A method of generating a clock signal from a moving medium includes detecting movement of at least one mark that moves synchronously with the medium. A generally sinusoidal output signal corresponding to movement of the mark is produced. The generally sinusoidal output signal is interpolated to provide a clock signal having a higher frequency than said generally sinusoidal output signal. The interpolation dynamically adjusts the phase of the interpolated output clock signal to correct for errors in the phase of said output signal. In one embodiment, servo tracks are written to a magnetic disk using a servo pattern signal, the phase of the servo pattern signal being adjusted relative to a clock signal. In another aspect, timing errors in a servo track writer are characterised.