To bass manage or not to bass manage

[audiobrad]

Well Tom, I don't think there's much competition here really. I totally yield to your superior expertise......................................... .. IN GOLF! I would need at least a 3 iron and a hurricane gust off the river! I'll have to get back to you on that crossover frequency.

Brad

[minister]

I'll have to get back to you on that crossover frequency.

brad, where ever your monitor manufacturer has their crossover in the bass redirect (which is most likely 80Hz), DOLBY defines the LFE Channel, for 5.1 Discreet Surround Digital (Dolby Digital) as being between 20Hz - 120Hz. so, when you are doing sound for a film, and will use a Dolby license, that is the bandwith you have to work with for that channel. doesn't mean you can't choose 80Hz. surround channels are defined as as stereo full range. i am talking MIX here, not monitoring.

Dolby | Technical Library

[1150Post]

brad, where ever your monitor manufacturer has their crossover in the bass redirect (which is most likely 80Hz), DOLBY defines the LFE Channel, for 5.1 Discreet Surround Digital (Dolby Digital) as being between 20Hz - 120Hz. so, when you are doing sound for a film, and will use a Dolby license, that is the bandwith you have to work with for that channel.

Just making certain that we are all clear on the difference between the redirect from the 5 sats, and the LFE channel. Usually, the redirect crossover is specified according tho the response of the 5 sats, with the filtering happening prior to the redirected signal making it to the sub. The LFE, generally, would have no filtering built into the monitor system, with the low pass filtering happening in the session or between the session and the monitors. The redirect and the LFE are seperate items, though they are both combined and "played out" of the sub.

I know this is basic stuff, but it seems like the explanation of what is happening in a proper Bass Management (BM) circuit is a little blurry between the various posts.

Also, Georgia has stated that when mixing on her stage, utilizing a Tascam Monitor Controller, she checks the mix both BM'd and not BM'd. I'd be curious to hear again her experience on that, and what the differences are in playout.

As a side note, it's funny that when we were authoring some of the early DTS 5.1 music discs and DVD-Audio 5.1 discs, it was surprising to see how many masters were not low pass filtered on the LFE. Also, in discussing with Dolby some of the finer details of their Dolby-E (broadcast) encoder, they said they encopde into the Dolby E data (which is in essence, a packeted AC-3 stream) up to 500hz for the LFE. I found that curious as well. But that's another discussion.

Brad McIlvaine

[abluesky]

Hello All -

I thought I might chime in (I hope nobody minds)...

Here are the basic points that I would like to convey.

THE LFE: The LFE is a channel and is not the same as the subwoofer. It’s actually a 120Hz bandwidth limited channel (80Hz for DTS) that has an additional 10 dB of electrical headroom above the 5 main channels. It was created to enhance low frequency sound effects, such as large explosions and other LF content which exceeded the headroom of the main channel (this is an important point - it was created to extend the headroom, not frequency response). Hence the name LFE: Low Frequency Effects and not low frequency bass channel. So, unless you are recording Tchaikovsky’s 1812 Overture with REAL cannons, it doesn’t need to be used in most music applications, but does make sense for action movies etc. Additionally, never put anything critical to the mix exclusively in the LFE channel. This is because when a Dolby Digital decoder folds down a 5.1 source, it typically drops the LFE channel completely (poof!).

BASS-MANAGEMENT - Is a function of the monitoring system and is analogous to a full-range speaker (bass-management = full-range). Bass management uses filters to extract low frequency information from the main channels and then reroutes that information, along with the LFE channel, to a single mono subwoofer (more than one sub can be used to reproduce this signal for greater flexibility and output). The advantages are overwhelming and include smaller main speakers that are easier to place, better LF response, reduced inter-modulation distortion and more repeatable LF response from room to room (small or large).

The point about "more repeatable LF response from room to room" is one that people often don't fully understand, or overlook completely. However, how low frequencies sum, from two or more sources, will depend greatly on room acoustics, the relative distances between the speakers, the relationship between the speakers and the boundaries in the room, which speakers are playing, what the phase relationship between these signals are, and where the listener is in relation to the speakers in the room. These factors are highly complex and will be different from room to room and will vary greatly between Mix A and Mix B.

