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What about the LFE channel
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mosc
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PostPosted: Fri Apr 14, 2006 9:31 am    Post subject: What about the LFE channel
Subject description: Do we even need to use this?
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There seems to be a little confusion about the .1 or LFE channel in X.1 systems. The misunderstanding seems to be that the LFE channel is for the subwoofer and that the other channels are not full-range.

As I understand it, the X.1 systems are intended for all of the channels to be full range. If there is a subwoofer, it should be derived from the main channels, usually the front left/right. The LFE is a special theatrical channel of Low Frequency Effects. If you monitor LFE on commercial DVDs it is rarely used - only for guns and explosions and stuff.

Why should we use this in music?

Any thoughts.

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neilwilkes



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PostPosted: Fri Apr 14, 2006 10:15 am    Post subject: Reply with quote  Mark this post and the followings unread

A few, really.
The main thing about the LFE was that it was originally intended to extend the bass response for movies, especially in things like explosions and the like.

The recommendations for music mixing say that it should not really be used - but now the "rule" has been made, let's break it:
I just finished off a really nice track in 5.1 by the Bombay Dub Orchestra - think Zero 7 meets Enigma, and now add in a 28 piece Indian String section, with Tablas & sitars that sound as if they are being played by Jeff Beck.
There are a lot of low, rumbling synth pads, and these work really well when fed into the LFE as well, and there are certain items I have piped exclusively into LFE (againsy all the rules, I know - but it sounds great)

The thing to remember is that the LFE & a Subwoofer are 2 different things that are often confused with each other.
As long as the resulting mix is checked on simple sub/satellite type systems, to ensure compatibility - there should be no problem at all with using the .1 in music production. It can give the bass a real kick where needed.

In most music, the choice is down to the producer - but to use an LFE like this it is critical for the 5 main channels to be all full-range, or things WILL go awry, badly at that.
If you are mixing on a sub/satellite setup then avoid placing anything at all directly into the LFE channel, as you will not be using it as an LFE but as your only source of bass playback.

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PostPosted: Fri Apr 14, 2006 10:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting. Thanks...

So, on commercial satellite systems, if you don't supply the LFE it will derive the subwoofer channel from the other channels, but if you do it will just send the LFE to the subwoofer? Is that the idea?

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PostPosted: Fri Apr 14, 2006 11:14 am    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
Interesting. Thanks...

So, on commercial satellite systems, if you don't supply the LFE it will derive the subwoofer channel from the other channels, but if you do it will just send the LFE to the subwoofer? Is that the idea?


Basically, yes.

On a sub/satellite system, the 5 satellites are incapable of reproducing any significant bass information, so it's internal crossover will route everything they cannot handle to the Subwoofer (It will not be an LFE in these systems).
IMHO, these setups are a con - and suitable for a computer playback system - if that - only.
The biggest problem with these is when mixing on them there is simply no way the mix will ever translate - the crossover points are arbitrary, and vary wildly. You cannot second guess these things, so trying to use an LFE as an LFE in these conditions is futile IMHO.

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PostPosted: Tue Apr 18, 2006 10:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi neil,

neilwilkes wrote:
On a sub/satellite system, the 5 satellites are incapable of reproducing any significant bass information, so it's internal crossover will route everything they cannot handle to the Subwoofer (It will not be an LFE in these systems).
IMHO, these setups are a con - and suitable for a computer playback system - if that - only.


Are you saying that the LFE channel is not used in these systems? I think the LFE, more than likely being below the cross-over point, would be sent to the subwoofer. It does on my systems anyway Smile

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PostPosted: Wed Apr 19, 2006 5:24 am    Post subject: Reply with quote  Mark this post and the followings unread

No - I was saying that in these configurations, there is no LFE - it's being used as a subwoofer.
The difference is subtle, but there.

Where the diffculty will come is in attempting to mix on these systems with an LFE track - you cannot tell what is happening, as what should be the LFE is being used for all the bass, not just the LFE content.

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PostPosted: Wed Apr 19, 2006 6:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, this is confusing and that's why I started this topic. I'll try to summarize.

The 5.1 format is 5 full range channels and one LFE (low frequency effect) which is a separate channel only low frequencies.

For the 5 full range channels, you can use 1 to 5 subwoofers. Subwoofers are orthogonal to the format. If you use full range speakers or satellites and subwoofers, that's a detail of the playback system, not the format. A bass management system is usually required to produce the signals that go to the subwoofers. Again, bass management is not a part of the 5.1 format any more than it is for stereo.

The LFE is an independent 6th channel. It is not the bass of the other 5 channels.

Commercial home theater amps usually provide 5 sets of speaker connectors. Nominally, these are set up as full range signals. They usually provide a line level subwoofer output (usually labeled subwoofer, sometimes it's subwoofer/LFE). There are usually some settings you can make to determine what the roll off frequency is depending on the quality of the unit.

Most of these commercial amps don't provide separate LFE output from the subwoofer output. If a LFE signal is present on the source material, they usually send that signal to the subwoofer if it is being used or mix it in somehow to the other 5 full range signals. On the better units, you can specify how to do this. On the cheaper units, you may never know.

