So here's my question:

Say you have a 6.1 track, DTS-ES or something, but you only have a 5.1 receiver. What happens with the channel containing the rear center information. Does the 5.1 receiver read the signal and play it over both the left and right surround to create a pseudo-rearcenter or does the information contained in that signal just not get played at all?

If the information does not get played, then it would be ideal to choose the DD 5.1 track if available then becuase you'd be getting all of the sound information encoded in a way setup for 5.1.

Thoughts???

I think if you have a 5.1 decoder you get the 6.1 soundtrack in regular 5.1.

6.1 is a bit of a misnomer since the 7th channel is not discrete. In Dolby 6.1, the 7th channel (rear center) information is matrixed; i.e. encoded on the rear R/L channels. When a 6.1 soundtrack is played back on a 5.1 system, no information is lost—the rear center sound is directed to the rear R/L channels. Some receivers, Sony for example, have a sound field that attempts to create a virtual rear center channel when a physical speaker is not present.

DTS ES Discrete is a true discrete 6.1 format although there are very few DVDs with this soundtrack.

You are, of course, correct and I should have included that exception in my comment.

What happens when decoding a DTS 6.1 ES track on a DTS 5.1 receiver?

(Fellowship of the Ring: Extended Edition is an example of a DTS 6.1 DVD.)

No information is lost; it is played via the rear L/R channels. The format is 100% backward compatible.

Yeap!