  We simulate Gamma-Ray Bursts arising from internal shocks in relativistic
winds, calculate their power density spectrum (PDS), and identify the factors
to which the PDS is most sensitive: the wind ejection features, which determine
the wind dynamics and its optical thickness, and the energy release parameters,
which give the pulse 50-300 keV radiative efficiency. For certain combinations
of ejection features and wind parameters the resulting PDS exhibits the
features observed in real bursts. We found that the upper limit on the
efficiency of conversion of wind kinetic energy into 50-300 keV photons is
$\sim$ 1%. Winds with a modulated Lorentz factor distribution of the ejecta
yield PDSs in accord with current observations and have efficiencies closer to
$10^{-3}$, while winds with a random, uniform Lorentz factor ejection must be
optically thick to the short duration pulses to produce correct PDSs, and have
an overall efficiency around $10^{-4}$.
