ATMOS has many complex issues that are hard to convey quickly and simply. The main idea I have always had, even from the time I visited Dolby to discuss the ATC systems they bought to design ATMOS on, was the important first step to three dimensional audio that could actually work with available delivery systems someone might be able to own at home. Front to back (5.1) and left to right (stereo) we already have; Up and down is missing and is a very important part of 3 dimensions.
The center channel is 99% dedicated to dialogue with movies but on ATMOS music (readily available on blu ray and others) the mixer can use the center for whatever they want (and they do).
The overheads are designed to be aimed at the mix position only, not the floor. In your home you would aim them at your listening position. 2 in front of you L/R and 2 behind you L/R. This would bring up the same dispersion issues your main speakers face when addressing your listening position. Wide dispersion would be an advantage and speaker position would be important to consider. You can’t just throw them up there anywhere.
There is a variant of ATMOS "ready" speakers that uses a specially filtered speaker to bounce audio off the ceiling. You can recognize these as they are angled and are usually designed to be placed on the main L/R. IMHO this method cannot get you to the same place as overheads and no one uses this method to build ATMOS mixes I am aware of. But in fairness, if you don’t want to cut holes in your ceiling it’s a possibility that does work better than nothing. The workaround I prefer is full range direct radiators (regular speakers, preferably the same brand/model as the sides and rears) high on the walls aimed down to your listening position. Ive tried 2 speakers L/R high on the front wall and 2 L/R high on the back wall; Ive tried 2 speakers (front and back) high on the two side walls. The same 20x15x9 typical home listening room was used in both. In all cases you have to locate the speakers so your listening position is within the dispersion pattern of the speaker. Also know that boundary mounting will boost bass and ceiling speakers mounted flush do not have this issue. If you mount these height speakers on front/back or sidewalls, pushed up to the corner of wall and ceiling, you can boost bass as much as 6dB and this can really mess with EQ. (not a lot of bass used in ambience) You can also get in trouble by mounting a speaker designed for vertical use and rotating it to horizontal use as a height speaker because now the designed horizontal dispersion is now rotated 90 degrees to be vertical dispersion. You don’t want wide vertical due to excessive reflections off ceilings and walls. Since so much of what is placed in the overhead channels is ambience/space/motion there is a lot of high end in it and you won’t get the effect if you are missing top end via narrow dispersion.
In music, anything can be in the overheads, as Giles Martin showed us in his expert ATMOS mixes of Beatles and others. He is the best! Giles’ ATMOS systems are ATC SCM100s at Abbey Road. Same speaker in every channel. Using verticals can be very effective in giving the mix a sense of height instead of this flat two dimensional plane of audio you get in 5.1. {Phase tricks and speakers that are very good at imaging are not enough compared to actually placing audio overhead). Some mixers are using overheads for exciting artist driven effects such as motion or isolated events previously buried are now [surprise!] overhead. The prince "When Doves Cry" Atmos mix is like that. It was done at Blackbird in Nashville on a big 9.1.4 ATC rig they installed the day before. Im not sure I like it used like that but every artist has their own idea of what they want. I think Giles use of vertical playback is tasteful and what I imagine they might have done if they could have when recorded. But I digress.....
There is also an issue as to the location of side speakers. In movies, ATMOS is used to convey motion- front to back is the most dramatic and important. 5.1 can do this and has been used since the first Jurassic Park 5.1. Locating the side speakers in the right locations is important (first side speaker cannot be too far back or it fails to give you that smooth image without major gaps from Mains to side to rears).
ATMOS is very room dependent (as are all speakers), so planning is important. Object based does allow the processor to fold down all the info into whatever speaker arrangement you select on the processor, but that doesn’t mean it gives you the same effect as the discrete channel 7/9/11 output. 5.1 for example, has no up and down (vertical) component and 7/9/11 all do.
ATMOS can be stunning done properly on a good, properly done system. But as all playback systems, there are many issues with what gear, how it’s installed, what the room is like etc. Some of the naysayers of ATMOS I suspect have never heard it done properly (which admittedly is rare in consumer audio world). I’m lucky to have heard it right, but I’ve also heard it be awful on certain playback systems, methods (Apple Spatial does have some real phase weirdness on earbuds) and rooms. Ive spoken to mixers doing important legacy mixes who have been frustrated by the software, but Ive also heard some of these figure out how to use and develop spectacular ATMOS mixes. I have heard from several of them the Apple Max headphones are a lot better than any other ATMOS headphone/earbud.
So I always go back to the three dimensional idea: if we could have 3 dimensional audio in all audio, that would be ideal as this is the way real sound works. ATMOS isn’t perfect, but it’s a lot closer to the target of 3D than anything that’s come before and can actually be delivered and mixed/produced. Having been involved in the SoundField B Format mic, Ive been chasing this 3D audio creation thing for a while. It’s a very big challenge and every attempt so far has failed either in engineering flaws or in the inability to execute on playback. ATMOS is a step forward in my view and despite its challenges, it’s worth implementing if you think you can do it right. Some of the source material is great, some not, just like stereo.
Brad