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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

dtparams=i2s=on

dtoverlay=hifiberry-dac

in /boot/config.txt

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.

: enter image description here

How can I fix this?

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The adafruit instructions are for replacing the internal audio output with i2s. You'll need both devices enabled and working to send audio to both at once. So places where you remove the snd_bcm2835 device you'd instead leave working.

You can create an asoundrc that appears as a single sound device to your audio applications, but outputs to two hardware devices. The first net result I found was http://www.6by9.net/output-to-multiple-audio-devices-with-alsa/

Getting up to speed with alsa isn't easy, so you might be better off with a hardware amplifier or splitter.

Good luck!

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