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Discussion Starter · #1 ·
My question/theory is:


Is the LFE channel an essential element of Dolby digital/DTS playback on DVD's if all your other speakers have the ability to play 20-20KHZ effectively?


My answer is yes, and my theory is, because without the LFE channel activated, LFE channel information would be directed to the left and right main speakers simultaneously causing those speakers to see "Different bass information" thus having a negative impact on the clarity of bass overall.


What do you think about this question and theory of mine......?
 

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It depends on what you mean by "different bass information". If you mean a difference between the two channels, then no, because the LFE is mono, and (presumably) in phase between both fronts.


If you mean different from the left- and right-front signal, then that's true, but speakers play all kinds of different signals simultaneously anyway; what's one more? (We're talking speakers that can handle the LFE signal, right?)
 

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I've done a lot of playing around with this. I don't run a sub... my mains are easily capable of playing to 25Hz even at high SPLs.


I've hooked up a sub to see what the difference was, and there didn't seem to be any. Playing movie excerpts with a lot of bass info, like the part in Finding Nemo where the little girl thumps on the glass or when the sub falls, I don't notice any difference.


But not many mains are capable of playing like that. These days, speaker manufacturers are happy to leave the low end to the sub. It makes engineering thier speakers that much easier since the low end can be so demanding.
 

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Az,


Just curious, what are your mains and what sub did you try? Without knowing what you are comparing it is hard to know how to relate your response. Just as you say, not all mains are the same, neither are all subs. Providing the additional information will give a good reference point.
 

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Mains are homemade. 15" horn loaded bi amped (actively). Similar to a Klipschorn. Using the same horn driver, as a matter of fact.


The sub I tried wasn't the greatest, but I was mainly just trying to get a feel. It was a Yamaha. (Actually a decent sounding sub, just not good for really high SPLs)


The mains have no problem with deep powerful bass. My HT is in the basement, 3 sides undergroud, concrete floor, on a 1 acre lot. My next door neighbor informed me that his windows were rattling one day when I was putting the speakers through the test.


I set the system up with an RTA, and the speakers play down to 25Hz +/- 3 db at well over 120db.


My point is that it's all relative to what you're doing. It's obviously much easier to get some really nice speakers and a good sub and get excellent results. But it's not impossible to have real full range mains.
 

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Az,


Thanks. That helped. In my previous setup I had a Boston Acoustics sub that I thought was pretty good until I moved up to an SVS sub. As you say, it is all relative but now knowing what your main's are capable up gives me a better reference to understand your point.
 

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Discussion Starter · #7 ·
Quote:
Originally posted by Larry Fine
It depends on what you mean by "different bass information". If you mean a difference between the two channels, then no, because the LFE is mono, and (presumably) in phase between both fronts.


If you mean different from the left- and right-front signal, then that's true, but speakers play all kinds of different signals simultaneously anyway; what's one more? (We're talking speakers that can handle the LFE signal, right?)
OK, what I mean by "Different bass information" is:


When the master recording is created by the sound engineers for a particular movie. What happens when a particular scene calls for extremely demanding bass signals to be routed around the front channels and LFE channle to achieve a desired affect? varying in signal amplitude and frequency, sometimes simultaneously , sometimes not, and the sounds are all occurring within milliseconds.......

Consider this; The engineer wants to have you believe a helicopter is flying around the room, while gunfire, grenades, bombs ect... are all occurring, even a full range hearty score in the background....now how can even a fully capable main speaker that can even play 20-20 KHZ be expected to do this with the same clarity and speed that it could if it didn't have to deal with all that extra LFE signal?!.....

It's simple physics here at work isn't it?


The drivers just can't respond as quickly and with the same speed that they could otherwise...correct?


What are your thoughts?


I mean why did DD and DTS design every channel with the capability to play 20-20KHZ and the LFE also just for low freq effects in the first place...??
 

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I would have to say not correct. Sure, the more drivers making any sound at a given level, the more drivers, the less each has to do. Keep in mind that the LFE channel may or may not contain what is coming from the others; it is a discrete channel, and it can contain 20 to 20KHz if desired.


