TOPIC:

Hi,

I am planning to build an 4 way speaker stereo speaker setup, and need an 8 channel active crossover for this. So I researched the market for potential solutions for this project, and found Flex Eight interesting. Although I have a question regarding phase linearization on the Flex Eight, which I hope to get some feedback on in this forum.

If we compare the regular Flex to Flex Eight, one of the main differences is that FIR filtering is available on the output channels of the regular Flex, where on Flex Eight FIR filtering is instead available on the 2x input channels.
That means that linear phase FIR crossovers is not available on the Flex Eight, as those would have to be implemented on the output channel for each driver.
However after googling around I found there may be another valid aproach to active speakers and for phase linearization in crossover region, which is inverted all-pass filters. Unfortunately inverted all-pass filters using IIR filtering is "non-casual", so it's not possible to implement those in practice because they are unstable biquads. But as I understand it is indeed possible to implement an inverted all-pass filter using FIR filters, which could then linearize phase in the crossover region.

Active speaker phase linear strategy on Flex Eight:
1. EQ each driver with minimum-phase IIR filters to flat'ish response ~1-2 octave below/above intended crossover frequency (depending on driver capabilities)
2. Output level match and time align all drivers
3. Use IIR high/low-pass filter for each driver - for example LR 24db
4. Implement one minimum phase inverted all-pass FIR filter for each crossover frequency on input channels calculated by for rePhase (Minimum phase filter tab, using mode compensate (inverted) + shape 2nd order all-pass Q0.707 for LR 24db example).

My question is if step 4 would be possible on the Flex Eight? 
I know FIR filters has a lot of use-cases, and some requires FIR to be on output channel, and other use-cases can be done on input channels.
Question is if use-case in step 4 of "unwrapping" phase from IIR crossovers can be done on "shared" input channels, when IIR filters are applied on seperate output channels, or if the phase unwrapping with FIR should ideally be done on the output channels?
It's my understanding this can be done on input channels, but just seeking confirmation / feedback from this forum

I know the strategy above does not factor in FIR correction for room modes or driver anomalies/excess phase caused by cabinet etc, but that is not my intend for now. 
 

There are already multiple threads on this topic.  Here's one example:
www.minidsp.com/forum/flex-series/20823-...n-flex-eight-outputs

Your objective is certainly possible with the miniDSP Flex 8.

Dave.