Electrical summation of the LF signals (as is done with bass-management), is a very predictable and repeatable way to get consistent LF response. LF phase issues between channels are resolved in the most absolute and accurate way - electrically. As a general rule, bass-management should be used in any professional recording studio that is creating content for direct consumer consumption (music, broadcast, radio, DVD etc.).

In large rooms, such as movie theaters, these summation issues become less of a problem, because the main problem in a big room is reverberation time (RT60) and not LF room modes (standing waves etc) which are a huge factor in smaller spaces. However, even in a large room bass-management can help ensure better LF translation between spaces. As a side note, I believe THX's new digital crossover for movie theaters and dub stages actually does allow for bass-management of the surround channels and also the main channels.

In general I believe people need to consider the complete monitor / room interface (room acoustics) and use a monitoring system that is designed to compliment the realities of their studio.

That is just my take...

Cheers!

[audiobrad]

Yeah. What he said. Thanks Pascal. Good summary.

Brad

[Rick Sanchez]

Thanks for some useful information. It seems that we are all in the same boat dealing with the ambiguous nature of home theatre systems and my original question was about mixing or remixing specifically for home "theatre" releases.

Considering the fact that 95% of the home theatre systems redirect everything below a "set" frequency is to the sub, wouldn't it make sense in the remix to avoid (in most cases) using LFE exclusive material? I realise that for those of us who have a higher end home system with relatively full range sats and a properly set up room may be disappointed by the results of this mix, but I've found that mixing on a properly set up "full" range theatre style system has not translated as well as remixing on a bass managed system.

How are other people dealing with this problem?

[audiobrad]

Tom, me thinks your jousting with me. I originally said:

why would you use bass re-direct (which I assume crosses at 80Hz) in the first place? (Unless you are simulating a satellite/sub system).

--the key word here is redirect (or crossover). I did not say LFE.

You said:

Dolby recommends 120 Hz

----which is Dolby's recommended band limit for the LFE (which wasn't what I was talking about).

I said:

I'll have to get back to you on that crossover frequency

-- meaning just that, my system crossover frequency - not LFE cutoff. I wanted to verify that my M&K's cross at 80 Hz. They do.

You said:

brad, where ever your monitor manufacturer has their crossover in the bass redirect (which is most likely 80Hz), DOLBY defines the LFE Channel, for 5.1 Discreet Surround Digital (Dolby Digital) as being between 20Hz - 120Hz. so, when you are doing sound for a film, and will use a Dolby license, that is the bandwith you have to work with for that channel. doesn't mean you can't choose 80Hz. surround channels are defined as as stereo full range. i am talking MIX here, not monitoring.

Here, you're in essence quoting back to me what I said in earlier posts about not confusing the system (along with bass management) with use of the LFE track. Pascal nicely delineated and clarified the point.

Brad

[minister]

long day of mixing.... brain tired...

i agree with what pascal said, it is what i understand.

brad, i am not jousting you about. i did not confuse the LFE channel with the SUB as part of a monitor system. if you think i was making comments to you via a moving target, that was not my intention. yes, your monitor system crosses over at 80Hz, i think my genelec system will crossover there when engaged too. and then, yes, i was talking about the LFE channel (not the SUB monitor, or where the crossover for it vis a vis the georgia satellites, not YOU, georgia , but the band) as dolby defined it. i can see how you would think i might be confusing you with this because i left it dangling out there.... sorry for not making that absolutely clear.

you were seeming to make a point about never turning off the bass redirect even if you were sending a mix to a dub stage. i was trying to say that that is a mistake. one mistake being that once you are in an environment where there is no bass management, and you weren't careful about what and how you sent things to the LFE channel, your mix might me missing some lows. if content in a bass redirected system is merely sent to the satellites (like you sort of mix 5.0), the low end might sound perfectly fine. but if you took that to a non bass redirected system, it would not play back the same. that would be something to definitely watch out for when mixing in a bass re-directed system for something destined for the cinema. let me give you an example. one of my first mistakes was mixing a rocket launch with bass redirect on. sounded big and boomy in my room. (a small room btw.) but then i took that mix to a theater as a test. what happened to the low end? in the bass re-directed system, all frequency content that was in the channels going to the satillite was being played in the room. i didn't think to send any to the LFE as the bass redirected system was telling me, all systems go (or so i thought). but in the theater, nothing was being redirected. rather than leaving redirect on, i shut it off and bussed some filtered low end from the rocket launch sound i had to the LFE and then i was in business. and maybe you wouldn't do that, (i am saying ONE might) but i did.

