Trace Arithmetic - Questions and Validations

sam_adams

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Thread Starter
Joined
May 23, 2017
Messages
285
@John Mulcahy, just a few questions as a sanity check, kind of an "Am I doin' this right?".

Here's the scenario:

Make a measurement of a speaker as a control. Make some changes to the device. Remeasure the device. What I want to see is only the magnitude difference between the two measurements.

Using the Trace Arithmetic panel in the All SPL graph, I take the initial control measurement, Control, and divide it by itself, A/B, Control/Control, to produce a 0dB reference line.

I then take the the second measurement, Sample1, and divide that with the Control measurement, Sample1/Control, to produce the difference between them. The results seem to be correct.

Is this the correct procedure to display just the magnitude difference between the two measurements?

If this is correct, when examing the magnitude difference between the two measurement, the result of Sample1/Control, the magnitude difference from the A/B division as displayed at the bottom of the All SPL graph doesn't seem to jibe with the displayed magnitude difference of the individual measurements.

Is this caused by a rounding error in the math or is it the result of something like the screen scaling/resolution; or is it caused by some other factor?

I've attached a couple of screen shots and the .mdat file of the test measurements.

Your thoughts and explanations would be most helpful both to me and hopefully, other members.
 

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John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
7,213
Yes, A/B is fine for getting the relative magnitudes. The trace at any single cursor position is often not single-valued, within the span of a pixel there may be many data points. In those cases the graph trace for that pixel shows the min-max span of those data points, the trace legend shows the maximum value within the pixel span.
 
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