BruceD

A Question of Balance

Hoping I can get some discussion on this topic from some of the DIY sub gurus here:

One of the biggest obstacles to good integration between main speakers and subs is the implementation of the digital crossover on an HT processor.

Most of the receivers and prepros on the market were designed with an electrical high-pass @80Hz, 2nd order, 12dB/octave rolloff bass management algorithms to fit THX certified speakers.

If your speakers are anything other than THX certified (not my cup of tea) good crossover integration across the crossover boundary (between those speakers and a sub) is nearly impossible. Great musical sub integration with an HT processor's internal bass management and non-THX speakers is almost an oxymoron.

Personally, I prefer a high quality active crossover with symmetrical 24dB/octave L-R high-pass and low-pass filter slopes.

There are a number of HT processors that allow more sophisticated bass management than what I mentioned, i.e. Theta, Meridian, Lexicon to name a few.

What I'm referring to is the assumption made in the digital bass management algorithms about your speaker's low frequency rolloff characteristics.

I find it very hard to blend a speaker with a 18-36dB/octave low frequency rolloff/slope --that many ported speakers have-- plus the 12dB/octave electrical slope of the HT processor for the high-pass side of this crossover equation (equals 30-48dB/octave slope) with the low-pass side of this equation which the HT processor models on a 24dB/octave slope.

The anomolies (peaks and dips >3dB) at and around the crossover frequency described above make this setup less than desirable for a smooth main-to-sub blending for the best musical enjoyment.

My point is NOT to use a sub's internal crossover, but instead to use an active external standalone crossover (like a Marchand ) to optimize speakers, sub, and room.

To do this means re-directing all bass from Small center and Small surrounds to the main Large L&R preouts and re-directing the LFE to the main Large L&R preouts as well. Then the xover is only done once; from the Large L&R preouts to the active external crossover and then to the L&R mains and sub(s).

I find this provides the best musical optimization of a sub for 2-channel and still works great for HT. For my speakers, room and sub this happens @60Hz.

I've been doing it this way for the last 4 years.

Dave Milne

Bruce,
This is indeed a complex and interesting topic.


I have struggled with this myself. However, since most receivers and pre-pros have symmetrical slopes, one must assume that they beleive the actual speaker rolloff is far enough away (in frequency) from the crossover to be insignificant. The filter design experts would say you should be a decade apart to fully eliminate interaction. Nobody makes a main speaker with 8Hz rolloff, so this clearly is not the case. In my experience, however, if you're an octave away, it's pretty safe. A lot of main speakers do have 40Hz capability.

The idea of matching the crossover frequency/slope with the speaker rolloff characteristics is not new, but is rarely considered or implemented. It's just not practical for the mass market. As you pointed out, receivers and most pre-pros have limited crossover options... and few main speakers have precise 12dB, 80Hz rolloff. It's hard enough to educate the masses on picking the right crossover frequency...let alone matching asymmetrical slopes to speaker rolloff characteristics.

One of my favorite designs, on paper, is to match a passive RC filter (between pre and main amps) to sealed mains for a true third-order rolloff. This avoids active crossover circuitry in the signal path to the mains and minimizes noise and distortion. Then just use a third-order low-pass on the sub for seamless integration. I've done a few of these designs and they work well. A variation on this is to use 2nd order highpass with sealed mains and 4th order sub lowpass for matched Linkwitz-Reilly responses -but this requires active filters in the main signal path.

However, I've had equal success with symmetrical 24dB LR filters (using my own analog active filter designs or the 24dB LR option in my Lexicon MC-12). In fact, my current theater system has symmetrical 24dB 70Hz slopes with 12db/52Hz speakers . This measures quite flat through the crossover region in my room. The key here is that rooms often have ragged response this region that easily swamps out a few dB of peak or dip in sub integration. If you have some test gear and are willing to experiment a bit, you can often flatten overall room response with a deliberate crossover mismatch!

Happy experimenting!

My home page with theater and speaker details

RichardHOS

You can also muck up phase response with a crossover mismatch.

I think the best solution is to use a 6dB/oct symmetrical active crossover that is at least two octaves away from the speakers' natural roll-off points.

Since that isn't practical (except according to Vandersteen, but I don't buy his claim that he has found drivers that allow that to work properly), the next best option IMO is symmetricaly 24dB/oct active crossovers that are at least an octave away from the speakers' natural roll-off points.

It's really the only way to maintain a semblance of phase coherency throughout the full frequency range.

I also don't understand the common discomfort with using active crossovers in the signal path. Passive crossovers are also in the signal path, and they do more nasty things to the signal than an active crossover could ever imagine doing!

RichardHOS

BTW, I have an interesting idea for a solution to the problem with phase/slope/frequency response that Vandersteen and Dunlavy claim to have conquered. My solution is a lot more complex, but on paper it looks very promising. Hopefully I'll get a chance to build a prototype next summer and make some measurements.

BruceD

Dave,

I think this is False.

Most HT processors have a THX inspired;
high-pass = 12dB/octave slope (depending on speaker for other 12dB)
low-pass = 24dB/octave slope

Now because they implement these in the digital domain, they may be able to avoid phase offsets, but the asymmetrical shapes of the two curves approaching each other causes real problems at and around the crossover frequency in most rooms when trying to blend mains and subs.

I think this is one reason why many are unhappy with their subs when comparing music vs. HT sources (albums sound like they have bass peaks at different frequencies), which is also contributed to by the well known room-mode problem.

