fft module - power spectrum density output

[edfritsch]

I am running the following worksheet with the FFT module set to give me power spectrum density (PSD) output:

I observe that as I increase the size of the vector length in the Data Window module, I get finer frequency resolution on the Y/t Chart module.  I can see the effect of vector size on the "quality" of the Y/t Chart; but regardless of the chart quality, the Y/t Chart module displays only one chart.

However, if the vector length is small (relative to the sample file I'm reading in) the outputs from the Y/t Chart module, as sent to the Write module, will provide more than one set of data (a multiple of 2) that can be plotted (outside of DASYLab) as PSD curves. 

What is the relationship between these curves from the written data and the chart produced by the Y/t Chart module? i.e. Can they be combined somehow to create a chart which matches the Y/t chart?

Or, is there a relationship between these curves and the size of the data sample I'm reading in? i.e. Do the multiple curves represent sequential analyses of the original data sample which is too "wide" to fit, all at once, into the narrow window defined by a relatively small vector length?

[enderWiggin10]

Hi edfritsch,

Please post toDASYlab's online support at:  http://www.dasylab.com/content/mydasylab.php?action=start  for all DASYlab that do not involve NI products.  Thank you.

Best,

[CJ_Butler__DASYLab_Pro]

Hi,

The Y/t chart is simply displaying a single block of the data, or the most recent snapshot. As you've noted, the block size helps to determine the resolution of the FFT computation.

To see more than the most recent sample, you can turn on the Waterfall feature, which will display the most recent block at the front bottom, and offset the previous blocks up and to the right.

Take a look at the Examples folder, yt_3d_chart.dsb