Pros and Cons of Using Individual Crossover Frequencies for the Various 5.1 Channels

Discussion in 'Archived Threads 2001-2004' started by Joe, Nov 3, 2002.

  1. Joe

    Joe Stunt Coordinator

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    Thought you might find this interesting...


    Pros and Cons of Using Individual Crossover Frequencies for the Various 5.1 Channels

    September, 2002 by Brian Florian

    Do you know how a subwoofer output is produced? Most everyone assumes that bass is taken from channels set to "Small", combined with the LFE channel and bang, you have a subwoofer output. Nope!

    FULL RANGE copies of all channels set to "Small" are combined together with the LFE channel, and the sum is low-passed. Think about that. Strictly speaking, any processor with a sub/sat crossover frequency set lower than 120 Hz is "discarding" the upper end of the LFE channel. THX units are NOT exempt from this. With the standard THX 80 Hz 4th order crossover, the top of the LFE channel gets chucked.

    Wait! Don't panic . . . yet.

    This has been going on since day one, and virtually nobody has noticed . . . with good reason. I've said many times before, and I will say it again: THX did not pull their crossover out of thin air. It is the product of much development, and, when used in concert with THX speakers (or others which exhibit the correct roll-off), represents the best overall compromise of minimizing localization, extending dynamic range, and yes, minimizing LFE truncation. THX looked at an inordinate number of modern 5.1 soundtracks and guess what they found in the LFE channel: not much at all in the region of 80 Hz - 120 Hz. Dolby Digital's LFE channel has a digital brick wall at 120 Hz, not a roll-off, so content creators almost always roll-off their stuff, usually somewhere around 80 Hz. Therefore, chucking the top band of the LFE is no big deal.

    But what if you set your crossover to, say, 40 Hz in an SSP (Surround Sound Processor) that has a selectable crossover frequency? The sub output will be low-passed at 40 Hz, and you're now throwing away 40 Hz - 120 Hz of the LFE channel.

    What has all this got to do with independent crossover frequencies for the various channels? I'm glad you asked.

    Once you wrap your head around the fact that you are setting a high-pass on each main channel and a single low-pass on the sub, things begin to focus. Lets take an extreme scenario, just to illustrate the point.

    We set the high-pass on the main left and right to 35 Hz because we think its in the best interest of our massive tower speakers. We set our center channel high-pass to 100 Hz because it isn't very big. What is the subwoofer low-pass in the processor going to be?

    If set at 35 Hz to complement the main speakers, the center channel signal will have a huge hole from 35 Hz - 100 Hz. Whoa! Lots of bass on that channel we don't want to miss out on. So let's try setting the subwoofer low-pass to 100 Hz. Oops! Now we have IN-ROOM 6 dB too high from 35 Hz - 100 Hz on the main channels because BOTH the main speaker and the subwoofer are voicing it. You CANNOT correct for this. If you lower the subwoofer level, you lower it for everything, and now you don't have enough bass from the center channel.

    By now some of you are thinking, "Why not low-pass a copy of each main channel at the various frequencies I want and sum that with the full LFE channel?". Well, this is a can of worms. Phase issues become monumental. Bass is often common to the front three channels and even more often common between the LFE channel and the fronts. Summing different low-passed copies of the same material would by definition result in a messy frequency response. The THX design manual references the Dolby Digital licensing manual which mandates that the subwoofer output be arrived at the way it does for good reason.

    An SSP designer can, within licensing guidelines, take a low-pass copy from the center (in our extreme example, at 100 Hz), add that to the front left/right and still high-pass those at 35 Hz, the balance going to the subwoofer (though you still waste 35 Hz - 120 Hz off the LFE channel). However:

    - You still have phase issues. You get a proper in-room phase response with a single crossover frequency, all of which would be undermined if the bass was handled any other way (multiple crossover frequencies and one subwoofer).

    - When mixing channels digitally, S/N is lost (approximately 6 dB when two channels are added for example), because after the summing, the combined level has to be attenuated to the original level.

    Our example is indeed extreme and few are the systems that would be set so radically. There is a widespread misconception that if your speaker manufacturer quotes the performance of your towers as -3 dB at 35 Hz for example, that we must therefore splice them with a sub at 35 Hz. Nothing could be further from the truth. All crossovers assume response below the crossover point and for such a speaker, 70 Hz may very well be a better choice.

    Before some of you start throwing rotten fruit at me, I acknowledge that a different crossover point for each speaker is a desirable thing from the point of view of real world acoustics and dynamics. The different positions of the speakers in the room virtually dictate it, and the various members of a mismatched speaker set will each have different points of intersection for increasing dynamic range and maximizing bass performance. But without also having a selection of slopes in the SSP and some VERY expensive measuring equipment, one is likely to end up further behind than ahead. [Editor's Note: The Theta Casablanca and Mark Levnison No. 40 have crossovers and slopes for each channel, but you might need to fill out some forms at the bank before you get them.]

