Spectrograms are produced by a procedure known as the STFT (Hlawatsch and Boudreaux-Bartels, 1992). During an STFT operation the entire signal is divided into successive short time intervals or frames (which may overlap each other in time). Each frame is then used as the input to a DFT, generating a series of spectra (one for each frame) which approximate the "instantaneous" spectrum of the signal at successive moments in time. To display a spectrogram, the spectra of successive frames are plotted side by side with the frequency range on the abscissa and the magnitude of the frequency component (frequency contribution amplitude) represented by greyscale or colour values.
A given STFT can be characterised by its frame length, usually expressed as the number of digitised amplitude samples that are processed to create each individual spectrum. The frame length of a STFT determines the time analysis resolution (i) of the spectrogram (Figure 2.18). Changes in the signal that occur within one frame length (e.g. the end of one sound and the beginning of
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