I'll kick this off. THX is more of a certification than an actual processing mode. Put that off to one side.
Dolby Digital 5.1, DTS 5.1
For DVDs there are two 5.1 standards -- Dolby Digital (DD) and DTS. All DVDs must include Dolby Digital although it can be 2 channel instead of 5.1 (surround). DTS is optional. You choose the standard via DVD menu (the DVD, not the DVD player) and the receiver switches automatically to the right mode.
If the DVD you are playing has both give DTS a try, some think DTS sounds a bit better. Both DD and DTS take 5.1 data from the DVD and play it on 5.1 speakers, ie no expansion from 2 to 5 channels etc...
I'll talk about DD 2.0 later.
Dolby Digital EX, DTS-ES (>5.1)
Dolby Digital and DTS both have new modes to support 5.1 and 7.1. I think Dolby EX is the 6.1 DD and DTS-ES is the corresponding DTS 6.1. The receiver will auto-switch into these modes but I think you have to enable them first on a Yamaha. I'm pretty sure it's the same surround "modes" which can take 5.1 content and process it to 6.1 or 7.1.
General consensus seems to be that a good 5.1 system will be better than cutting any corners to get 6.1/7.1, but a 6.1/7.1 system can sound a bit better than 5.1 but you need lots of space around/behind your listening position to get the benefit.
OK, so far we've talked about playing 5.1 DVDs on 5.1 speakers (DD / DTS), 6.1 DVDs on 6.1 or 7.1 speakers (DD-EX, DTS-ES) and 5.1 DVDs on 6.1 or 7.1 speakers (same).
Dolby Pro-Logic II (expanding 2 channels to 5)
Dolby Pro-Logic (PL) is an older surround processing mode, pretty much replaced by PL II (and, I assume, PL IIx). It takes two channel inputs from CD, DVD (the 2 channel track), FM, tape, phone etc..) and synthesizes 5.1 channels by processing the two channels against each other.
Multi-channel sound tracks can be "processed down" to matrix more channels onto a 2 channel recording; most DVDs with 2 channel soundtracks are processed this way and the hidden info can be extracted via PL II Movie mode.
PL II Music mode is intended for stereo input which does NOT have multiple channels encoded down to two channel. In other words, most 2 channel DVDs should be heard using PL II Movie while everything else should be heard using PL II Music.
Logic 7 is HK's proprietary equivalent to PL II Music but was introduced quite a bit earlier. DTS Neo6 is the DTS equivalent. In other words, PL II Music, Logic 7, and Neo6 are all interchangeable. Most people have a slight preference for one of the three but the difference seems to be partly a function of how loud your center channel is set
Again, all of these modes take two channels on DVD or other media and expand them out to 5.1. I imagine you can also go 2 channel to 6.1 or 7.1 but I don't know which mode would do that.
Bottom line :
- you have lots of choices with 2 channel input. Use DD PL II Movie for most DVDs; use DD PL II Music for all other 2 channel input. Try Neo6 as an alternative to PL II Music and use whichever you prefer. If you readjust speaker levels do the Neo6/PLII Music compare again.
- you don't have a lot of choices with 5.1/6.1. Whatever the disk is recorded in is what you play in, but many DVDs have the option of more than one surround mode, selected via DVD menu.
- don't rush into 6.1/7.1. Make your 5.1 system as good as you want first. If your room is too small don't even bother
- if you buy 6.1/7.1 speakers and you don't seem to be getting back channels check to see if you have to enable something on the receiver via remote. If I remember correctly it's one of those stupid "only on the remote" things.
- if the surround modes sound crappy with 2 channel content (CDs) then your speakers may not be matched properly, either different brands with different sounds or the setup levels are not right
- and finally, the listening room probably makes more difference than all this stuff combined. Make the room sound good in stereo first (placement, wall coverings, carpets, etc...) before you start looking for improvements via surround modes.
Enjoy !!
