Film-Tech Forum ARCHIVE: Difference between IMAX D and IMAX film for sound

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Posted by Terry Lynn-Stevens (Member # 7349) on 08-23-2013, 08:55 PM:

Can anyone quickly tell what are the differences between 15/70 and IMAX Digital when it comes to sound?

Say I watch Star Trek Into Darkness in 15/70, will the sound be different than Star Trek Into Darkness in Digital IMAX? Can someone also comment on the equipment?

Posted by Edward Havens (Member # 4715) on 08-25-2013, 11:57 AM:

My understanding is that 15/70 prints before the early 1990s used a type of interlocking 35mm magnetic film soundtrack not unlike the one used for Cinerama in the 1950s. With the advent of digital sound, IMAX 15/70 prints moved to a CD-ROM (then DVD-ROM) based time-coded soundtrack not unlike DTS. IMAX digital uses the same kind of digital sound DLP projection uses.

Since a movie like ST: ID would have a digital soundtrack regardless of whether the film was presented digitally or on 15/70 film, I would assume the soundtrack would be the same.

Posted by Terry Lynn-Stevens (Member # 7349) on 08-25-2013, 01:29 PM:

Edward, I understand what you are saying. But would the speaker lay out and equipment in a digital IMAX be the same in a 15/70 cinema?

Posted by Marcel Birgelen (Member # 6801) on 08-25-2013, 11:11 PM:

The Digital IMAX locations that have been converted from "ordinary" theaters usually don't have a height channel deployed. The rest of the channels seem to be the same.

Regarding speakers: According to IMAX, all speakers are designed and manufactured specifically for that auditorium. I cannot say if that is true in any way, but the speaker cabinets themselves for the D-IMAX locations that I've seen them, all look exactly the same. Please note that I didn't have the possibility to look inside of them and I might also be misjudging sizes from memory.

What I also do know from my own perception is that the average converted D-IMAX theater usually plays movies much louder than the regular 15/70 IMAX venue. I cannot say if that's due to the fact that the larger venues just cannot create the same wall of sound or just the 15/70 exhibitors being more reasonable.

Posted by Ron Lacheur (Member # 1126) on 08-26-2013, 02:26 PM:

Most multiplex IMAX locations are eliminating the top-center channel. Other than that, it's pretty much the same. The big disappointment with the local Cineplex IMAX,was they "upgraded" the two way coaxial system with the three-way "tower of tinnitus " speakers as I like to call them. These shriek so bad, I won't be coming back.

Posted by Ivan Plamondon (Member # 7637) on 08-29-2013, 06:16 PM:

Sound levels and quality certainly play a big part on whether the whole experience will be pleasurable or not.We still have the old style speakers at IMAX Niagara with digital eq's to process them. Features tend to venture into the 100 dB zone at times, but overall the sound system is great for full length films

Posted by Marco Giustini (Member # 4544) on 09-07-2013, 02:48 PM:

In a Dolby calibrated room when a movie is run at 7.0 a single stage speaker will be capable of 105dB, a single surround channel will play up to 102dB and the subs will be capable of 115dB.

Considering you are playing these channels together, I would expect more than 100dB in the loudest peaks.

Posted by Claude S. Ayakawa (Member # 1401) on 09-07-2013, 05:27 PM:

Just being a patron and not a film theatre technician like most of you here, I have found the sound system of 15-70 and digital IMAX sound just about the same-Superb!

I have seen IMAX 15-70 films at the Kennedy Space Centre IMAX, Lincoln Centre IMAX IN NYC, Niagara Falls IMAX, Polynesian Centre IMAX , Waikiki IMAX and the Dole Cannery 18-IMAX -RPX here in Honolulu in Digital IMAX.

-Claude

Posted by Jonathan Goeldner (Member # 4854) on 10-19-2013, 08:19 PM:

quote: Ron Lacheur
Most multiplex IMAX locations are eliminating the top-center channel. Other than that, it's pretty much the same. The big disappointment with the local Cineplex IMAX,was they "upgraded" the two way coaxial system with the three-way "tower of tinnitus " speakers as I like to call them. These shriek so bad, I won't be coming back.

I'd find that counterintuitive to eliminate the top center when the new systems like Auro and Atmos pride themselves with height channels - personally I'm finding IMAX's 5.0 setup antiquated compared to other system setups.

Posted by Marcel Birgelen (Member # 6801) on 10-20-2013, 06:46 PM:

From IMAX's point of view, the elimination of this speaker makes perfect sense...

Those Digital IMAX screens usually are just 1:1.77, so the height channel doesn't add very much. Since Digital IMAX venues almost never play original IMAX content, they're stuck with content that was originally mixed in 5.1 or 7.1.

But yes, Digital IMAX's current sound system is nothing more than some glorified 5.1 system. Although with very powerful surround speakers [Wink]

Posted by Gordon McLeod (Member # 33) on 10-20-2013, 09:22 PM:

historically Imax used bass management and the stage channels had a fairly steep cut off and summed to a large sub array
when connecting a 35-70 processor to it one had to insert the sub signal back into the the three stage channels and let the imax filters do their job

Posted by Terry Lynn-Stevens (Member # 7349) on 10-26-2013, 12:03 PM:

quote:

But yes, Digital IMAX's current sound system is nothing more than some glorified 5.1 system. Although with very powerful surround speakers [Wink]

Marcel. I am under the impression that IMAX modifies the sound similar to what they do with the DMR process for the image. In other words, the IMAX mix is slightly different than the studio original. Do you have any info on this?

Posted by Marcel Birgelen (Member # 6801) on 10-26-2013, 05:05 PM:

The soundmix in IMAX DCPs is different. How different is IMAX's secret sauce. But they actually rather open to the fact that their DRM process also involves "remixing" the sound.

As Gordon already stated, the IMAX sound system uses bass management exclusively for the subs, so there is no LFE channel in the mix. Essentially it's a 6.0 system (and not really a 5.1 system) with a rather large subwoofer array. In most modern systems, it's a 5.0 system, because of the missing height speaker.

The bass management approach of IMAX is actually not all that bad if you ask me. I don't understand why modern, post SRD/DTS sound formats still retain a separate LFE channel.

Posted by Jonathan Goeldner (Member # 4854) on 10-26-2013, 10:46 PM:

quote: Marcel Birgelen
I don't understand why modern, post SRD/DTS sound formats still retain a separate LFE channel.

isn't the reason that unlike IMAX speakers which are larger and can produce bass in all the channels - typical 'standard' theatrical speakers can't reproduce low levels due to their size and the number that are arrayed in an auditorium would be a tad excessive. Since bass is omni-directional, one or two bass speakers behind the screen is more than adequate.

Posted by Marcel Birgelen (Member # 6801) on 10-27-2013, 03:09 AM:

Yeah, but any modern cinema sound system already uses bass management. The frequencies the main and surround speakers cannot handle, should already be redirected to the sub.

The separate LFE channel was useful in 6-track magnetic soundtracks, as to avoid distortion of the other channels. SRD and DTS might have kept it for backward compatibility reasons.

But in modern installs, the LFE channel should be totally obsolete.

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marcel Birgelen (Member # 6801) on 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.

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marcel Birgelen (Member # 6801) on 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.

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marcel Birgelen (Member # 6801) on 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...