Mijn excuses voor het Engels, maar we hebben deze vraag al meerdere keren gekregen en willen hierbij ons antwoord geven. Er is sowieso wel een interessante link over luidspreker correcties;
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DSP & liniear phase:
The initial software and firmware only support minimum phase crossovers and delay. A later update (the unit is user-updatable) will include an option to linearise the sum using an FIR inverse allpass filter before the crossover. If this technique is employed alongside LR2/LR4 filters the individual driver responses will become linear phase as well.
The first aim of a digital crossover is to afford detailed response correction. In an analogue crossover you can't have thirteen biquads to EQ a driver unless you're happy to run a signal through that many components and unless you can afford the parts cost and board space. The second aim of a digital crossover is to take advantage of the fact that sources are digital these days. An analogue crossover requires the signal to be converted to analogue first and then processed in the analogue domain. With a digital crossover, the signal goes from the DAC straight into the amp with no further processing. The third aim of a digital crossover is to have a universal product that people can actually use in a variety of projects. You can't tweak or configure an analogue crossover by software. An additional advantage is the availability of delays.
By no means all digital crossovers are linear phase. Commercially available ones that are, are not particularly good because they are brute-force FIR corrections which have all sorts of pre-echo problems. Please have a look at the presentation at
http://www.hypex.nl/docs/Bruno%20Masterclass/slides.htm . There is a section about loudspeaker correction starting from slide 101. I hope you will understand why we prefer to use IIR filters for the actual crossover and EQ, with optionally a FIR filter only correcting the sum phase. This works as follows. After delay correction, loudspeaker drivers are nearly perfectly minimum-phase. This means that you can use minimum phase EQ to fit the response to another minimum-phase target. No FIR is necessary for that. If you do this to EQ the woofer to a LR4 lowpass function and the tweeter to a LR4 highpass function, the phase response of the two drivers becomes highly regular. The sum will be a second order allpass filter with a Q of 0.7 at fc. So what separates a correctly designed crossover with LR4 acoustical slopes from a linear phase system is an inverse second order allpass, which can be approximated using a short FIR filter. As I mentioned in the previous reply, the individual driver responses become linear phase too, as you will appreciate from the pole/zero locations. Listening tests show that correcting the phase response of an LR4 system is only audible to some trained listeners and barely so. It is by no means spectacular.
This shows that linear phase is by no means the core of digital crossovers, and that it would be irrational for us to defer offering a product to the market only because the software does not yet include an easy way to insert the FIR section, especially when the FIR add-on is under development and user-updatable.
Now, the biggest advantage of the IIR xo/eq and FIR phase correction method is that the resulting response is completely free from pre-echos. Several companies sell boxes of the "shove a mic in front of the speaker and compute inverse FIR filter" type, and market these with snazzy looking impulse response plots which, crucially, were measured with the microphone in the exact same spot. Dare to move it by an inch and the response falls apart in both directions. Brute-force corrected speakers do not sound good, although they can be made to look very nice on paper.
From what we have seen published I gather that the sales literature of some of these companies tries to convinced that linear phase is the thing with digital. I hope that I've been able to explain that we *really* know about how to do digital crossovers.
Bruno, Chief R&D Hypex
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Jan-Peter
