Someone explaining the function of an Anthem pre/pro raised what I consider to be a good question.

It seems that every filter introduces a phase shift that is a function of the knee frequency (among other things). A bass management system that applies a single universal crossover frequency introduces the same phase shift into each channel, and it is relatively easy to resynch.

But in a multi-channel BM system (like my 950), each different crossover frequency introduces a difference phase shift. It seems to me that these would be a pain to re-synch before combining.

How does the 950 deal with the problem?

It doesn't. No pre/pro does. And, get this: even for pre/pros that do simple 5.1 analog pass through: even if you adjust phase correct for DD/DTS/CD if you're using a coax or optical connection to your pre/pro, you still might not be in phase for the 5.1 analog inputs, even if you have the player setup as "all large", "sub on".

BTW, the phase between the mains and sub *more* depends on the slopes of the high pass filters (to the speakers) and the low pass filter (to the sub), and not so much the crossover freqs themselves.

The best suggestion I can offer you, is to pick up the DVD-A setup disc offered by Chesky, "The Ultimate DVD Surround Sampler & 5.1 Setup Disc". This disc has DVD-V and DVD-A phase tests that at least you can characterize your system with.

Since we are talking about phase, and wrt the 950... I had previously reported that the 950 reversed the phase of the 5.1 analog inputs when using its analog 80 Hz crossover with respect to the internal digital crossovers (for DD/DTS/CD). I now know that what the 950 does is unknown. What I do now know, is that if you are using a Pioneer player (the 45A and 47Ai at least), and you compare phase out of the analog inputs vs going digitally to a pre/pro, *then* you are already out of phase. I.e., using the analog outputs on the player, even if "all large", "sub on", the phase relationship between the mains and sub *is* different than going digitally to a pre/pro. I had assumed it was the 950, but it's the player. What I don't know, is how this works with any other DVD player's 5.1 analog outputs...

Theorectically, "all large", "sub on" does not do anything to the phase relation between the sub and mains. But most digital crossovers use a 24 db/octave low pass, and a 12 dB/octave high pass filter, and, usually you get a 90 deg shift for every 6 dB/octave of slope. So *that's* where the *difference* in phase comes from, between the analog outputs on a player, probably *any* player, and a pre/pro using its digital crossovers.

And to confuse things even further, obviously some players do have BM for SACD and DVD-A, but those slopes might not be the same between SACD and DVD-A, or even the same as what the pre/pro is doing!

I'm just surprised that more people and HT mags haven't picked up on this...

Now, there actually are (at least) 2 solutions. One, and I think some Anthem pre/pros do this, is to run "all large", "sub on" to the pre/pro, have the pre/pro digitize those inputs, and then use the same crossover scheme as for DD/DTS for DVD-A and SACD. This is a kludge IMO, because that extra A to D, processing, and D to A conversion *will* affect the sound quality. The 2nd is ... i.Link. You deilver everything to the pre/pro in its native digital format, and then the pre/pro can apply the same exact BM to everything.

I'm hoping that the next Outlaw pre/pro has i.Link...

I would think that the three basic types of crossovers, passive, active and digital, would handle the division of frequencies in different ways. Not being a crossover designer, I may be speaking out of hand, but I’ll offer my thoughts in simplistic fashion for discussion.

If a single stream of data is divided by some means into two or more streams based on frequency, and then each stream put through the same type of D/A conversion, the resulting outputs, even if different in phase from the original input, would, in a decent design, still be properly in phase with each other.

Similarly with an active analog crossover, where the division of frequencies are usually made by complimentary resistor-capacitor networks in association with op-amps, there is a phase shift, but my understanding is that all frequencies shift in the same direction, and at the point of crossover, by the same amount. The points of crossover are the most important, where some small portion of the signal will be sent in two directions and would hopefully be reproduced in phase with each other.

Passive crossovers, now there’s a different animal. At each crossover point, both a capacitor and a coil are usually used in the division of frequencies, the phase shift being in opposite directions. Depending on the complexity or simplicity of the crossover, if the output to one driver leaves the crossover in near opposite phase to another, one driver may be wired to the crossover in opposing phase to another driver in order to help restore the proper phase relationship of the moving drivers.

In a pre/pro or other device dealing with bass management, it would seem to me that three reasons exist for not giving the consumer phase adjustment controls. One, the implementation is unnecessary. Two, the implementation is complex and/or would add cost deemed by designers/manufacturers not to be an advantage when marketing a device to consumers, unless that device is meant expressly to deal with the phase issue. Three, giving most consumers more to adjust might lead to increased frustration for some users who would rather have things as simple as possible.

Most powered sub manufacturers give the consumer the option to switch the phase relationship by 180º, and the same option exists for passive subs by reversing the speaker leads. Some powered subs have a variable phase adjustment. If the method of dividing the frequencies is consistent for all frequencies, a one time adjustment of a subwoofer should be all that is necessary. I don’t that kind of consistency is too difficult to achieve.

As far as I can tell the Anthem allows one to set separate high-pass freqs for each channel, but uses a single low-pass for all channels, keeping the phase of the combined result coherent. Problem is, that leaves coverage gaps between the high and low freqs and you end up throwing info away.

If the whole idea is that the loss of info is compensated by the coherence of the bass, then phasing seems to be a big problem (at least in their minds).

