  Time-frequency representations such as the spectrogram are commonly used to
analyze signals having a time-varying distribution of spectral energy, but the
spectrogram is constrained by an unfortunate tradeoff between resolution in
time and frequency. A method of achieving high-resolution spectral
representations has been independently introduced by several parties. The
technique has been variously named reassignment and remapping, but while the
implementations have differed in details, they are all based on the same
theoretical and mathematical foundation. In this work, we present a brief
history of work on the method we will call the method of time-frequency
reassignment, and present a unified mathematical description of the technique
and its derivation. We will focus on the development of time-frequency
reassignment in the context of the spectrogram, and conclude with a discussion
of some current applications of the reassigned spectrogram.
