SID-Maths:

  Frequency:
  ----------

   To calculate the frequency corresponding to the 16-bit value in
   $D400+$D401, $D407+$D408, $D40E+$D40F use the following formula:

    Freq = 16Bit-Value * Phi2 / 16777216 Hz

   where Phi2 is the system-clock, 985248 Hz for PAL-systems,
   1022727 Hz for NTSC-systems.

   A good approximation for both systems is the formula:

    Freq = 16Bit-Value / 17.03

  Pulse-Width:
  ------------

   To calculate the pulse width (in %) corresponding to the 12-bit value in
   $D402+$D403, $D409+$D40A, $D410+$D411 use the following formula:

    PulseWidth = (16Bit-Value / 40.96) %