Let’s talk about IMAX. Originally a 70mm film format for documentaries in special venues, IMAX has successfully transitioned over the last 25 years to become perhaps the premium large format for exhibiting blockbuster commercial movies. As such, it was only natural that the company make a foray into the home market, which they did in the form of high-end private theaters and the launch of the IMAX Enhanced program for home audio/video equipment and media about five years ago.
Since then, IMAX Enhanced has been developing slowly but steadily domestically in a world dominated by the commercial behemoth of Dolby Vision and Dolby Atmos. Initially partnered primarily with Sony for studio support, IMAX Enhanced released a handful of discs along with digital titles on the Sony Pictures Core service (formerly Bravia Core and exclusive to Sony TVs, recently expanded to PS5), later augmented by Marvel titles on Disney+.
While the expanded IMAX aspect ratio picture has been widely distributed in the home market, what has remained elusive is the unique IMAX sound. IMAX cinemas use proprietary 6-track or 12-track audio formats and sound systems that do not conform to the traditional 5.1 or Atmos layout. IMAX 6-track lacks the LFE channel found in 5.1 but adds a top-of-screen channel that is not in 5.1 (because IMAX screens are, you know, really tall). For the 12-track format, IMAX adds one channel on each side wall (between the screen and the iconic single point-source surround speakers in the back corners of the room) and four channels overhead.
These differences necessitate a separate IMAX mix for titles that are released in IMAX cinemas. Of course, IMAX Enhanced wanted to deliver IMAX sound to the home rather than pairing the IMAX visuals with Dolby audio, so they partnered with DTS to create a specific subset of the DTS:X format customized for IMAX. IMAX DTS:X is not a direct port of the IMAX mix; it would perhaps be more accurate to say that it is a new format specifically designed to simulate the IMAX sound in home theaters — most of which are configured to play Dolby, because, well, that’s all we’ve really had. Until now…
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IMAX DTS:X comes in two flavors: a lossless version on disc that is delivered by the DTS-HD Master Audio codec and a lossy (perceptually lossless) version for streaming delivered by a relatively new codec dubbed “DTS:X for Streaming” or “DTS:X Profile 2.” The lossless version is object-based and can be rendered to a number of different speaker layouts (e.g., the ubiquitous 7.1.4). The lossy version is delivered in a 5.1.4-channel configuration — more on that later.
The Why and How of IMAX at Home
So, how does any of this relate to our business? Well, Atmos has been wildly popular, but it is over 10 years old. You can’t really draw in customers to hear the “new” Atmos sound in your showroom or upgrade their home theaters to the “latest” format because Atmos is simply not new anymore. On the other hand, IMAX has maintained and perhaps increased the mystique of its premium format in recent years (thanks in no small part to Chris Nolan). IMAX continues to be a buzzword among audiences, representing a worthwhile upgrade to see the biggest epic movies even though it costs more. Why not leverage the IMAX reputation to get customers back into the showroom to experience IMAX Enhanced and discuss some upgrades? You offer premium services to your clients; IMAX is a premium format. Sounds like a perfect match, right?
Let’s break down how you can pull this off, because it isn’t without challenges. The biggest issue facing IMAX Enhanced right now is delivery of IMAX DTS:X audio. Sadly, there are too few discs (only ten I know of that really qualify — all from the initial 2019–2020 release slate) to be relevant. Yes, you can use them to demo IMAX DTS:X in its full lossless glory in your showroom, but your customers need more than that to take home.
Here’s where things get tricky, and we’re really hoping and relying on IMAX and DTS to help us out. As of this writing, the DTS:X for Streaming codec (which is being delivered by Sony Pictures Core and, as of May 2024, Disney+ for select titles) must be transcoded by the playback device into a format that is recognized by existing decoders in AVRs and surround processors. The only devices that will currently perform this transcode are…televisions. Obviously, we can’t use a TV as the source device in a high-end custom home theater with a large front-projection system or screen wall. We need third-party streaming devices (e.g., Apple TV 4K, Chromecast, Roku, Fire TV, etc.) to support the DTS:X for Streaming codec. Fortunately, the IMAX Enhanced camp has indicated that DTS:X for Streaming support is coming to such devices as early as Q4 2024, although we can’t say exactly what form this will take or which devices will be included. The PS5 would appear to be a logical step (it’s made by Sony, which is heavily invested in IMAX Enhanced; it supports Sony Pictures Core and Disney+, the two services instrumental in delivering IMAX DTS:X; and it has the processing power to transcode DTS:X for Streaming). However, that is just speculation on my part.
It may seem a bit presumptuous to promote IMAX Enhanced before IMAX DTS:X is really viable for high-end dedicated home theaters. (I mean, you could always show it for a smaller “media” room with a TV and couch, but to me, at least, that kind of defeats the purpose of “IMAX.”) However, if you want to use IMAX Enhanced as a marketing strategy for big projects, being first to market is the name of the game; it may behoove you to plan ahead. Your call.
About That Bass
If you do decide to take the IMAX Enhanced plunge, I’d like to highlight a couple of things in the audio format. The first is the bass management system. Remember how IMAX 6-track and 12-track don’t have an LFE channel? Instead, IMAX cinemas use a 70 Hz crossover in the playback chain to redirect bass from the 6 or 12 main channels to a large stack of subwoofers. Yes, IMAX cinemas are bass managed! (It may surprise you to hear that traditional cinemas typically are not bass managed — though a form of bass management for the surrounds is outlined by Dolby guidelines for Atmos cinemas.) This concept is carried over to IMAX DTS:X, although the implementation is different.
