Raspbian on a Raspberry Pi 3. I want to output all the audio that would normally go to the audio jack output also go to the I2S interface (on GPIO pins 12 BCK, 35 LRCK, 40 DOUT).
Most answers on the internet mention uncommenting
I was able to uncomment dtparams=i2s=on but dtoverlay=hifiberry-dac was not present in /boot/config.txt on Raspbian. Despite that I added the line dtoverlay=hifiberry-dac and rebooted after that, I did not see changes in voltages on these 3 I2S GPIO pins after playing sound. After that I tried what was described here: https://learn.adafruit.com/adafruit-max98357-i2s-class-d-mono-amp/raspberry-pi-usage Which did seem to output I2S on the GPIO pins now, but the audiojack out did not output sound.
How can I fix this?