Fir Measurements; Conclusions - Crown I-Tech 4x3500HD DriveCore Series Operation Manual

16 Application of FIR Filters to Loudspeaker Crossovers
The remaining two LR crossover graphs in Fig. 17.6 show the

16.13 FIR Measurements

reversed-polarity on-axis response and the ±25° off-axis response.
The Crown I-Tech HD amplifier was used to create a linear-phase FIR
note that when the tweeter leads are reversed, the response exibits a
crossover with a 3.67 ms latency. This latency allows crossovers to be
deep null at the 3 kHz crossover. However, the null is also accompa-
created with filters whose corner frequencies can extend down 500 Hz.
nied by a wide two-octave depression in response. The second graph
Selections of higher latencies allow sharper cutoff filters to be created
of the pair shows the 25° off-axis null due to driver spacing. Not
with correspondingly lower corner frequencies. FIR filters with
surprisingly, the response is similar to the on-axis response when the
latencies from 0.92 ms up to 29.33 ms can be created with the
polarity is reversed.
amplifier's software with minimum corner frequencies ranging from 2
kHz down to 62.5 Hz. the 3.67 ms latency FIR filter provides a relatively
sharp rolloff filter with reasonably short latency.
10 Figure 7 shows two graphs illustrating the measured frequency
response magnitude (left) and phase (right of the 3.67 ms latency FIR 3
0 kHz crossover generated by the HD amplifier. The right graph shows
the measured phase of the crossover with the phase rotation due to the
latency and speaker-to-microphone transit time subtracted out. It
-10
shows a linear zero-phase characteristic over most the measured
range.
-20
Note the extremely fast filter rolloffs in the left graph which approach
-30
150 dB/octave and the exceptionally-narrow one-third octave
20 100 1k 10k 20k
Frequency - Hz crossover span.
10
0
-10
-20
-30
20 100 1k 10k 20k
Frequency - Hz
Fig. 16.6 Reversed-polarity on-axis response (left) and ±25° off-axis
response of a two-way system with a 4th-order 3 kHz crossover.
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I-Tech HD DriveCore Series Power Amplifiers I-Tech HD DriveCore Series Power Amplifiers
16 Application of FIR Filters to Loudspeaker Crossovers
10
0
Low Pass
-10
High Pass
-20
-30
20 100 1k 10k 20k
Frequency - Hz
10
0
-10
-20
Low Pass
High Pass
-30
20 100 1k 10k 20k
Frequency - Hz
Fig. 16.7 Magnitude (left) and phase (right) of a 3.66 ms latency 3 kHz
Fig. 16.8 Measured individual driver responses (left ) and overall
FIR crossover filter generated by an I-Tech HD amplifier.
summed on-axis response (right) of the two-way system of Fig. 5
driven with 3.67 ms latency 3 kHz FIR crossover filter generated by an
I-Tech HD amplifier.
Figure 16.8 shows the measured individual driver responses of the
same two-way system driven by the FIR crossover as produced by the
Figure 16.8 shows the measured individual driver responses of the
Crown I-Tech HD amplifier and the resultant summed 1m on-axis
same two-way system driven by the FIR crossover as produced by the
response. Note the extremely sharp filter rolloffs in the left magnitude
Crown I-Tech HD amplifier and the resultant summed 1m on-axis
graph. Note also that the right on-axis response is essentially identical
response. Note the extremely sharp filter rolloffs in the left magnitude
to the previous LR on-axis response.
graph. Note also that the right on-axis response is essentially identical
to the previous LR on-axis response.
The final two graphs, shown in Fig. 17.9, illustrate the reversed-
polarity on-axis response and the ±25° off axis response for the FIR
crossed-over system. Note the extremely narrow crossover regions due
to the rapid filter rolloff of the FIR filter. Psycho acoustic tests reveal
that these narrow high-Q dips in the response are much less audible
than low-Q wider dips.
Operation Manual Operation Manual
10 10
0 0
-10 -10
Woofer
Tweeter
-20 -20
-30 -30
20 100 1k 10k 20k 20
Frequency - Hz
10 10
0 0
-10 -10
-20 -20
-30 -30
20 100 1k 10k 20k 20
Frequency - Hz
Fig. 16.9 Reversed-polarity on-axis response (left) and ±25° off-axis
response of the two-way system of Fig. 5 driven with 3.67 ms latency 3
kHz FIR crossover filter generated by an I-Tech HD amplifier.

16.14 Conclusions

Crown's powerful DSP processing power in the latest I-Tech HD Series
amplifiers allow sophisticated crossovers to be implemented using
Finite Impulse Response (FIR) filters. FIR filters offer a number of very
strong advantages when used to implement DSP electronic
loudspeaker crossovers.
16.15 Refereces
For those who want a more in depth treatment of the topics in this white
paper, here are three good reverences.
[1.] A.V. Oppenheim, R. W. Schafer, Digital Signal Processing,
(Prentice Hall, Englewood Cliffes, NJ, 1975).
[2] E.C. Ifeachor, B.W. Jervis, Digital Signal Processing, A Practical
Approach (Addison-Wesley Publishers, Reading, MA, 1996).
100 1k 10k 20k
Frequency - Hz
[3] G. J. Krauss, "Advantages of FIR Filters in Digital Loudspeaker
Controllers," Presented at the 120 convention of the Aud. Eng. Soc.,
th
Convention Paper 6779. (May 2006, Paris)
100 1k 10k 20k
Frequency - Hz
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