Abstract: An apparatus for servo track writing on a magnetic medium comprises a clock head (12) and a regular head (14). The regular head (14) writes an initial approximation of the clock track so as to leave a small gap between the last written pulse of the clock track and the first pulse of the clock track. Then this gap is measured by gap measurement system (33) and the frequency of clock track is updated using a direct digital synthesis system (35) so as to fill a full clock track during the next writing. The initial clock track is read by regular head (14) and its output signal is supplied to a phase locked loop. The output of the phase locked loop drives a direct digital synthesis system (35), which generates a modified frequency on its output. A signal of this frequency, in turn, is supplied to the clock head (12), which writes a new improved clock track on the disk.