If I've got this wrong, somebody please correct me. The existence of the LFE has to do with movies. I never quite got it. Maybe someone can explain where it came from. It seems like it is unnecessary, especially for music.

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diatonis



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PostPosted: Wed Apr 19, 2006 6:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
The 5.1 format is 5 full range channels and one LFE (low frequency effect) which is a separate channel only low frequencies.


Yes it is one possibility. You don’t have to have full range speakers. I mention this mainly because most people can’t afford full range speakers. Instead of speakers you can use the word ‘channels’.

mosc wrote:
For the 5 full range channels, you can use 1 to 5 subwoofers. Subwoofers are orthogonal to the format. If you use full range speakers or satellites and subwoofers, that's a detail of the playback system, not the format. A bass management system is usually required to produce the signals that go to the subwoofers. Again, bass management is not a part of the 5.1 format any more than it is for stereo.


If you don’t use bass management only the LFE channel will be heard in the sub.

mosc wrote:
The LFE is an independent 6th channel. It is not the bass of the other 5 channels.

Yes .


mosc wrote:
Commercial home theater amps usually provide 5 sets of speaker connectors. Nominally, these are set up as full range signals. They usually provide a line level subwoofer output (usually labeled subwoofer, sometimes it's subwoofer/LFE). There are usually some settings you can make to determine what the roll off frequency is depending on the quality of the unit.


Yes - The cross-over frequency should match the speakers used – you basically want to create a full range system using satellites (nearfields) speakers and a sub-woofer. If your nearfield monitors go down to 60Hz (-3dB corner) then 60Hz would the cross-over point.

mosc wrote:
Most of these commercial amps don't provide separate LFE output from the subwoofer output. If a LFE signal is present on the source material, they usually send that signal to the subwoofer if it is being used or mix it in somehow to the other 5 full range signals. On the better units, you can specify how to do this. On the cheaper units, you may never know.


Hopefully this diagram will help:


bass_manager_diagram.gif
 Description:
Bass Management Diagram
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bass_manager_diagram.gif



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PostPosted: Wed Apr 19, 2006 7:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the clairification. That diagram is helpful, but it's only one way commercial units work - most of them though. Very Happy
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PostPosted: Thu Apr 20, 2006 7:13 am    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
Thanks for the clairification. That diagram is helpful, but it's only one way commercial units work - most of them though. Very Happy


Yeah I guess it's a numbers game.

For instance if you are using full range speakers (20Hz -> 20kHz) and then add a sub-woofer you'll more than likely get phasing problems caused by using added and unnecessary sub-woofer. On a full range system such as this the LFE channel would only be sent to the subwoofer and not re-directed to the front mains.

I would say that most people who listen to music in surround sound don't have full range speakers. From what I've found is most of them are 'inexpensive' consumer level home theater systems. I see no real benefit to creating a surround mix for the top end and mix using bass management. I guess I have to because I don't have full range speakers. The point of the whole thing is very simple: to hear everything from hopefully (20 Hz to 20 kHz). This is very similar to what is done in stereo.

NOTE: By top end I mean a system with full range speakers and a dedicated LFE subwoofer.

I would rather stress that surround sound isn't that difficult to get into and enjoy. It's a little more complicated but not much more than doing stereo.

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PostPosted: Thu Apr 20, 2006 6:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

diatonis wrote:
I would rather stress that surround sound isn't that difficult to get into and enjoy. It's a little more complicated but not much more than doing stereo.


That's good to hear because I think it is a big opportunity to push the envelope on our music. I hope this forum will help.

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PostPosted: Mon Apr 24, 2006 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Everything that diatonis has said is correct, but I'd like to add some extra clarification to this thread.

The word subwoofer can be misleading, and is often misused. A subwoofer is a speaker. There is technically no subwoofer channel, though a signal for a subwoofer can be created by an amplifier when no LFE channel is available, or for other reasons depending on the speaker configuration. The LFE (Low Frequency Effects) channel, which is a true discrete channel, is often fed to the subwoofer, if one is present. This is why LFE is often mistakenly referred to as a subwoofer channel.

The LFE channel in Dolby Digital generally has a frequency response from 20 Hz up to 200 Hz, depending on how the decoder is implemented and how the audio is mixed. Because 20-200Hz is 1/10 of 20-20,000Hz (it's actually a bad approximation either linearly or logarithmically, but I digress), the LFE channel is often referred to as a the "point one" in an X.1 because it contains one tenth, or 0.1 of 1 channel's frequency range. This will also represent about 1/10 of the data of a full-frequency channel, when encoded. A minor detail, but I thought I'd explain it just in case anybody was curious and didn't know where the terminology came from.

The LFE channel in Dolby Digital Plus (newer version which will be used on HD-DVD and Blu-Ray) can go up to about 500 or 600 Hz, which will allow more mixing and post-processing options for those with limited or unusual speaker setups. This is also a minor detail, and may never actually get used by those mixing audio. Time will tell on that one.

