Wow, this is fascinating - glad I stumbled on this thread. Thanks to our two pros for sharing their inside knowledge, and all for a lively discussion.
Glad to be of service; it also helps me clarify thoughts that otherwise rattle around in my head unchallenged!
This would have nothing to do with LFE though, right? This is just summation of LP'd content from the channels. However wouldn't Atmos include some inherent mitigation (pan law?) to reduce the disparity in the level of the full range channels if an object/channel was to be routed to multiuple channels based on how many speakers are defined in the decoder? Wouldn't that counteract this build-up?
Yes, it's primarily the LP'd content. But, if the LFE content were in phase with the LP'd content, you can see how the two would then sum more aggressively in BM than they do in room. In the initial cinema mix, we try not to duplicate too much content from the main channels in the LFE because we know this can cause issue down the road, but obviously there's always some sort of relationship phase-wise, and generally more positive than negative.
But no, there's no mitigation beyond the -3dB pan law. All the Atmos decoder does to render the "speaker" outputs from the objects is apply the age-old "Make the signal 3dB lower when sending to two speakers simultaneously" but it does so in 3D. The BM is after this panning, or "rendering", except in the case of Object Bass Management (earlier in this thread somewhere).
Is there always limiting happening in the HT side in the AVR? On the HT side I assume the BM happens after the signals exit the decoder, so it's really up to the implementation in the AVR (and the inherent limit of the speaker system I suppose) - wouldn't this be another stop betwen the 'chip' and the (even) wilder realm of room acoustics and calibration? I presume that you don't ride output limiters while mixing, so if a HT system had enough sub capacity, they should not experience this effect, right?
In Atmos, Dolby have built the limiters & BM built in on their side so it's not manufacturer specific. Like I say, knowledge of exact implementation in AVRs isn't my thing, but it occurs to me that even though the BM is part of the Atmos ecosystem, an AVR manufacturer
could set it to "OFF" and implement BM post Dolby. I'm not sure if Dolby's "built-in" limiters can be bypassed - I suspect not.
We normally mix with the Dolby spec limiters engaged in our monitoring chain, so we should be hearing the same result as in the consumer unit (for a given speaker config). This threw me to begin with because like, what!? "We're putting compression between our mixing desk output and our monitors!? You're kidding?" But since the implementation is the same at the consumer end, it makes some sense.