The wonderful world of LFE

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ssully

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An LFE is not the same thing as a subwoofer is. I cannot overstate the importance of the distinction between the 2 any more than by linking to the Dolby Professional document that describes the reality.
Quite so, but I hope no one in this thread is confusing an LFE channel with a subwoofer, for all the reasons you cite.

Subwoofers are loudspeakers for outputting low frequency sounds. Those sounds can be the (low frequency) contents of an LFE channel, and/or low bass derived from other channels. A sub isn't designed for full-range input or output. (and an LFE isn't 'meant' to have full range contents).

But for better or worse, non-filtered LFE appears to be standard in the music-only BluRay releases I've checked, and as I note, it's present in quite a few DVDA and SACD releases as well. It's up to us consumers to deal with it, apparently. Since systems with subwoofers typically have a lowpass filter somewhere before subwoofer output, the users most 'at risk' for unintended playback effects are those who don't use a subwoofer, who fold the LFE into the main channels.

It's especially curious to see tracks using the 'correct' LFE practice mixed with others using full-range LFE on the same release (Brain Salad Surgery DVDA, released 2000 and then again on SACD in 2008).
 

jhw59

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Very interesting discussion. I'm running dual SVS 2000 subs in my setup (all other speakers are Klipsch). I like bass so how should I being setting LFE to allow the subs to do their magic?
 

J. PUPSTER

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Yes, very interesting conversations here with knowledgeable folks.

I thought I'd mention something I noticed on a recent Blu-ray concert I got in lately. I just finished working on it for some Re-mix/Up-mix of it to get more of the wide instruments back into the rears for more interest, as the original has just a shadow of sound from the fronts. That may be fine for most folks as it is a concert after all, and that's what you'd expect to hear typically. And just to be clear I love this concert, but I have some questions as to its mix/mastering.

Some of the audio credits-
-Live Audio Mix: Ryan Hewitt
-Post Production Audio Mixed by: Kevin Shirley
-Mastered by: Bob Ludwig
-Blu-ray Authoring: Scott Long

This is how the song (ripped from the BD dts-HD Master Audio files) looks in Audacity-
*(The LFE is the fourth from the top BTW. And it looks like the fronts were Hard Limited.)

ROYAL TEA BD ACWA.jpg


Here is what the LFE Spectrogram looks like:

ROYAL TEA BD SPECTRO ACWA.jpg


and here is a Plot Spectrum section at around the 7 min. mark-

ROYAL TEA BD PLOT SPECTRUM ACWA.jpg


I've also attached that section of LFE audio untouched at around the 7 min. area below so you can hear it.
* So am I missing something here, or don't understand what's going on, otherwise it just seems like there's some questionable choices being made? And is this typical sound engineering/editing for current releases?

 

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ssully

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Very interesting discussion. I'm running dual SVS 2000 subs in my setup (all other speakers are Klipsch). I like bass so how should I being setting LFE to allow the subs to do their magic?
They probably already are doing it. Are you noticing anything strange?
 

ssully

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And is this typical sound engineering/editing for current releases?
All you needed to do was listen to the LFE alone -- the file you provided. That tells you right away that it's not a 'standard' LFE, i.e. band-limited, lowpassed. A standard LFE as per movie spec is just a series of 'thumps'. What's going on here is that this LFE contains a full range bass guitar part, plus some vocal.

The spectrogram and histogram show you how far 'up' the significant content goes-- well beyond 10kHz in this case.

It is typical of rock music BluRay music-only releases that I have sampled. I'm not surprised to learn that it's happening on BluRay concert (video + audio) releases too.

It appears that the makers of these products are increasingly assuming that they will be listened to on systems that will apply a low pass filter to LFE --which has been true for virtually every system that includes a subwoofer, for many years now. But as noted, there are use cases where it can cause a problem.
 

Owen Smith

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Some of the audio credits-
-Mastered by: Bob Ludwig

(And it looks like the fronts were Hard Limited.)
The fronts are hard limited because that is how Bob Ludwig, utter idiot that he is, does things. It is time he retired, or crawled under a rock to never darken our doors again.
 

