     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) %