I believe you mean that the fewer different frequencies a driver is being asked to reproduce, the better job it can do on the remaining sounds. I guess that's true, at least in theory, but sounds, whether music or not, contain very complicated waveforms and combinations, and speakers manage to reproduce them.


I'm using my subs for left and right bass AND LFE duty; I set bass mgmt. to 'no sub' and L & R to 'large', and my stereo subs are being driven from a pair of L & R line-level outputs; LFE in mono from both, and bass in stereo, at the same time. Any speaker that can handle what it's being asked to reproduce will work.


I would enjoy my HT less without subs, for sure!
 

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Billy, implementing your Bass Management feature in your pre/pro activates the LFE in mono and limits the sub frequency to what you set it at. Normally set it at 80HZ and the sub plays from 80HZ down. Then setting the speakers to small lets them play 80Hz and up to the human range of hearing. Bass Management was designed so that all 6 (or...) speakers play from 20-20KHZ. You think you hear 80HZ and below from the sat's, but they do not play it. Sure, full-bandwidth CAN be sent to each speaker, but with video less distortion is heard using Bass Management. Go to www.mksound.com and read their article called:"Bass Management Explained" it simplifies it.
 

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Quote:
Originally posted by Billy Gun
now how can even a fully capable main speaker that can even play 20-20 KHZ be expected to do this with the same clarity and speed that it could if it didn't have to deal with all that extra LFE signal?!.....

It's simple physics here at work isn't it?

They manage. Tweeters manage to make cymbals shine at the exact same time they have to play the high frequency info from a dynamic rim shot. The number of waveforms they have to play really isn't the issue. What I think you're trying to say is that the low frequencies are so demanding that the system has to sound better with a seperate driver and amp specifically for those sounds.


In a word, usually. Yes, that's a difficult thing to get right, and most speaker manufacturers are more than happy to give up trying.


But within the parameters of a properly designed speaker, that is to say a speaker that is designed to handle all that hard work, it will be able to do it. Just because most speakers built today can't doesn't make it impossible.


One of the reasons is pure size. Everyone wants thier little easily hidden, or at least not too big and fairly decorative speakers. So smaller cabinets means longer throw woofers to get the low bass. Well, the only problem is the long throw woofers cannot produce midrange tones well at all anymore because of doppler distortion and other effects. How do you get real bass without the cone taking a trip into the next county? You make it big, and you make it rigid. The woofer needs to move air. You can either make a small woofer that moves a long way or a gigantic one that only moves a small amount. Both will move the same amount of air. But which one is going to respond to transients better? Which one will have more realisitic mids? Which one will hide behind the ficus?


So you see the dilemma. And you see why speakers get smaller. It's what everyone wants. And it means you have to have a sub. And there's nothing wrong with that. It's just not the only way to get the job done.
 

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I miss the refirgerator sized speakers :(
 

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Discussion Starter · #12 ·
AZ Barber,

You seem to be the only one understanding what I'm getting at here.....


Yes, I'm not talking about mids and highs.


Yes, I'm saying that during certain demanding passages that not having the LFE channel activated will not provide the ultimate listening experience that was intended because the mains are also trying to play completely different signals..... how could this be possible that the mains would have the same clarity on bass as they would other wise playing just what they were intended to play? I believe not. A main speaker is for one channel of information. Just like all the other channels have a speaker for just that channle's information....that is by design. People make compromises all the time with their setups, which is understandable, but making those compromise comes at a price.


Disregard what Speaker manufactures are doing because they are not always designing in the interest of true sound Quality. Smaller, skinny, cheap boxes....whatever. I'm talking about having a set of FULLY CAPABLE mains, whether home built or purchased that would be in a true home theater environment (Not a spare bedroom or living room that serves a multi functional family room.)


Also think of it in the reverse scenario from what I mentioned in the beginning of my post (Also being a compromise in my opinion) activating the LFE channel and playing ANY other speaker in the "Small" setting would put all that extra signal routed to the sub(s) hooked up to the LFE channel. How can the LFE sub respond with the same clarity and speed that it could if it were not burdened with extra channels of information all occurring at different times, amplitudes and frequencies? It can't. Again physics.


I still have yet to hear anyone present a case with evidence that would blow my theory out of the water.


please let me know what you think.....Thanks.
 