i can see how a blue sky (or maybe your m&k's???) can only function with bass re-direct always engaged given how far the satellites go down...and they function great like that as a system...so what to do in that situation is a little unclear to me...what to do with the stuff between 80 & 120?...my LFE output on my 192 would go where, in this system, if i knew i was going to a stage and then a theater in SR•D with a 20-120 range LFE channel?? i guess let the blue sky (which i know is an excellent system) do it's thing, make judgments based on that and then, it ought to translate to the dub facility just fine? just trust that it is being represented properly between the sat's and the sub? (which it may be, i just ask.) is that what people are doing?

another question, again, not a hostile one, for pascal, or anyone else who understands this, completely related to the above: if there is going to be THX or other bass redirected system in a theater, and dolby defines the LFE as <120Hz, how do you deal with the content between 80-120? if the crossover of the system is 80, then i'd guess you would simply have to know that before hand and mix that way. right? or no? again, trust that the sat's and sub represent it just as well as a discreet system.

[Eric L]

Considering the fact that 95% of the home theatre systems redirect everything below a "set" frequency is to the sub, wouldn't it make sense in the remix to avoid (in most cases) using LFE exclusive material?

Rick,

As Pascal pointed out (and I agree 100% with him) using "LFE exclusive material" should indeed be avoided at all times, for all mixes, regardless of monitor systems. This is not really a bass management issue, so much as a foldown issue.

LFE material should augment existing sound from the main speakers, to extend the dynamic range of your system. There should never be a sound effect only in the LFE track for reasons previously mentioned. Home systems use the sub to help extend the "frequency response" of smaller satellite speakers, not the LFE. Two different issues.

However, you should indeed employ the LFE track when required for broadcast programming as you still want to service those with a proper 5.1 living room. Just make sure your covered in the LT/RT, stereo and mono.

I personally find the low end easier to control and more accurate in smaller rooms when bass management is employed, as opposed to running the mains full range and adding a discreet sub.

[Eric L]

one of my first mistakes was mixing a rocket launch with bass redirect on. sounded big and boomy in my room. (a small room btw.) but then i took that mix to a theater as a test. what happened to the low end? in the bass re-directed system, all frequency content that was in the channels going to the satillite was being played in the room. i didn't think to send any to the LFE as the bass redirected system was telling me, all systems go (or so i thought). but in the theater, nothing was being redirected. rather than leaving redirect on, i shut it off and bussed some filtered low end from the rocket launch sound i had to the LFE and then i was in business. and maybe you wouldn't do that, (i am saying ONE might) but i did.

hi Minister,

From your posts, it sounds like you may be confusing bass management (bass re-direct) with the LFE channel as Dolby defines it.
Bass management should be employed in small rooms with smaller monitors for all the reasons explained by Pascal. This is part of the rooms monitor system and should not be turned off ever. The room should be pinked and tuned with bass management engaged. Bass management, simply put, is an attempt to emulate a large full range speaker system with smaller transducers. The goal is, the flattest most accurate monitoring you can achieve in that space.

If you have larger, full range speakers (whatever that might mean to you) then bass management may not be required. But if you are in a small room you should use it.

Your problem quoted above, sounds more like a room dependent problem or a miscalibrated system. As Pascal pointed out, room modes may have been excited by low end coming from different point sources and causing a build up of low energy at mix position.

All things being equal, material designed or mixed on a bass managed system should translate to a larger room within reason. Obviously, the rooms will sound different, and adjustments need to be made, but I would not think that defeating bass management is a proper technique for dealing with it. All that does is cripple your monitoring system for the room you are working in, and may cause more problems than the one you are trying to remedy.

JMO