sentences:  Only regions of slow phase variation (stationary phase) will contribute significantly to the reconstruction, and the maximum contribution (center of gravity) occurs at the point where the phase is changing most slowly with respect to time and frequency.The time-frequency coordinates thus computed are equal to the local group delay, formula_20 and local instantaneous frequency, formula_21 and are computed from the phase of the short-time Fourier transform, which is normally ignored when constructing the spectrogram.These quantities are "local" in the sense that they represent a windowed and filtered signal that is localized in time and frequency, and are not global properties of the signal under analysis.