Originally Posted by Dan Hitchman
Atmos would be at least 7.1 since that's it's core bed (that or 9.1). There is no 5.1 Atmos. It's all in the speaker configuration in your room. If you only have 5.1 with no tops, then I don't see much of a benefit between Atmos and regular surround.
Technically there is no 7.1 or 9.1 Atmos either, since the top channels are counted separately from the basic channel bed speakers. Atmos requires at least two top channels and a channel bed of at least 5 speakers. The Onkyo receivers that feature Dolby Atmos support the following configurations...
5.1.2 (5 discrete channels for the channel bed, 1 discrete LFE channel, and 2 discrete top channels)
5.1.4 (5 discrete channels for the channel bed, 1 discrete LFE channel, and 4 discrete top channels)
7.1.2 (7 discrete channels for the channel bed, 1 discrete LFE channel, and 2 discrete top channels)
7.1.4 (7 discrete channels for the channel bed, 1 discrete LFE channel, and 4 discrete top channels)
9.1.2 (9 discrete channels for the channel bed, 1 discrete LFE channel, and 2 discrete top channels)
Keep in mind that the above are channel configurations only, not soundtrack formats. AFAIK
, all Dolby Atmos soundtracks can actually contain information for more than 12 discrete channels. The cinema version can support up to 64 discrete channels.
If you do not use any top channels then you aren't really utilizing the object-based capabilities of Atmos. I suspect that if your receiver does not detect top channels (or if you manually turn them off in the settings) then it will just default to the standard Dolby TrueHD soundtrack, utilizing whatever speakers you do have connected.
If the intent of the original question was to ask what sounds better between a 5.1.2 setup and a standard 7.1 setup when playing a movie with a Dolby Atmos soundtrack then the answer would usually be the 5.1.2 setup. Again, this isn't a question of quality since the quality of the sound coming from each speaker would be identical. It's a question of immersion. The lack of the rear surrounds in the 5.1.2 setup can be compensated for to an extent, by playing a sound from both the left and right surround speaker to give you the impression that the sound is coming from behind. The effect might not be as convincing as it would if you actually had rear surrounds, but it can be simulated. On the other hand, a standard 7.1 setup cannot simulate the effect of sound coming from above at all.