Joe
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- Joined
- Jul 27, 2002
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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.
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.
(See "Denon 3803 Info" or something.)
