FF 58 04 nn dd cc bb
Time signature is expressed as 4 numbers. nn and dd represent the "numerator" and "denominator" of the signature as notated on sheet music. The denominator is a negative power of 2: 2 = quarter note, 3 = eighth, etc.
The cc expresses the number of MIDI clocks in a metronome click.
The bb parameter expresses the number of notated 32nd notes in a MIDI quarter note (24 MIDI clocks). This event allows a program to relate what MIDI thinks of as a quarter, to something entirely different.
For example, 6/8 time with a metronome click every 3 eighth notes and 24 clocks per quarter note would be the following event:
FF 58 04 06 03 18 08
NOTE: If there are no time signature events in a MIDI file, then the time signature is assumed to be 4/4.
In a format 0 file, the time signatures changes are scattered throughout the one MTrk. In format 1, the very first MTrk should consist of only the time signature (and tempo) events so that it could be read by some device capable of generating a "tempo map". It is best not to place MIDI events in this MTrk. In format 2, each MTrk should begin with at least one initial time signature (and tempo) event.