$D418/54296/SID+24:  Select Filter Mode and Volume

   +----------+---------------------------------------------------+
   | Bits 7   |   Cut-Off Voice 3 Output: 1 = On, 0 = Off         |
   | Bits 6   |   Select Filter High-Pass Mode: 1 = On            |
   | Bits 5   |   Select Filter Band-Pass Mode: 1 = On            |
   | Bits 4   |   Select Filter Low-Pass Mode: 1 = On             |
   | Bits 3-0 |   Select Output Volume: 0-15 (linear steps)       |
   +----------+---------------------------------------------------+

   This register is write-only!

Kernal-Reference:

 STX $D418   : $FDC4