     $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