  The noise of signals or currents consisting from a sequence of pulses,
elementary events or moving discrete objects (particles) is analyzed. A simple
analytically solvable model is investigated in detail both analytically and
numerically. It is shown that 1/f noise may result from the statistics of the
pulses transit times with random increments of the time intervals between the
pulses. The model also serves as a basis for revealing parameter dependences of
1/f noise and allows one to make some generalizations. As a result the
intensity of 1/f noise is expressed through the distribution and characteristic
functions of the time intervals between the subsequent transit times of the
pulses. The conclusion that 1/f noise may result from the clustering of the
signal pulses, elementary events or particles can be drawn from the analysis of
the model systems.
