waveforms and spectra


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Introduction

This tool allows investigation of the relationship between the time and frequency domains. Users load in signals, and then see spectra of selected portions updated in near to real time. In the speech field, this can be used to illustrate the effect of window size, type and placement as well as short and long-term spectra.

The tool

Type 'wavspect' to launch the demo. When the window appears, use the load menu to load a signal file. Supported formats currently include .wav, .snd and .au sound files. The signal and its spectrum will appear. The spectrum is always computed from the portion of the signal between the two cursors.

The control buttons (3) allow the user to zoom in and out. Additionally, the cursors can be linked (that is, to move together) by checking the control (6). In this mode, the signal displayed will be shifted right or left when one or other cursor reaches the current display boundary.

Cursors are moved by dragging. The portion between the waveform will be played back if the checkbox is ticked (2).

If the 'recalc' checkbox (5) is ticked, the spectral display will be updated on-the-fly (as fast as possible).

By default, a rectangular window is used to select the signal between the cursors. A Hamming window can be applied instead (4).

When a new signal is loaded, the option exists (1) to add it in to the existing signal. This allows the spectra of signal mixtures to be investigated.

Things to investigate

  1. Load a sinusoid and investigate its spectrum as a function of the window size, type and placement.
  2. Load a speech signal. Examine its long-term spectrum. Now zoom in on a few 'pitch periods' and compare the spectra which result from careful placement of the cursors on the precise start and end of the fundamental period. How many pitch periods are necessary to see evidence of the fundamental frequency in the spectrum?
  3. How easy is it to find formants in speech spectra? What is the optimal window size/placement to reveal formants?

Further reading


Credits etc

Produced by: Martin Cooke

Release date: October 5th 1998

Permissions: This demonstration may be used and modified freely by anyone. It may be distributed in unmodified form.