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production partner
12/09_01/10
sonderdruck
Dynac
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orange curves representing the FIR variants
are characterized in general by more details
and in the active version considerably grea-
ter steepness.
If you look more closely at the FIR setup in
the IRIS software, you will notice at once
that in the lower frequency range there are
an additional two IIR filters, a bell filter and
a high-pass filter. These minor reinforce-
ments are necessary because the FIR filters
no longer afford sufficient resolution or fre-
quency selectivity in this range.
Depending upon the selected number of
coefficients of the FIR filters, there is an
increase in the total latency of the system,
which otherwise, independent of the num-
ber of signal processing functions, is 1.03
ms at a sample rate of 96 kHz and 2.3 ms at
48 kHz. With a sample rate of 48 kHz and
512 coefficients, for example, the increase in
latency attributable to the presence of a FIR
filter in the signal path comes to 5.3 ms
when the filter is designed to be linear pha-
se. This value can be calculated by dividing
the number of coefficients by twice the
sample rate – in this case, then, by dividing
512 by 2 x 48.000 1/s. At this point, it must
be emphasized once again that the latency
is not conditioned inexorably by the princi-
ple of the FIR filter but by the filter function
itself. If the FIR filter is designed with a
minimal phase function, its latency can be
just as long or short as that of an IIR filter.
The latency is only a by-product of the line-
ar phase approach. What is needed, therefo-
re, is to find a good compromise with a suf-
ficiently short linear phase FIR filter combi-
ned with an IIR filter. The IIR filter is then
used where the short FIR filter for want of
frequency selectivity can no longer interve-
ne – i.e. at the bottom end of the frequency
range. This is the thinking here behind the
combination of a 512-tap FIR filter and two
additional IIR filters. These configurations
can be nicely appraised using the IRIS-Net
software (Fig. 15), in which the amplitude
and phase of all the filter elements can be
viewed singly and together – if necessary
in conjunction with the frequency respon-
se of the loudspeakers. At the present time,
as far as we know, the IRIS-Net software is
the only controller software that offers
such possibilities together with the measu-
red values of the loudspeakers (see Fig. 16).
Fig. 7: Vertical isobar curves of the 60° coaxial system. Discrepancies
between the horizontal and vertical curves are attributable to the
shape of the enclosure
Fig. 8: Maximum level at 3 % (blue) and 10 % (red) THD along with
the calculated curve (green) for the maximum supplied power of
900 W referenced to 4 ohms
Fig. 9: Controller functions for the passive version (above) with IIR
(blue) and FIR filters (red) as well as for the active 2-way version
(below) with IIR (light blue) and FIR filters (orange)
Fig. 10: Frequency response of the CXM 15 in passive mode without
the controller (green) as well as with the controller and IIR (blue) or
FIR filters (red)
