REW’s Curve Fitting Algorithm of Impulse Response - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-25-14, 03:19 PM Thread Starter
Shackster
Randy

Join Date: Apr 2011
Location: Los Alamos, NM
Posts: 8
REW’s Curve Fitting Algorithm of Impulse Response

The graphs of Impulse Response data points in REW are always displayed with a fit curve. This curve is necessary to interpolate intermediate points between actual sampled data as in the option “Sub-Sample Timing Adjustment”. Although phase could be determined by the ratio of the Input/Output DFTs as recorded via the chirps in REW, Impulse Response data requires “up-sampling”, i.e. more points mathematically generated between actual sampled data points. For instance, at 20 kHz a phase of 5 degrees is ~ 0.7 usec yet sampling at 48 kHz would provide points separated by 20.8 usec. REW states that it resamples the data.

Below is an overlay of a typical Impulse Response and the Exported Text data smoothed via Excel. The REW Impulse Response data is in Green and overlaid with a fine Black line from Excel using its standard smoothing function. After searching extensively on Google, Excel appears to use (undocumented by Microsoft) possible spline fits such as a four-point Bezier (equally spaced points) or Catmull-Rom in displaying smoothed data. Since I want to also process REWs Impulse Data offline with programs such as Matlab or MathCAD, what is the curve fitting algorithm used by REW?
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Old 01-26-14, 12:16 PM
REW Author

John

Join Date: Apr 2006
Location: UK
Posts: 6,310
Re: REW’s Curve Fitting Algorithm of Impulse Response

REW uses a Hermite interpolator with a tension of zero, unless there is a large deviation at the beginning or end of the interpolation section in which case tension is forced to 1 to prevent excessive overshoot. The interpolator is only used to draw the graphs, exported data contains the raw response data points.
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 Tags algorithm , curve , fitting , impulse , response , rew’s

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