    If you want consistent bass response from each channel of your 5.1 system, in the opinion of this writer, you're best to set all speakers to "Small", set them all to the same crossover point, and set that point no lower than what you are comfortable throwing away from the LFE channel. If your main left and right speakers are genuinely full range (be honest now!), then you are better off running them full range as opposed to high-passing them at a ridiculously low frequency. Short of that, high passing floor-standing speakers at 70 Hz is not "wasting" them in any way shape or form and in fact will more than likely extend their dynamic range thanks to the relief they'll be getting from the high-pass. Alternatively, setting center and surrounds as "Small", the mains as "Large", subwoofer as "None", and implementing an external two channel crossover to the subwoofer is a valid, and in some situations an advantageous way to go.
     
  2. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    "If you want consistent bass response from each channel of your 5.1 system, in the opinion of this writer, you're best to set all speakers to "Small", set them all to the same crossover point, and set that point no lower than what you are comfortable throwing away from the LFE channel."

    While I am still not sure whether I agree or disagree with Brian's beliefs in this post, the last part of this quote is the issue that bothers me the most. It's one I am dealing with right now with the Anthem AVM-20. Even with a global crossover of say 60 Hz, I lose the LFE (.1) channel above 60 Hz (subject to the crossover slope).

    It's too bad there isn't an option to sum the LFE channel before OR after the crossover for the subout. I imagine there might be some phase issues that prevent this, but it can't be any worse than losing some of the LFE channel, or can it?

    I like the LFE and sub setup for the Lexicon MC-12. The LFE channel has its own output, if you chose to select it that way, while the bass from "small" speakers can be sent to a separate output or two (selectable as mono or stereo).

    Michael
     
  3. Kevin C Brown

    Kevin C Brown Producer

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    This has been covered in previous threads. [​IMG] (See "Denon 3803 Info" or something.)
    Bottom line for me, is that I don't want a pre/pro-receiver manufacturer making the decision to exclude that feature for me. I want that capability to use it or not.
    I had a Sony TA-E9000ES pre/pro for 4 years. I now have the Outlaw 950. Both of them have individual crossovers. In my system, I have found that I prefer the sound with different settings. 120 Hz front center, 60 Hz mains, 80 Hz surrounds/rears. I have never noticed a problem, and I suspect Brian is actually confused on some of his points.
    For example, this he gives no support to the following blanket statement:
     
  4. Travis G

    Travis G Stunt Coordinator

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    Michael,
    If you were to cross your sub @ 100 Hz and your L/Rs @ 60 Hz you would get a 4 or 5 dB hump in the frequency response between 60-100 Hz
    Kevin,
    Brian is right. The more crossover frequencies that you add, the more phase issues that you have to deal with. If these issues are not dealt with the frequency response will not be smooth. This is true wether you are mixing electrical signals or acoustical signals.
    The phase difference between woofer & tweeter is generally not audible under normal listening conditions. Research has proven it as far as I'm concerened. This is assuming that the phase has been compensated for in the crossover.
    Travis
     
  5. GregLee

    GregLee Stunt Coordinator

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    This is not quite on topic, but I'm curious about opinions on using a receiver's subwoofer=PLUS/BOTH setting with full-range speakers set to "Large". If this sends LFE info to all the speakers and a combined LFE + 5 full range channels to the sub, with the sub's crossover set to a low value, it seems this would avoid throwing away part of the LFE. But I don't know whether this is, in fact, what this setting generally does.
     
  6. AaronBatiuk

    AaronBatiuk Second Unit

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  7. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    Travis,

    "If you were to cross your sub @ 100 Hz and your L/Rs @ 60 Hz you would get a 4 or 5 dB hump in the frequency response between 60-100 Hz"

    I understand completely what you are saying here. That's why I stated is was an issue for me now, as I am trying to find a suitable mains/sub crossover setup I can live with.

    Aaron,

    "The subwoofer and LFE have separate low-pass crossover settings. You can set your 'subwoofer' to cross at say 70 Hz and the LFE to 120 Hz. Then the subwoofer will reproduce all of the bass from the main channels at or below 70 Hz as well as the LFE channel in it's entirety."

    The issue here, if I understand correctly, is that the bass from the "small" speakers can actually be duplicated in the LFE channel, which has a higher dynamic range than the regular channels. Once the "small" speakers go through any crossovers to send bass to the subwoofer, the signals have been phase shifted by the crossovers. Add this phase shifted bass to the LFE channel and you can have significant peaks and valleys in the bass being send to the subwoofer.

