note # 3
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.
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.
Figure 3 plots the spectrum of the selections in figures 1 and
2, obtained from a 32k points FFT.
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.
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.
Figures 5 and 6 represent the spectrum (magnitude and phase)
of the equalized impulse response, much flatter than previous
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
the amplifier level was
increased in order to obtain a louder MLS stimulus.
Figure 9 plots a Y zooming of the frequency data shown in figure
7. Note that the spectrum is flat within +/- 0.5 dB.
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!
PDF file (384K) Shift-click
Back to Application Notes