sentences:  The Wigner–Ville distribution is
  highly concentrated in time and frequency, but it is also
  highly nonlinear and non-local.Consequently, this
  distribution is very sensitive to noise, and generates
  cross-components that often mask the components of interest,
  making it difficult to extract useful information concerning
  the distribution of energy in multi-component signals.Cohen's class of
  bilinear time-frequency representations is a class of
  "smoothed" Wigner–Ville distributions, employing a smoothing
  kernel that can reduce sensitivity of the distribution to
  noise and suppresses cross-components, at the expense of
  smearing the distribution in time and frequency.