    Michael
     
  8. Travis G

    Travis G Stunt Coordinator

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    George,

    If you set the XO to a lower frequency the LFE gets thrown away. It's a seperate channel meant to go to a subwoofer. The reciever can copy bass from the L/R/C (left, right, center speakers) and send it to the sub but it can not send LFE to the L/R/C.

    Travis
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    Travis- Neither you or Brian explain *why*. It's a simple addition of the magnitudes of the sounds at different frequencies. In this regard all of the speakers can be though of as independent. (Obviously, the pre/pro/receiver has to do crossing over, summing, then time alignment.) There is absolutely no reason why individual crossover points can't work just as well or better than a global setting. The crossover on each channel works for the high pass *and* low pass portions of the signal.
     
  10. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    Kevin,
    "The crossover on each channel works for the high pass *and* low pass portions of the signal."
    For the majority of receivers and processors, I do not believe this to be true. When the bass from "small" speakers is sent to the subwoofer, in fact what is happening is that the full range signals from the "small" speakers are summed along with any present LFE (.1) channel, which is then passed to the subout's crossover where it is low passed.
    Michael
     
  11. BruceD

    BruceD Screenwriter

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    Travis,

    You said;

     
  12. Jeff Kohn

    Jeff Kohn Supporting Actor

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    The thing I find troubling with Brian's articles is that he makes some blanket statements about all receivers and pre/pro's, but doesn't really have anything to back his statements up with.
    And if you take everything he says as gospel, then it seems to me that setting all speakers as small with a single X-over frequency would not be the best approach. Instead, it seems like it would be better to set the mains to large, sub=no, and redirect all bass + LFE to the mains, which could then be passed through the subwoofer's internal x-over. That way you wouldn't be throwing away any LFE.
     
  13. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    BruceD,
     
  14. Tom Grooms

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  15. BruceD

    BruceD Screenwriter

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    Michael,

    No phase problems specifically. Since my M&K sub which has a 0 & 180 degrees phase switch and is located only about 4-5 feet from my Main left speaker (mains are about 8'-9' feet apart) I don't seem to have any phase problems with mains/sub integration.

    The Marchand XM9-L xover I use has a number of rotary level controls: Left high-pass, Right high-pass, Left low-pass, Right low-pass, and the exact xover frequency itself, which for my mains (-3dB @32Hz) I have set @60Hz. It also has a L+R SUM switch for mono bass output to the sub.

    This xover is a 4th order (-24dB slope) symmetrical Linkwitz-Riley design that SUMs the high-pass and low-pass signals to a zero SPL peak at the xover frequency, with a zero degree phase offset. It's the best tool I have found to smoothly integrate sub(s) with my mains. It's also got a great noise floor, i.e it's quiet and doesn't impact the quality of the signal as far as I can tell. It gets great reviews over at AA.
     
  16. Kevin C Brown

    Kevin C Brown Producer

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    If I was to design a crossover network for a receiver or pre/pro, here's how I would do it:
    You have separate paths for all 6 channels, 5.1.
    I take all of the "main" channels, and cross them over individually. So now I have 5 (a) + 5 (b) plus 1 signals.
    So far so good. I can apply whatever crossover setting I want to the original 5 full range signals. Doesn't have to be the same either. Each signal is low and high passed according to the crossover freq chosen. (MM: why *wouldn't* the high passes and low passes all correspond to the actual crossover freqs? It's not hard. It doesn't make sense to do it otherwise, because then you'd knowingly be throwing away a portion of the signal.)
    Now I adjust the volume all of the individual 5 (b) low passed signals, individually, so that they will add into the LFE properly. Then I sum them. (I also give the user a way to independently control the level of the LFE as it is combined with the low passed portion of the mains. I also allow the user to set a "high cut" for the LFE signal. The Sony had both of these settings.)
    Now I time align all of the signals that remain: all of the individual high passed feeds 5 (a)'s, and the combined LFE + 5 (b)'s.
    Then ship them out of the pre/pro to the amps.
    Until someone can educate me otherwise, this is the easiest, simplest, and makes the most sense approach to me. Why a pre/pro or receiver manufacturer would do it any other way, would have to be explained to me. [​IMG]
    And finally, a real world example. Case 1) I set all my speakers up to x-over at 80 Hz. (I was high on "THX" when I 1st set up my system.) There's a panning test signal on Avia that's kind of white noisish (or pink, doesn't matter for what I'm going to point out), and as it pans through my center, I can *hear* the level of bass decrease. At first, I though it was a problem with how (at that time) the TA-E9000ES was rerouting bass.
    Case 2) I change the center to 120 Hz. I repeat the pan. Now, the sound level is a lot more seamless as it moves from my left to center to right speakers. No "hole" in the bass as the pan goes through the center channel.
    Real world example... [​IMG]
     
  17. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    Kevin,

    I do not disagree with your design concept of an "ideal" bass management setup. I am just stating for those who believe their systems work this way that they be in for a surprise.

