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Author Topic: Difference between IMAX D and IMAX film for sound Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-28-2013 06:26 AM                     Marcel, You don't understand the LFE channel at all. It isn't bass management, it is an effects channel all to itself. It does not carry the bottom end of the main or surround channels. You may have been referring to either DTS (in film) or ATMOS for the surrounds (where the surrounds get their own subwoofer systems...one for the left and one for the right surrounds). In IMAX...it really was bass management...it merely played the bottom end of each channel that it couldn't otherwise play. If the stage channels could each handle down to 20Hz, they wouldn't have needed/used subwoofers...however, back when IMAX came out...that would have been Altec A4s and they are not going to play below 40Hz well. In conventional cinema, the modern "LFE" channel actually came out as a stereo bass channel for effects. The frequency range was from 20Hz to 250Hz and did NOT play what was on the main channels...think about it...it would have exaggerated the parts of the response that both could play. Since they had two channels to work with, LC/RC...they could also "pan" the bass if they so chose. Due to the lack of directionality in bass as one goes lower, the thought of stereo bass was very short lived and the notion of using actual subwoofers grew. Note, that for those theatres that had a CP200, subwoofers and LC/RC...LC/RC carried the information between 100 and 250Hz and the subwoofers carried 100 Hz and down. Thus one could have stereo bass in the frequencies where it could be heard and it also had the bass well covered in all of the frequency range. The LFE channel has an inherent 10dB boost in its pass band. This does make for careful bass management if one is going to use it (some processors offer this option, like the QSC DCP series) since one has to account for the 10dB boost. A conventional bass management system would not allow for this extra 10dB for effects, like explosions. So to sum up...LFE is not and never was bass management, they are two different animals to address two different things...while they can be combined, that is generally an inferior method. One should design a channel that can handle the signal that is handed to it (if you want the Center speaker play down to 20Hz...put in a speaker that will do it). An LFE channel is handling different things than the stage speakers.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 10-28-2013 08:06 AM                     quote: Steve Guttag You don't understand the LFE channel at all. It isn't bass management, it is an effects channel all to itself. It does not carry the bottom end of the main or surround channels. You may have been referring to either DTS (in film) or ATMOS for the surrounds (where the surrounds get their own subwoofer systems...one for the left and one for the right surrounds). Well, maybe you don't agree with what I've said, but I guess you've also at least partly misinterpreted my posts. I certainly know that the LFE channel is a separate low frequency effects channel. But in my humble opinion (and I'm not alone in this), a modern digital sound system, doesn't need a separate LFE channel anymore. Everything that the LFE channel does can be included in other channels (like the center channel) and be compensated with proper bass management and some good EQ. The dynamic range of a digital channel really isn't a blocking limitation anymore. That way, if you want to have stereo bass and LFE, you can have it if your sound processor supports it. You're referring to Atmos, and yes, Atmos already relies on this entirely in a far more advanced way. If you equip your theater with 3 subs at different locations, Atmos will use them and map the sound effects accordingly. quote: Steve Guttag One should design a channel that can handle the signal that is handed to it (if you want the Center speaker play down to 20Hz...put in a speaker that will do it). An LFE channel is handling different things than the stage speakers. Maybe that's the part were we differ in opinion. In my opinion, the actual channels in soundtrack don't need to match 1-on-1 to the speakers in the auditorium. The main reason why I don't agree with this, is because that way, almost every movie gets mixed to the lowest common denominator. It has been years before movies finally started to use the full potential of the surround channels, only to play it safe for the few remaining theaters that didn't have a proper surround configuration.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-28-2013 10:37 AM                     It may just be a disagreement...but honestly, I think you flat out wrong when it comes to bass management versus a dedicated LFE channel. With bass management, you would necessarily restrict localized bass information out of a channel and will create more complexity/work in ensuring that the levels match between those above/below the cut off while simultaneously allowing the effects stuff to have the extra 10dB of SPL for the effect that would benefit from it.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 10-28-2013 02:35 PM                     If you're trying to apply my train of thought to how it is currently being implemented and being mixed, then I agree, that's not going to play nice without some ugly hacks and I'm not advocating that. I was referring to the hypothetical case, in were the sound mix itself would not use a dedicated LFE channel. If you're using bass management exclusively you do not have to restrict localized bass/LFE information inside the mix. If you need to use bass management/LFE crossover, because your main speakers cannot handle what Dolby calls "full spectrum", your whole sound system is probably pretty shitty to begin with. The IMAX Digital Audio system is doing it without dedicated LFE channel and their bass is probably one of the more impressive parts of their sound system. I'm not entirely certain, but Dolby Atmos is probably also doing it without specific LFE information. That 10dB SPL boost that's being applied on the LFE channel is probably a remnant of the past. The dynamic range that can be achieved with modern digital formats should be sufficient to compensate for that artificial 10dB SPL boost.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-28-2013 02:58 PM                     And you'd be mistaken again. With the system referenced at -20dBFS...that 20 dB from reference is all you are going to get above...bass or no. With the way LFE is set up in all current, non IMAX systems, since noise/hiss isn't really an issue on a bass-only track, one can put in the extra 10dB of gain on the amplifier side and reference -30dBFS, in effect to get the extra 10dB of pass band...you are not going to accomplish that easily in a bass management system. Furthermore, you don't get anything for your efforts in a bass management system either (except the reduction of one dedicated audio channel). IMAX's bass is definitely no better than any system set up with comparable components. In general, I've never really been THAT impressed with IMAX's sound. It is good, but it has always been inadequate, in my opinion, for the size of the experience they are trying to create.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 10-28-2013 04:46 PM                     Yeah, if you're going to keep mapping my train of thought to current sound formats and their actual implementation, like SRD, DTS/DATASAT and DCI LPCM, I'm mistaken again, but I repeat myself, or maybe I wasn't being clear enough: I was not trying to do that, it was a theoretical observation. A modern digital sound system (designed from scratch if you will) does not need an LFE channel to achieve low frequency effects. If you reference the whole system at -20 dBFS, it is not going to work the same way it does now. IMAX for example, will certainly not reference their system at -20 dBFS. But actually, I never intended to take this discussion that way. I also agree that the benefits of changing this setup now, will probably be minimal and I was actually not advocating any change in current setups. Additionally, a format like Dolby Atmos, which I hope will become the defacto standard in a few years, will obsolete the LFE channel anyway. I agree with you regarding the IMAX sound system, it is definitly not better than anything that can be done with traditional non-IMAX sound systems. A big problem though is, it is seldomly done even as good as IMAX, which is rather worrying, but an entirely other discussion...  |  IP: Logged

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