I'd like to hear the Outlaws address this issue, too.

bang, that's pretty much how I understand the different types of filters to work, too. I think you're right about the output phase shifts being coherent between the two resulting streams. That isn't a problem when the stream is going to a standalone transducer, but when you have 7 streams potentially shifted to 4 different angles, then combined with a 5th stream into a sub out, what happens when the same signal is present on all 5 sources? I'd think it could get chopped up pretty good.

Another problem with phase shifting controls (adjustments) is that as the frequency goes to zero, the delay time goes to infinity. They'd have build in an absolute high-pass filter and stop quoting response down to DC.

I wonder whether the 'distance' configuration might be used to tweak bass stream phases? I wonder how you'd measure it without a 4-channel scope?

I’m not familiar with the Anthem’s handling of bass management, but my understanding of the Outlaw 950 leads me to believe that choosing different crossover points for main, center and surround does not produce a ‘gap.’ In digital mode, if L/R are set at 60Hz, the center at 80Hz and the surrounds at 100Hz, the sub receives all that is below each setting, channel by channel. When using analog bypass mode with bass management ‘on,’ all speakers get the same 80Hz crossover split.

As to timing and phase, I suppose an ideal adjustment could be achieved with enough course and fine control over time delay for each channel, but only for one point in space. Since some of us will be listening with two ears and those ears may move from place to place within the listening area, maintaining any exactingly obtained phase relationship for the listener would seem impossible. ‘Ballpark’ would be about the best one could hope for. Then there are the myriad effects of reflected sound and/or standing waves. There is almost no end to the things that need to be taken care of in the pursuit of an unattainable ‘perfect sound.’ I think we end up with the pursuit of an ‘acceptable’ sound.

Four imperfect solutions:

One, put the subwoofer/s under the chair or over the head of the listener. If that means the sub is four feet from the listener’s ears, then all the other speakers should be four feet away also, and all delays at zero.

Two, similar to one, however most speakers’ placement is normal and the delays are set normally. The sub, under or over the listener, would need to be delayed even more than the other speakers because it is closer to the listener.

But sub placement near the middle of the room is rarely good for low frequency response.

Three, each loudspeaker has it’s own subwoofer adjusted in phase with that one loudspeaker. But such subwoofer placement is also rarely the best.

Four, the subwoofer phase of one or more subs, some people liking stereo subs, some people liking a separate LFE sub, are set to agree with an aggregate of either the main L and R, or L, C, R, or all speakers in operation at once. As a listener shifts position, the listener receives the best compromise available within realistic means wherever they are.

My own real world example is the running of stereo subs via an Outlaw ICBM between pre/pro and amp, each sub not too far from, but not co-located with, the my main L and R speakers. Each sub is in general phase ‘agreement’ with the appropriate L and R speaker, which certainly helps with stereo listening. For surround listening, phase ends up approximately correct for the center speaker also, whose low frequencies are sent to both subs equally, so center and sub sounds reach the middle of the room at nearly the same time. The rear speakers are about the same distance from the center of the room as the fronts, so there is general timing agreement for those speakers as well, if one listens near the middle of the room. Listening from near either end of the room means ‘all bets are off’ if one is concerned with maintaining some type of exacting timing/delay relationship, but this hasn’t detracted from my enjoyment.

With no perfect adjustment for everywhere in the room, and with a fully enjoyable result obtainable with correct coarse adjustments, it doesn’t surprise me that most equipment treats this issue in a minimalist fashion.

One of the major reasons I use an external crossover (24dB slopes for all crossover points which yields in-polarity response at each crossover point) for the subwoofer-to-mains split is to maintain phase relationships regardless of which source, analog or digital I am listening to. This is essentially the same thing the ICBM does except for the 24dB/octave aspect.

Yes, that’s right. Now, go to your local guitar center to get yourself a 24dB/octave Linkwitz-Riley active crossover so you can stop using that bass management and run your system like big boys do!!

An external crossover won't solve the bass routing problems that most people have, but the ICBM will.

But this isn't how it happens. Like I mentioned, most pre/pros and receivers when they do digital BM use a 24dB/octave low pass, and a 12 dB/octave high pass. Because the slopes are different, the phase delays are different. That difference is 90 deg per 6 db/octave of slope. So ... for the low pass, the shift is 360 deg, and for the high pass, it's 180 deg. Hence the ubiquitous 180 deg phase switch on subs. But that isn't all there is. Unless your sub is right on top of a main speaker, there will be additional phase delay introduced by the actual difference in that distance. Plus, even though digital crossovers are designed to have the same phase characteristics of analog crossovers, there *is* an extra delay due to the digital processing itself. Hence the fact that it is really difficult to get all the phase relations to match up between using digital crossovers in a pre/pro, analog crossovers in a pre/pro (if present), or whatever you're doing with BM in the player (if anything).

This is why Soundhound is right on the money when he says he uses identical 24dB/octave slopes for HP and LP filters for all sources.

But... There is a small problem with the identical 24dB/octave approach. Real example. 80 Hz crossover (with low pass and high pass filters). 24dB/octave LP means 24dB/80 Hz for the LP slope. But the high pass slope is also 24dB/octave which is 24dB/40 Hz. They are not equal. Hence I don't see how you get flat freq response through the crossover.

The funny thing, is that after the 950 I went with a Lexicon pre/pro, which actually uses identical 24 dB/octave slopes. So there's something there, but no one has yet explained exactly how it works.