junh1024

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What the content on an LFE channel ought to be is bandwidth limited. Unless I am told differently - in writing - I will always, always add an 80Hz LPF at a 48dB/Octave slope to all LFE channels at rendering the mix time - the 80Hz is lower than Dolby's recommended 120Hz, but bear in mind Dolby's 120Hz is a brickwall filter - 120Hz will pass, 121Hz will not so we use the 80Hz on steep slope to make the transition smoother and avoid lumpiness.
NON filtered LFE run a very real risk of an MLP Lossless encoding failure - especially if a downmix coefficient is encoded (and unless the 5.1 is going to a PGC-Block group in the Audio_TS then you really do want to include downmix values or else some silly sod will certainly try to play it & then whine relentlessly in another forum I will not mention that his disc is somehow 'flawed' and isn't he oh, so clever to have found this) but that is not why you called (thanks to Alex Belfield for that phrase).
A bass managed subwoofer is an entirely different animal and one we should never even attempt to second guess, as I have seen sub crossover points from as high as 500Hz and suspect there are plenty that go even higher than this. When preparing for DVD-Audio (and Blu-ray & even SACD) we must follow the RPGA 5.1 guidelines so that means doing things the correct way - the .1 is a .1 because it is not full bandwidth. Period, otherwise the mix would not be 5.1 it would be 6.0.
Treating the LFE as full bandwidth is quite simply wrong. To finish off, I will quote a section from the RPGA 'Recommendations For Surround Sound Production (also attached here)
As noted in section 1, the LFE ("Low Frequency Effects") channel was originally introduced by the film industry because early theatrical speaker systems were
unable to generate loud low frequency signals without clipping. Sometimes referred to as the "boom" channel, it is used in film applications to add dramatic
effect, almost exclusively carrying the rumble of volcanic eruptions, spaceships thundering into view, and bombs and planets exploding.
In terms of multichannel music production, however, there is some debate as to whether the LFE channel is necessary at all. One can argue that the home
theater experience is heightened by having the walls shake whenever rocket launchers are fired, but is there really that much value in having the listener feel
every bass drum hit in such a similarly dramatic fashion?
There is no clear-cut answer, but the mere presence of the LFE channel almost dictates that it be used. The danger lies in overuse, because too much reliance
on the LFE channel to carry bass information can result in the loss of low end altogether on incorrectly configured or poorly designed home theater systems. In
addition, the LFE channel is discarded by most matrixed encoding systems (such as Dolby Pro Logic; see section 1.1) and downmixing algorithms (see section
5.5), including those used for HDTV broadcast. Therefore, the LFE channel should never be used to carry the bass content of the main speaker channels —
that is the job of bass management. Because bass management is employed by almost every consumer home theater system, placing too much information in
the LFE channel will effectively result in double management — total bass overload and probable distortion.
Instead, the LFE channel is best approached with caution. Only modest amounts of signal from specific instruments with significant low frequency content — kick drum, tympani, bass guitar, acoustic bass, low organ or piano notes — should be routed to the LFE, and in all instances those instruments should also be printed full range to the desired main channels as well. An experienced mastering engineer can help in correctly assessing the relative level of the LFE channel as compared with the main channels.

But for better or worse, non-filtered LFE appears to be standard in the music-only BluRay releases I've checked, and as I note, it's present in quite a few DVDA and SACD releases as well. It's up to us consumers to deal with it, apparently. Since systems with subwoofers typically have a lowpass filter somewhere before subwoofer output, the users most 'at risk' for unintended playback effects are those who don't use a subwoofer, who fold the LFE into the main channels.

It's especially curious to see tracks using the 'correct' LFE practice mixed with others using full-range LFE on the same release (Brain Salad Surgery DVDA, released 2000 and then again on SACD in 2008).
Discarding the LFE when downmixing is a valid approach for non-sub systems, and should be the default for 2.0 systems

DVDA/BD music is usually lossless, and if a lopass was applied on the LFE (@ encoding) , that would not be lossless.

You can barely hear the effect of a lopass at the mix stage, since it will be lopass anyway at the sub, or by a lossy codec. Also, a lopass will typically generate a phase shift, and hence if the LFE were downmixed into L&R, that may cause combing/phasing, so I would suggest leaving lopass off at the mix stage.

The dolby docs also raise a couple of good points, like risk of overusing the LFE for music, and unreliable LFE levels in reproduction, which is why I typically don't use the LFE anymore for music.
 

