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Application note # 3
Sample Champion

 

Equalization and pre-equalization

Sample Champion can perform a complex multiplication or division in the frequency domain of the MLS signal spectrum and/or of the calculated FFT data by a selected file. Here below there are some applicative examples of this feature.

 

Impulse Response

Figure 1


Figure 1 plots the impulse response of a loudspeaker. The anechoic part has been selected, rejecting the first reflected impulse. Figure 2 plots a Y zooming (8x) that helps in finding the reflection.

 

Impulse Response

Figure 2

 

Frequency Response (not equalized)

Figure 3


Figure 3 plots the spectrum of the selections in figures 1 and 2, obtained from a 32k points FFT.

This spectrum has been saved in a file and set as MLS compensation file ("divide by" option). The measurement has been repeated with the same configuration.

The result of the measurement with the equalized MLS is shown in figure 4. As expected, now the impulse is much more similar to a Dirac impulse. Note that only the samples within the selection have been equalized.

 

Impulse Response, after  MLS equalization

Figure 4


Figures 5 and 6 represent the spectrum (magnitude and phase) of the equalized impulse response, much flatter than previous one!

 

Spectrum, after MLS equalization

Figure 5

 

Phase, after MLS equalization

Figure 6


The following figures 7, 8 and 9, instead, report
the measurement performed in the same configuration as before, by using an uncompensated (flat) MLS stimulus and computing a post-compensation on the FFT calculated data, with the same previous equalization file. Note that the different level is due to filtering while previously the amplifier level was increased in order to obtain a louder MLS stimulus.

 

Spectrum, after post-compensation

Figure 7

 

Phase, after post-compensation

Figure 8


Figure 9 plots a Y zooming of the frequency data shown in figure 7. Note that the spectrum is flat within +/- 0.5 dB.

 

Figure 9


WARNING: In order to obtain a correct result, the FFT size, the number of spectrum data in the equalization file, the MLS length and the input buffer length must be the same!

 

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