The input I am looking at is the 5-pin type (https://www.adafruit.com/product/1699).
1. Is there a way to use both Left and Right channels from the jack? Say, to average them somehow before sending data to the Arduino?
2. Looking in this post (https://forum.arduino.cc/index.php?topic=476900.0) I've seen two different methods of processing the signal from the jack before sending the the A0 pin, one an envelope follower, and the other a sort of biasing circuit.
FYI - There is a chip called the MSGEQ7. It gives you 7 bands filtered and then output as a DC voltage (like a voltage follower). That makes your software a LOT easier if you can live with 7-bands. That chip doesn't require bias on the signal.
To summarize, I should put 10K resistors from both L and R pins from the jack, then run it through the bias circuit from before, omitting R1.
Would the MSGEQ7 do that, i.e give a value that can be put into analogWrite?