This is fantastic, but I wonder how could I miss it in the old days. I had the full C64 reference manual and all I found was an audio in that could only turn on

This is fantastic, but I wonder how could I miss it in the old days. I had the full C64 reference manual and all I found was an audio in that could only turn on and off the incoming signal, which of course was a lot less fun than letting it through the SID filters; was it a different input? I'm not sure I used the DIN connector though, I probably interfaced to the user port; can't remember.