    Here's a test for those who own receivers/processors that allow separate crossovers by speaker. Select a test DVD that has a center-only track or connect the analog output of a CD/DVD player into the center channel of the 5.1/7.1 analog inputs. The last setup requires bass management capabilities for the analog inputs. Also disable any bass "doubling" features you might have. For my test, I used the latter setup.

    Set the sub to "no", mains to "large", and initially set the center to "large". Playing a CD (one with decent bass) with this setup should only provide output to the center speaker. Turn off the center speaker's amp or disconnect the speaker wires to the center speaker (being careful not to touch the wires!). Now, set the center's crossover to its lowest value (in the AVM-20, it is 25 Hz). Surprisingly, I get a great deal more than the bass below 25 Hz. Even more surprising is that no matter what I set the center speaker crossover to (up to 160 Hz), I get the same amount of bass (probably up to 160 Hz) through the mains.

    I realize that the response from some is going to be that it sounds like a design deficiency with the AVM-20. Maybe, but I am most certain that many setups will experience the same result. Normally, this bass (in some cases, full range signal) gets sent to the sub's crossover, which in systems with global crossovers or fixed (80 Hz, 100 Hz, etc.) crossovers gets crossed over at the same frequency.

    Michael
     
  18. Travis G

    Travis G Stunt Coordinator

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    Maybe they would but I don't really care. There are alot of loudspeakers (the majority actually) that do agree. The research has shown that phase distortion is generally inaudible under normal listening conditions. There are a few exceptions. Phase shift between drivers is more audible with sounds like several clicks every second. Phase shift is more audible in anechoic chambers or listening with headphones. But in real life we generally listen to music and speech on our sound systems and not clicks. We also listen in typical household rooms and not in anechoic chambers. In my opinion the distortion caused by lower order crossover networks is more a problem than phase shift between the woofer and tweeter. You should read this page, written by Dr. Siegfried Linkwitz, as well.http://www.linkwitzlab.com/phs-dist.htm
    The speaker plots that you see in Stereophile are most likely Nyquist complex impedance plots, or simply the maximum capacitive phase angle along with a frequency response curve. These plots are to demonstrate how well damped the loudspeaker is and also indicate how easy or difficult a load it will be for an amplifire.
    Bruce,
    Thanks. I stand corrected.
    Travis
     
  19. Kevin C Brown

    Kevin C Brown Producer

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    MM: Fair enough. I need to think about this, because the 950 doesn't work this way, but a few Q's:

     
  20. Michael Mohrmann

    Michael Mohrmann Screenwriter

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    Okay, the test I described previously is more difficult than it needs to be to determine how your system's bass management functions. I had just tested the center channel by itself because it was the one speaker that I might set to a higher crossover than the other speakers, so I thought I would post my results here. Let's make this easier for everyone to participate, if they wish and their system is set up to (and, unlike another currently active thread on this forum, this one won't cost you! [​IMG])
    Select a CD or DVD player that is connected to your receiver or processor via a digital connection (you can also use an analog connection if you can apply bass management to it). Select a music CD with decent bass and select the 2 channel stereo playback. Be sure to disable any bass "doubling" features. If not possible, you may not be able to perform this test.
    Set the mains to "large" and sub to "yes" at its highest crossover or bypass mode, both on the sub and in the receiver/processor. Play a CD. Your initial results should show only music in the mains. Now, turn off the amp to your main speakers. Enter the menus and change the mains from "large" to "small". You should note some bass being played from the subwoofer. Depending on how high frequency-wise your sub can go, you may hear male voices.
    Now, adjust the crossover to the mains. On the AVM-20, I can go from 25 Hz all the way to 160 Hz. No matter what I select, I get the same output to the subwoofer. Only when I change the crossover to the subout do I start low passing the main's signal to the subwoofer.
    I realize I am going in circles here, but I hope I am making my point. I also hope this isn't coming across as "right vs. wrong". I am more interested in understanding how bass management works than arguing about it.
    Michael
    Edit: Note to Kevin, I made my post without reading your last comments. This one, "(I.e., the center's crossover setting should be construed as meaningless if the center is still set to large.)", leads me to believe I did not explain myself too well in the first example. I should have stated that the center speaker gets changed from "large" to "small", then the center crossover is changed. Also, the reason for "no sub" was because I was testing out BruceD's setup. Hopefully this post makes more sense.
     

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