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Quote:
Originally posted by Billy Gun
My question/theory is:


Is the LFE channel an essential element of Dolby digital/DTS playback on DVD's if all your other speakers have the ability to play 20-20KHZ effectively?
If you define "effectively" as reaching 115dB with acceptable distortion levels in the last octave (few commercial subwoofers can do this) it's not.


Otherwise you'll do a lot better with a big driver in a big box.
 

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Quote:
Originally posted by Billy Gun
Also think of it in the reverse scenario from what I mentioned in the beginning of my post (Also being a compromise in my opinion) activating the LFE channel and playing ANY other speaker in the "Small" setting would put all that extra signal routed to the sub(s) hooked up to the LFE channel. How can the LFE sub respond with the same clarity and speed that it could if it were not burdened with extra channels of information all occurring at different times, amplitudes and frequencies? It can't. Again physics.


I still have yet to hear anyone present a case with evidence that would blow my theory out of the water.
I will give it a shot:

1. Frequencies below about 80Hz when played by ideal speakers with no cabinet or driver resonances are not localizable by the listener. Therefore it doesn't matter where the source of the low frequency sound originates from because the listener will not be able to identify the location of the source. No matter how many sources are producing low frequency sound the user will only preceive a single non-localizable low frequency channel.

2. Since the low frequency sounds will not be localizable we can sum the low frequency channels together without it being noticed by the listener. The signals are going to be summed in the listenning room anyway so why not do so at the processor? This is the basis behind sub-satellite systems and why we do not need 5 subwoofers.

3. Since in #2 we are summing the low frequency channels into a single channel, why not play them with a single speaker? The sub is not having to deal with "more information" whether or not we sum the signals together. Our human ears may be *preceiving* more information but the sub is just doing it's job as usual translating an electrical signal below around 80Hz into a sound wave. I forgot most of what I learned in EE Signals and Systems but I believe when multiple low freqencys signals are summed together the resultant signal may have a higher frequency component. If this is true I am not sure how this would normally be delt with by the processor. Someone else will have to answer that part. In any case it would be a problem for the surround processor and not the subwoofer.

I did not mention room nodes, standing waves, nulls, and the benefits of having multiple subwoofers which can be discussed in another thread. In any case, I do not see any benefit between having multiple channels reproducing independent low freq signals over having multiple channels reproducing a single low frequency signal.


-Shawn
 

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Discussion Starter · #15 ·
Quote:
Originally posted by rader
I will give it a shot:

1. Frequencies below about 80Hz when played by ideal speakers with no cabinet or driver resonances are not localizable by the listener. Therefore it doesn't matter where the source of the low frequency sound originates from because the listener will not be able to identify the location of the source. No matter how many sources are producing low frequency sound the user will only preceive a single non-localizable low frequency channel.

2. Since the low frequency sounds will not be localizable we can sum the low frequency channels together without it being noticed by the listener. The signals are going to be summed in the listenning room anyway so why not do so at the processor? This is the basis behind sub-satellite systems and why we do not need 5 subwoofers.

3. Since in #2 we are summing the low frequency channels into a single channel, why not play them with a single speaker? The sub is not having to deal with "more information" whether or not we sum the signals together. Our human ears may be *preceiving* more information but the sub is just doing it's job as usual translating an electrical signal below around 80Hz into a sound wave. I forgot most of what I learned in EE Signals and Systems but I believe when multiple low freqencys signals are summed together the resultant signal may have a higher frequency component. If this is true I am not sure how this would normally be delt with by the processor. Someone else will have to answer that part. In any case it would be a problem for the surround processor and not the subwoofer.

I did not mention room nodes, standing waves, nulls, and the benefits of having multiple subwoofers which can be discussed in another thread. In any case, I do not see any benefit between having multiple channels reproducing independent low freq signals over having multiple channels reproducing a single low frequency signal.


-Shawn


Shawn,

These are good text book points on why sub sat systems should work. Great in theory, but whether or not people can localize frequencies around 70-80 HZ is also very open to debate. (I believe that freq is lower) I know that when I'm watching a movie and I hear a bomb going off in the back round and it plays through one or both of my back surround speakers I about jump out of my seat...cause there is so much low end coming from the surrounds, I've even taken the grill off them and watched the drivers launch themselves during scenes like this....no way is this stuff 80HZ and up!....there is also no way that a theater can achieve this affect without having speakers in the back playing FULL RANGE. 20-20KHZ.


