Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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The following is from the Daqarta Help system:



Spectrum Analyzer

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Spectrum Window Width

Controls: Spectrum Dialog >> Width
Macro: SpectWidth

This button defaults to "Width 1024" which means the window function is applied to all of the data used to create the spectrum. In general, you should just leave it at this value. Repeated button clicks toggle to 512, 256, and 128 data points (with the button shown depressed), before returning to 1024 with the button back out.

The window function is always centered over the 1024 data points used for the FFT that creates the spectrum display. If you set Width 512, for example, the first and last 256 points are padded with zeros and only the center 512 data points receive the narrower window function. This effectively reduces the FFT from 1024 to 512 points.

As the effective FFT width is reduced, spectral resolution is reduced in proportion. However, temporal resolution is increased by the same proportion. (See Fundamental Time - Frequency Domain Concepts.)

This is of no value when viewing ordinary spectra, since there is no time axis to allow any such measurements. Spectrograms, however, do allow you to see the timing of spectral events. Yet even here the use of narrow widths has limited utility with live data. (Try it and see; make sure that Trigger is off, so you get maximum throughput.)

The place where the Width control is useful is when reviewing DDisk file data. There you can set the DDisk Read Step Size to small values so that, with Trigger off or in Pause mode, each 1024-point analysis data set moves only a small amount in time. (That is, each set overlaps much of the prior set.) This gives much higher temporal resolution than the case where each 1024-point data set comes after the previous one. Often, this small step size is all you need.

But sometimes you need still better temporal resolution and are willing to give up some spectral resolution to get it. This is the situation that the Width control was designed for.

A good example of where this is useful is with speech sounds. With a small Read Step and Width set at the default of 1024 points, you can easily resolve the harmonics of a vowel as a stack of horizontal lines. By toggling to smaller Width values like 256 or even 128 you can no longer resolve the individual harmonic frequencies, but you can easily resolve glottal pulses as a series of vertical lines. These are the bursts of sound energy from the vocal cords slapping together at the speaker's fundamental frequency (around 100 pulses per second for men), which excite the formant resonances of the vocal tract.

Examples are shown on the Spectrogram page of the Screen Shots section of the Daqarta Website.

Macro Notes:

SpectWidth=512 sets Spectrum Window Width to 512. The value you supply will be rounded to one of the following available widths: 1024 (default), 512, 256, or 128.

SpectWidth=x advances to the next Width value just like a manual button click: 1024, 512, 256, 128, 1024...

See also Spectrum Control Dialog


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