Spectrograms

Can plot the short-term power in different frequency bands as a function of time - called the spectrogram. For example, figure 11.

  
Figure 11: A spectrogram of the utterance ``She had your dark''

Originally implemented by recording onto magnetic tape, then passing the tape over a rapidly spinning drum contain the playback head (one loop around was the ``window''), the resulting signal was fed to a bandpass filter whose centre frequency was determined by the position of a stylus on head sensitive paper - the resulting signal being used to mark the paper with the intensity of the signal at that frequency.

Serious choice of filter bandwidth:

• Narrow band analysis: resolves harmonics but blurs temporal detail such as burst onsets.
• Wide band analysis: resolves fine temporal detail but looses fine frequency detail.

In most applications this tradeoff is manifested as the choice of window size. Good rule of thumb is to include a few pitch periods, say 20ms to 32ms.