Again, that's why DD and DTS have these channels designated as 20-20KHZ channels....to achieve an effect that is otherwise just not achievable.


Routing bass from the surrounds or any other speaker for that matter to the LFE channel assures you get all that information yes, but it is not the way it was intened to be. This is one of those compromises that I spoke of above in an earlier post.


Another thing to consider is simply this;

If what you are saying is true, then why wouldn't Dolby and DTS decided on (5) 80HZ-20KHZ channels with a .1 LFE channel playing 80HZ and down....why the option to play either way?...which do you really think is best, let everything play full range or use "Bass Management" .......I think instead of Manufactures using "Bass management" as a term they should call it "Low impact" so if you want to run your system this way at least maybe consumers might ask, hey if I run my system using Low impact mode does tha mean my home theater will have less impact than it normally would? answer: Hell yea!


Sub sat systems that only play 80 HZ an up are no match for a full range Home theater setup with an LFE channel.


So back to my original post: (But not nearly as bad)

I believe that if you have all speakers setup to play full range and they even do it superbly.....(With no LFE channel activated/separate sub (s)) it is also compromising the sound presentation in that when all channels simultaneously have to perform the way they were intended, the extra LFE channel (being routed to front mains typically) will compromise their ability to do what they were designed to do, and that is play ONE CHANNEL effectively from 20-20KHZ.....I don't know how much more clearly I can say this.


Anyone else have an opinion or knowledge of this subject...This seems so simple to me, but yet it also seems nobody else agrees with me on this...??:confused:
 

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Discussion Starter · #16 ·
Quote:
Originally posted by Drew Eckhardt
If you define "effectively" as reaching 115dB with acceptable distortion levels in the last octave (few commercial subwoofers can do this) it's not.


Otherwise you'll do a lot better with a big driver in a big box.
Drew,

Do you think an LFE channel is necessary if all other speakers in the system can lay 20 -20 KHZ effectively?


Effectively means 2 things to me in this case:


1. -No audible distortion. (Assuming that the mains can handle the LFE channel being routed to them probably means there would be no distortion.)

2. -No difference in the sound when comparing the two cases below;

By this I mean; having the LFE channel activated along with all the other speakers full range at Dolby reference level during the most demanding movie passage that you can possibly think of.


Vs.


Not activating the LFE channel and routing the LFE signal to the front mains and playing the same demanding material


I just can't see how there would not be an audible difference during certain scenes....why would Dolby have all channels designed this way if there was not an advantage to using them the way they were intended to be used....?
 

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Physically locating "towers speakers", or "subwoofers" (anything that goes lower than 80Hz) in five (or...) locations in a room usually results in wildly varying bass response on the different channels (gross standing waves). This is why the gurus at Dolby and Lucusfilm innovated implementing Bass Management and this results in even bass response in a commercial, or home theater. Bass Management creates five full-range speakers with identical 20-20K Hz response, so refrigerator speakers are no longer needed in our day.
 

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Discussion Starter · #18 ·
hometheaterguy,


You're really saying the same thing Shawn said above. I can respect your thoughts on why you believe this to be true.


But, again, like I also said above....., "If what you are saying is true, then why wouldn't Dolby and DTS decided on (5) 80HZ-20KHZ channels with a .1 LFE channel playing 80HZ and down....why the option to play either way?.


I'm going to call or write Dolby about this issue...I'll post here after I talk with someone there or get a letter back from them.


Thanks for your input guys.
 

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You could find "Roger Dessler" in the forum and ask. He works for Dolby. Just do a forum search. A guy named Sanjay may know as well.
 

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Quote:
How can the LFE sub respond with the same clarity and speed that it could if it were not burdened with extra channels of information all occurring at different times, amplitudes and frequencies? It can't. Again physics.
Are you suggesting that sending two different signals to a speaker will cause it to not perform properly, in other words, it won't reproduce all of the signal input to it?

If that's the case, then you're misunderstanding something...i'm not sure where, but, do a search for 'Fourier transforms', it should illustrate how a driver reporduces complex waveforms.
 
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