Rather than the main channels containing full-bandwidth bass, IMAX DTS:X filters the bass below 70 Hz from the main channels and re-works it into a new “bass channel” that is then encoded in place of the traditional “LFE channel” in 5.1 and Atmos. But that is just step one. IMAX DTS:X expects a complementary bass management system in the AVR or surround processor to complete the process. This system is included in IMAX Enhanced products and automatically engages when IMAX DTS:X content is detected.
Is it a good idea to bake bass management into the format? I’m not going to address that here except to say that my technical editor, Chase Walton, has spent a good amount of time playing around with the limited amount of available IMAX DTS:X content and reports that he is satisfied with their bass management system when properly implemented. However, it’s equally important to note that the IMAX DTS:X bass management system can be bypassed; furthermore, IMAX DTS:X can be played through a non-IMAX DTS:X decoder. In both cases, the normal bass management that you have configured for the system will be used — just like for any other format. This means that IMAX DTS:X will physically play on a wide range of systems, but keep in mind that it’s not technically playing back the way it was intended unless you use IMAX DTS:X bass management. This topic warrants further analysis when more content is available.
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IMAX DTS:X uses a 70 Hz crossover point between the speakers and subwoofers, so you need to make sure the speakers can reproduce 70 Hz at the appropriate playback levels for the room. Furthermore, it may be advantageous to create a specific EQ profile for the speakers and subwoofers for IMAX DTS:X. At this point, the amplitude and phase relationships between [A] the pre-filtered main channels in the content, [B] the processor high-pass filters, [C] the pre-filtered LFE channel in the content, and [D] the subwoofer output low-pass filter are a bit murky. We have achieved good results so far by equalizing the speakers to flat well below the crossover (at least 50 Hz, –3 dB and ideally 35 Hz, –3 dB, which is one octave below the crossover) and equalizing the subwoofers to flat well above the crossover (ideally 140 Hz, which is one octave above the crossover), then applying the IMAX DTS:X bass management. This is not the conventional way to equalize, but IMAX DTS:X is nothing if not a conventional format!
Though it hopefully goes without saying, this is not the EQ profile you should run with other formats, as propping up the low-end response of the speakers with EQ will quickly blow them up. Use one EQ profile for normal content with normal crossovers and a second EQ profile for IMAX DTS:X content with IMAX bass management. As always, if you have doubts about what you’re doing, ask first.
Finally, IMAX DTS:X has made the somewhat curious choice to go 5.1.4 channel for the streaming format, even though IMAX 12-track has a 7-channel base layer and the lossless IMAX DTS:X format will render to a 7-channel base layer. I haven’t received an explanation for this, but I would guess it has something to do with keeping the bitrate reasonably low. DTS:X for Streaming reportedly runs at a typical bitrate of 448 kbps.
By comparison, Apple, Netflix, Disney+, and Movies Anywhere stream Dolby E-AC-3 JOC, the codec behind Atmos, at 640–768 kbps, while other providers like Fandango at Home — formerly VUDU — and Prime Video knock that down to 448-575 kbps. Note that bitrates do not translate to sound quality when comparing two different codecs. They are only relevant for providers in determining how much bandwidth is required for each stream.
According to multiple sources, including our DTS contact, the IMAX DTS:X 5.1.4 format will not use the Side speakers in the standard Dolby configuration; rather the two surround channels will be routed to the Back speakers. The Back speakers were chosen because, in IMAX’s estimation, they more closely simulate the presentation of the two point-source surround speakers in the back corners of an IMAX cinema. However, as with bass management, IMAX DTS:X mode can reportedly be disabled in favor of normal DTS:X, which will allow DTS Neural:X to upmix 5.1.4 to the available speakers. Do cut me some slack with the explanation, here, because the routing of DTS:X for Streaming is new, and we haven’t fully fleshed out how it works on a wide range of decoders. These things take time!
Get Back
The salient point is that you may face a situation where only the Back speakers are used for IMAX DTS:X 5.1.4. Thus, their positioning and coverage become somewhat more important than normal. If you are typically inclined to put the Back speakers toward the 150-degree limit of the Dolby guideline, then you might want to consider backing them down to the lower limit of around 135 degrees (i.e., closer to the corners). Of course, assuming you are doing multi-seat and multi-row, you will need to adapt that to what makes sense in the context of the entire seating area. The thing to guard against in this scenario is reversal of the sound image — a phenomenon where sounds that are supposed to image behind your head in fact invert and appear to come from in front of your head (or sometimes even inside your head, which can be quite unsettling).
I had quite a bit of experience with this early on while developing the Surround EX format, and it can be quite a serious problem with two speakers in the 135–150-degree range. The quick and easy way to check for this problem is to play 500 Hz to 2 kHz narrowband pink noise through both Back speakers, sit in various seats around the listening area, and make sure the sound image always comes from behind you. If it does invert, you have to move the speakers. Take that into account when you’re designing the back wall construction, speaker mounting points, and wire drops.
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We will likely need to take another look at some of the technical details of the IMAX DTS:X format (I haven’t covered everything we already know due to space) as it becomes widely available, but for now I think the focus should be on the marketing side — leveraging the IMAX name and reputation to bring customers back in to witness the premium experience only a high-end dedicated home theater can deliver.
When you get the chance, spread the word and let’s do all we can to encourage IMAX, DTS, and their hardware partners to bring DTS:X for Streaming support to streaming devices!
Anthony Grimani is co-founder of Grimani Systems loudspeakers (www.grimanisystems.com), and President of PMI Engineering (www.pmiltd.com) and MSR Acoustics (www.msr-inc.com). Chase Walton contributed to this column.
