Spectrogram: Difference between revisions

Revision as of 20:56, 19 April 2007

Revision as of 01:30, 20 April 2007

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Spectrograms are usually created in one of two ways; either with a series of [[bandpass filter]]s, or calculated from the time signal using the [[short-time Fourier transform]] (STFT).

The filter method is usually used in the [[analog (signal)|analog]], continuous version of measurement. The frequency range of the signal (an audio signal, for instance, would have frequencies in the range of 20 Hz - 20 kHz) is divided into equal sections, either linearly (0-100, 100-200, 200-300, ...), or logarithmically (10-100, 100-1000, 1000-10000, ...). The signal is input to a corresponding [[audio filter|filter]], which removes most of the signal that does not fall within its frequency band (imperfect [[window function]]s and limited frequency resolution will cause some "bleeding" btweenbetween adjacent frequency bands). The magnitudes of each filter's output are recorded as functions of time. Each recording then corresponds to a horizontal line in the image; a measurement of magnitude versus time for a specific frequency band.

To calculate the spectrogram using the magnitude of the STFT is usually a [[digital (signal)|digital]] process. Digitally [[sample (signal)|sample]]d data, in the time ___domain, is broken up into chunks, which usually overlap, and Fourier transformed to calculate the magnitude of the frequency spectrum for each chunk. Each chunk then corresponds to a vertical line in the image; a measurement of magnitude versus frequency for a specific moment in time.