The bass management that diatonis was referring to is a mixing and filtering system designed to compensate for different types of speaker systems. For example, if you have two large front speakers and two small satellites, but no center or subwoofer, the center channel and LFE channel, as well as any bass from the surround channels, can be mixed into the left and right. It would look something like this:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Well, actually, it would look exactly like that. If it didn't, I'd fail it and it wouldn't make it to production Wink

Oh, and pardon the lack of distinction between the traces. Many of them overlay each other. Audio Precision is not very flexible software for displaying or reporting data, either. They make great measurement hardware, though!

Anyway, that's an example of how it works in a Dolby Digital implementation. Bass management can also be used to redirect bass to the subwoofer, or center, or other speakers, depending on which ones are big enought to deliver the bass. It's rather flexible that way. But it really depends on how the receiver was designed, because sometimes the user interface isn't that great and it's hard to set up. Oh well.

Some simpler amps, purely analog amplifiers for example, with only two speakers and a subwoofer will simply use an analog crossover to generate a signal for the subwoofer. It actually varies quite a bit depending on the equipment used, the number of speakers, the format of and number of channels in the audio signal, and other things like user settings and encoded audio with bitstream flags that change settings. It can be complicated, or it can be simple.

I think the sheer variety of setups that can use subwoofers to generate bass has led to some confusion surrounding exactly how and when a subwoofer should be used, and what the differences between the "subwoofer" and the "LFE" are. If anybody has any other questions, please ask. I haven't been working at Dolby for very long, but I think I know all of this stuff pretty well already.
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PostPosted: Mon May 08, 2006 5:30 am    Post subject: Reply with quote  Mark this post and the followings unread

1 - I was under the impression from both the encoders and from published Dolby documernts that the .1 in Dolby Digital is limited to 120Hz. There is even an HPF in the encoder for this express purpose.
Where does the figure of 200Hz come from???

2 - Dolby Digital (bog standard version) is also available for both BRD & HD DVD - it is not limited to Dolby Digital Plus.

3 - IMHO, extending the LFE to 500 or 600Hz is a serious mistake. All this will do is muddy the already murky waters.

Just my personal opinions.

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PostPosted: Mon May 08, 2006 5:57 am    Post subject: Reply with quote  Mark this post and the followings unread

IMO not capping the LFE at 120 or 200HZ will allow this channel to be used for various "effects"/sounds that will play better when "uncapped"(1) but then this opinion is based on the LFE not being a subwoofer feeder as such.

Trivia: Anyone who have ever tried to play some bassy texture rich synth sounds on a PA subsub with actibe filtering ( and then having muted the other feeds ) will remember how disappointing that was. A plain PA sub with a soft start at 800-600 hz will sound more convincing. ( I am not dissing the subsubs. These are wonderful in a full PA rig. I am simply trying to illustrate that that another octave or two can be useful. Using this well in a surround mix is another issue altogether, but I can easily see that more bandwidth in the LFE can be useful. )

(1). I am not a big fan of excessive band filtering/routing. I can easily imagine bassy growling sounds that will not work well if these are split between the LFE and the front speakers. Regarding music, certain ideas would be hard to translate well to a capped LFE surround setup.

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PostPosted: Mon May 08, 2006 6:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, so the LFE is a special purpose channel used for movie effects. Mixing music to send the bass to the LFE would be a tragic mistake. That is really the point of my starting this tread - to clear up that popular misconception that 5.1 is 5 satellites and a subwoofer.
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PostPosted: Mon May 08, 2006 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

neilwilkes wrote:
1 - I was under the impression from both the encoders and from published Dolby documernts that the .1 in Dolby Digital is limited to 120Hz. There is even an HPF in the encoder for this express purpose.
Where does the figure of 200Hz come from???

2 - Dolby Digital (bog standard version) is also available for both BRD & HD DVD - it is not limited to Dolby Digital Plus.

3 - IMHO, extending the LFE to 500 or 600Hz is a serious mistake. All this will do is muddy the already murky waters.

Just my personal opinions.


1. My mistake. 120Hz is usually the crossover frequency. It actually depends on the implementation, though. Given a full frequency range input to the encoder, some will pass higher frequencies up to about 200 Hz. We generally allow this as long as everything else works right. You'd be amazed how horribly some people can mess up our code.

2. I agree. I can't really figure out why they extended that range. The uses for that would be limited. However, it is unlikely that anything above about 100 or 200 Hz would be used anyway. The bass management curves will not change. If somebody wants to mix midrange into the LFE channel, I suppose they could... but why?

3. Yeah, I know. HD DVD will also use TrueHD, a lossless encoder. The implementations *have* to support Dolby Digital Plus, though, even though standard Dolby Digital will always be an option to content creators.
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PostPosted: Mon May 08, 2006 10:54 am    Post subject: Reply with quote  Mark this post and the followings unread

elektro80 wrote:
(1). I am not a big fan of excessive band filtering/routing. I can easily imagine bassy growling sounds that will not work well if these are split between the LFE and the front speakers. Regarding music, certain ideas would be hard to translate well to a capped LFE surround setup.


Agreed. If something is mixed for stereo, listen to it in stereo. I think multichannel speaker systems should be saved for content with discrete channels, mixed specifically for surround playback.
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