Confused: Dolby Surround on DD+ input?

[ypan07]

While watching DD+ input sources, my Denon receiver gives either DD+ or (DD+ and DolbySurround) as available sound modes. The latter sounds punchier and is the default option with Auto mode. It was my understanding that Dolby Surround is meant to upmix 2-channel audio to 5.1 and that begs the question: what could it possibly be doing on a DD+ signal? My setup is 5.1 anyway so what is there to upmix on a DD+ input signal? Any ideas? Which mode should be used?

 

[buxx]

the DSU replaces all other forms of legacy Dolby Prologic Mixers etc..
It should ideally be used only when you have Atmos speakers, it will then upmix 5.1 to add height effects.
Else you could just use DD+ option.