BruceD

Richard,

By the way, I do like 1st order 6dB/octave crossovers.

My Dynaudio Contour 2.8 towers use 6dB/octave crossovers between the drivers.

I selected 24dB/octave for sub(s) primarily for better control over room modes, plus the 360 degree phase offset of a 4th order Linkwitz-Riley was easier to deal with than the 180 degree phase offset of a 2nd order.

Jonathan M

I agree with most of the comments above.

Many receivers do use the THX inspired 12dB butterworth highpass and 24dB Linkwitz-Riley lowpass option, and in my opinion this is the best possible option for people who don't want big tanky speakers all around.

My speakers were designed as sealed with an F3 of 80Hz for precisely this reason. Not only is it easy on the receiver (Only running from 80Hz up with a 2nd order highpass), it's also easier on the speakers (Most 5-6" speakers don't like too much below 80Hz due to excursion required and the distortion that implies.)

Unfortunately, most speakers on the market (Probably 90% or more) are ported enclosures, so the above isn't so good. Usually though, when room behaviour is taken into account, it works fairly well though - the THX guys got this right IMO.

As for the suggestion to reroute all low bass to the mains then cross to the sub, the only problem there is that you have cascading xovers and more chance of out of phase problems causing nulls and peaks in the signal before it even reaches the speakers - this does depend somewhat on how the processor handles the digital delays applied to the speakers as well. I have yet to measure this on my NAD T752 - I have a method prepared, it's just finding the time to do it. I did notice, however, that when one bundled all the bass and sent it to the mains, then the response was no where near flat - I can't recall if I had any delays on the rears, however.

Feel free to check out my system with comments on my bass integration on my site by either clicking the www link above, or the link below.

\"Price and quality are not correlated\"

DIY Home Theatre

BruceD

Do you mean the DSP chip and bass management algorithm suppliers (approved by Dolby, DTS, and THX) like Cirus and Motorola (who supply most receiver and prepro manufacturers) screw up the bass management to the main L&R speakers when no sub is selected in speaker setup?

Can you explain how they screw it up?

Can you list which receiver's and prepros don't do bass management correctly?

I'm not sure I follow how you calculate a cascading crossover from an HT processor's L&R main preout (with LFE + re-directed bass already digitally integrated and time aligned) to an active crossover with the high-pass to main L&R and the low-pass to sub(s).

Jonathan M

Hey Bruce,

No, they don't screw it up exactly I don't think. Assuming that any digital delay (To compensate for closer rears or whatever) is applied before the digital xovers, then YES, that is a screw up. The delays should be applied to the highpassed signals only (As that's the only part of the signal that is coming from a speaker that is closer). Many receivers don't have delays for each individual speaker anyway, and even fewer have it for the sub (Which phase control can often remedy to a certain extent).

The problem with the cascading xovers is this:

The pre-pro is digitally crossing over the centre and surrounds (And anything else set to SMALL) and sending it, together with the LFE signal to the mains. If I remember correctly, the LFE gets it's 10dB boost at this stage. There is nothing wrong with this. Note though that the bass from the speakers set to small is crossed over (With a 24dB LR lowpass at say 80Hz or whatever it's set up as) whereas the mains are not lowpassed at all - they're fullrange.

OK, we now send this fullrange+otherchannels lowpassed bass signal offboard to an active xover such as a Marchand or DIY job.

This highpasses this signal and extracts the mains signals. Note that this signal will include some (albeit not much) midbass and bass from the speaker set to small. The xover also lowpasses the signal to send the bass to the sub where it belongs. Let's assume this is a 2nd order xover (For example). The mains' bass is crossed at 12dB/octave to the sub. The speakers set to small, however, is being crossed at 36dB/octave to the sub. Thus one MAY have phasing problems due to the different slopes involved.

Hope this explains things. I personally don't believe this'll be much of a problem when one considers room effects anyway. I do think that keeping things simple with all speakers identical, sealed at 80Hz and using the pre-pro's digital xover is the easiest and best solution. (It's even better than 5 fullrange speakers IMO as it has less bass cancellation issues when one considers a typical home theatre sized room.)

I plan to do some further testing in the future to see how and when the digital delay's are applied on my NAD T752. I've figured out a way to do it - I just have to get the time, and hope I can get sufficient resolution to notice the difference.

\"Price and quality are not correlated\"

DIY Home Theatre

BruceD

Jonathan, I think if you re-read my earlier posts you will notice I specifically state that receivers and prepros already screw up the phase with their digital 12dB/octave 2nd order high-pass. This is especially bad with ported speakers that are likely to have 18-36dB/octave low-frequency acoustic rolloff.

Then I go on to say that to avoid this phase problem I use a Symmetrical 24dB/octave high-pass and low-pass crossover (note no phase anomolies here). Let's not assume the external crossover has a 12dB/octave slope like you said above.

Does that make more sense?


I don't disagree with this statement at all.

The problem is how many people do you know that fit this scenario? Me, I don't know many that do, and I sure don't myself. So my suggestions are mostly for people who don't have the ideal speaker setup.

Yes, I think you optimized your speakers to match your HT processor. I didn't want to do DIY speakers, so I'm forced to deal with the speakers I have, Dynaudio Contour 2.8 towers, which I really like by the way.

If your NAD uses the cirrus DSP chips, you can read the cirrus application notes about bass management on their website.

RichardHOS