Guy Robinson

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I am extremely happy with the output from my 3 x 12 inch powered subs for both music and movies. I have placed them so that the bass seems to be equal no matter where you sit in the room. One is in the front facing the seating and the other two are closer to the back wall facing the back wall. I have them plugged into the LFE output on the AVR and I have all the speakers set to small with a 50 hz crossover point within the AVR. I have the limiter on the subs also set to 100hz just to limit anything that is specifically sent to the LFE. All three subs are different manufacturers including SVS and Velodyne. It seems to work very well that way. Bass for music is usually not overemphasized (except for things like the Lennon Blu-Ray release and Steve Hackett concerts) but is tight, low, detailed and very satisfying. LFE effects for movies rattle everything in the house and sometimes make me fear for the integrity of my house. I have one setting on the individual subs for music (higher volume output) and one for movies (lower volume output). These settings work for almost everything except for releases like the ones mentioned where I have to dial things down further. The way I look at it is that if you have too much bass you can always dial it down but if you have too little you cannot correct it without more (or bigger) subs.
 
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Wagonmaster_91

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I've also attached that section of LFE audio untouched at around the 7 min. area below so you can hear it.
* So am I missing something here, or don't understand what's going on, otherwise it just seems like there's some questionable choices being made? And is this typical sound engineering/editing for current releases?
You are correct, JP. That is NOT the way the LFE channel should be used by the mixers. It is the full frequency bass guitar (and some vocal leakage, which really makes no sense). As SSully said, the LFE channel should just be thumps - the attack and lowest frequencies of a given sound. The "E" stands for "effects" and that is what it is used for in a proper mix - just the "punch" of a kick drum, bass note, rumble of thunder, explosion, etc. - whether the source is music or movie. In the case of music, unless you know the song really well, it should actually be difficult to tell what song it is if you only listen to the LFE channel.
"Professional" sound mixers are making some weird choices lately. I watched some of Foreigner's "Double Vision Then & Now" concert the other night and couldn't believe how weird it was mixed. The rear channels had almost nothing in them - not even ambience - but the real head scratcher is the center channel. When they are introducing songs, the voices are in the center channel but when the songs start the signal switches to just the bass guitar and the vocals go to front left & right. What???
 

Owen Smith

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When they are introducing songs, the voices are in the center channel but when the songs start the signal switches to just the bass guitar and the vocals go to front left & right. What???
That approach to surround mixing is at least 20 years old, I have some 5.1 DVD-As mixed that way eg. Metallica Black Album on at least some tracks. The mixers seem to think on a cheap sound bar plus satellites the soundbar or centre speaker will have better bass response so put the bass guitar in it, and put the song in left and right to get a stereo image on the satellites. I don't understand why they don't just let bass management sort these things out.
 

Mr. Afternoon

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That approach to surround mixing is at least 20 years old, I have some 5.1 DVD-As mixed that way eg. Metallica Black Album on at least some tracks. The mixers seem to think on a cheap sound bar plus satellites the soundbar or centre speaker will have better bass response so put the bass guitar in it, and put the song in left and right to get a stereo image on the satellites. I don't understand why they don't just let bass management sort these things out.
These days I have to clarify that when I'm talking about surround sound systems I don't mean a soundbar, which confuses a lot of people. That should say enough about the majority of the population.
 

Owen Smith

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I will never consider a soundbar or wireless speakers of any sort a true surround system.
I disagree. In my kitchen I have a pair of Dynaudio Xeo 3 wireless speakers. They were £1200 a pair and sound absolutely fantastic as a stereo pair. If I was using them instead of my rear speakers in my lounge surround system they would provide a substantial upgrade in sound quality.

And I didn't say a soundbar, I said a soundbar with satellite speakers. If there are 4 satellites it surrounds you. It doesn't matter how crap it sounds, it still a surround system. You can have really crap stereo systems but they are still stereo.
 

Mr. Afternoon

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I disagree. In my kitchen I have a pair of Dynaudio Xeo 3 wireless speakers. They were £1200 a pair and sound absolutely fantastic as a stereo pair. If I was using them instead of my rear speakers in my lounge surround system they would provide a substantial upgrade in sound quality.

And I didn't say a soundbar, I said a soundbar with satellite speakers. If there are 4 satellites it surrounds you. It doesn't matter how crap it sounds, it still a surround system. You can have really crap stereo systems but they are still stereo.
Ah, my bad. Do these wireless speakers do lossless? Last I checked they only did lossy.
 

Owen Smith

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Ah, my bad. Do these wireless speakers do lossless? Last I checked they only did lossy.
Lossless 16/44.1 or 16/48 depending on what you feed the little transmitter box (takes optical, stereo analogue or USB in). Newer models of Dynaudio wireless speakers run the link at up to 24/96 lossless, though they also now accept a Blutooth (ie lossy) connection. Mine don't do Bluetooth.

I agree entirely with you about lossy wireless speakers, I'm not interested. And most, but not all, wireless speakers are Bluetooth only. So you do have do look for lossless ones.
 
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