As long as the bass management is done in the digital domain there will be no phase errors. That's one of the beauties of digital filters. Check out this technical article for a good explanation. The discussion is centered around CD player "brickwall" filters but the concepts are the same when applied to digital filters implemented as a crossover. The most commonly used type of digital filter for audio use is the FIR (finite impulse response) which is further discussed here . This not to say that digital filters are perfect, they have their own set of problems but the introduction of phase errors is not one of them.
Bass management done in the analog domain can introduce phase errors and having staggered cutoffs for the different channels could have some interesting results. However, in the case of the 950 analog bass management is only done on the 6ch input AND the crossover point is fixed at 80Hz for all channels. Therefore, any phase error introduced by the analog circuitry will be uniform across all channels. I also believe that these phase errors would be largely swamped by the effects of the room and speaker placement when considering the long wavelenths at 80Hz and the distances between the speakers and the sub(s).

I don't believe that that's true.

1) I was told by someone in the industry, that the digital filters used for BM are designed to mimic analog filters.

2) If you hunt around Secrets, there is an article by Brian Florian on why multiple (individual) crossovers are a bad idea. I can actually refute most of his arguments, but he does state that when you use individual crossovers, the shifts are different for the different frequencies. That has been backed up by tech support people at a few companies I know.

Plus, Anthem (one of them) has a special setting in their software to do the following. They have individual high passes but a global low pass. The wrong way to do it IMO, because you either leave holes in the coverage or overlaps unless you set all the filters to the same exact number. But anyway. There is a scenario that allows the full range LFE signal to be combined with the globally low passed info from the mains. But with the shift that takes place for the low passed info from the mains but not the full range LFE info, you get phase effects (and cancellation). So there is a special setting to artificially adjust the delay of the full range LFE info so it combines more in phase with the globally low passed mains info.

Now, if the global low pass filter didn't introduce any phase effects, you wouldn't need that special adjustment. But it's there nonetheless.

But personally, I don't understand *why* digital crossovers are designed this way. But that seems to be the way it is.

TD- your first link is for an antialiasing filter which is an FIR filter. Obviously, a different application than crossovers in a pre/pro. Good info, but I can only conclude that this *isn't* the type of filter (usually) used for digital crossovers for BM. I still have my connections at Anthem and Lexicon, so maybe I'll ask. Good info!

SH and BB4TB- Yes, I understand the phase effects. Same slope, same phase shift. I guess I need to do lttile bit more research into:

a) If a L-R 24/24 crossover is the best, why don't more companies use it?

b) Why my simplistic argument about the matching slopes (by freq) on either side of the crossover doesn't work.

Don’t forget the crossover used inside your passive speakers (If you have one). That is the most critical part.

One reason is that the bass drivers used in most speakers today are so small (6-8", which hardly qualifies as a woofer in my opinion) that they don't have enough bass extension to allow such steep slopes. Ideally, the response of a speaker should extend an octave above or below the point where it is to be crossed over. If this is not the case, the rolloff of the speaker itself is added to the slope of the crossover netowrk. A crossover that has a 12dB/octave slope when used with a typical speaker, yields a much steeper slope than the crossover alone.

I see SH’s point. If my speakers have a -12db/octave roll-off starting at 60-80Hz, and my crossover is has a –12db/octave roll-off at a similar frequency, my effective acoustic roll-off will be –24db/octave. In order to have acoustic continuity using an –24db/80Hz crossover, a full-range loudspeaker would need a rating down to 40Hz.

One method sometimes used to help fill in a gap in a situation where there is a 80Hz crossover point used with a full-range loudspeaker not useful down to 40Hz is to build a mild custom boost into the crossover, as in a Linkwitz Transform Circuit, to the lower end of the desired frequency range. This helps compensate for a combined roll-off of crossover and loudspeaker, but is usually not something that can generally be applied. The designer must know the characteristics of the loudspeaker in question before determining what frequency and amplitude to use in such a compensating circuit.

In general, it’s better to have speakers where such compensation is not needed. Also, if the speakers performance is too limited, too much compensation leads to distorted performance. I suppose that such compensation only works when the assistance the loudspeaker needs is only mild to moderate

SH- Wait- with steeper slopes, you don't need as much bass extension. With a less steep slope, you need more.

But I do agree with the "rule of thumb" that dictates setting the crossover a half octave to a full octave above the -3 dB point of the speakers specifically such that the natural roll off of the speaker doesn't "add" to the imposed slope from the crossover. I personally believe that a lot of people have their crossovers set too low.

Soundhound

I sent you a PM.

If the speaker itself is already rolling off in the vicinity of the crossover point, then it's slope is added to that of the crossover. If the speaker's natural rolloff occurs an octave above/below the crossover point, they are offset enough so that addition does not take place - there is minimal interaction.

Yes, a steeper crossover does take place over less bandwidth than a crossover with a gentler slope, but nonetheless if the speaker is already rolling off in the crossover region, then it's slope is added to the total slope. A steep crossover is best if there is adequate excess bandwidth available below or above the electronic crossover point.

This is an issue because many satellite speakers simply don't have solid response down to the 40Hz that would be required for no interaction with an 80Hz crossover network.

The manufacturers really have to design for the worst case, that that worst case is the reality that many speakers are already rolling off in the vicinity of 80Hz by themselves because of their small woofers and cabinets.

The 950 manual indicates a HPF slope of -12db/octave using 2nd order Linkwitz-Riley and a LPF slope of -24db/octave using 4th order Linkwitz-Riley.

The ICBM manual indicates, in ‘normal’ setting, both HPF and LPF slopes are 2nd order Butterworth.

Assuming that the ICBM’s Butterworth arrangement will cause a gentle rise to a +3db boost approaching the crossover point with a sum of the outputs, this would tend to compensate for a loudspeaker characteristic that already is -3db down at the chosen crossover point. If a loudspeaker and sub are down -1db at the crossover point, the acoustic in-phase sum would be +2db. However, a sub and loudspeaker will be slightly out of phase in all but a few ideal spots in the room, so I’m guessing there wouldn’t be the full +2db acoustic boost in reality.

Seems the ICBM designers were making some assumptions about real world equipment/environment and figured if the end result of the crossover would keep things within +/- 1db at the crossover point acoustically, they were doing pretty well – similarly for the 950.

SH- That's one thng I don't understand at all. The large number of speakers that are supposed to be used as surrounds and rears that are only rated to -3 dB at 80 Hz. That might work for a THX rated speaker where the speaker's roll off is purposefully combined with the crossover, but most people don't use a THX crossover setup with non-THX speakers. I even specifically asked Mirage about this with the OM-R2's, the speakers that are supposed to be used with what I have up front, and they either didn't understand the question, or pretended not to. So I went with their Omni 60's which are -3 dB to 45 Hz. And I even run that crossover a little high at 100 Hz, because 3 of the 4 are up against walls. The higher crossover helps deal with boundary low freq reinforcement.