Downmixing - How can you tell?

[spaceboy710]

Hey everyone!

I've been seeing lots of mentioning of tracks being "downmixed" from the 5.1 to 2.0 stereo or mono. 

How are people able to tell it is a downmix as opposed to a separate mono mix? Is there an analysis tool people are using? Just very curious.

[Stamper]

One way to tell is by hearing some sort of whooshing over the track, as usually happens when mono is expanded to 5.1 artificially, then downmixed back (you can even hear it listening to the 5.1 in stereo).

[spoRv]

I guess, in theory, that downmixing 4.x or 5.x to stereo, upmixing back with Dolby Surround/Pro Logic should, always in theory, result in some residual center and/or surround properly placed.

[BDgeek]

I think the most usual, fail safe and very empirical way of identifying a downmix is detecting new or missing foley effects (wooshing included) and/or music cues on the track.

Remixes usually incorporate new sound effects and in many cases some original effects and music cues are forgotter or left inaudible.
That's the case with a lot of famous remixes like Jaws, The Terminator, Conan.

Even recent 5.1 movies when remixed to atmos have these collaterals. For instance, Sony's Spiderman (2002) Atmos has missing foley.