$D417/54295/SID+23:  Filter Resonance Control / Voice Input Control

   +----------+---------------------------------------------------+
   | Bits 7-4 |   Select Filter Resonance: 0-15 (linear steps)    |
   | Bits 3   |   Filter External Input: 1 = Yes, 0 = No          |
   | Bits 2   |   Filter Voice 3 Output: 1 = Yes, 0 = No          |
   | Bits 1   |   Filter Voice 2 Output: 1 = Yes, 0 = No          |
   | Bits 0   |   Filter Voice 1 Output: 1 = Yes, 0 = No          |
   +----------+---------------------------------------------------+

   This register is write-only!