hi, just saw this post - maybe my inputs will be helpful. or maybe they'll just open up a can of worms!
IMO, THX is fraught with much more problems than it aim's to solve - on both, a practical and a theorectical level.
...The basic problem with thx is the requirement for speakers. Because of the vertical directivity consideration (where speakers must radiate their sound energy into the room and not into the walls), all thx speakers must have 3 drivers for the satellites. viz 2 mids placed vertically over a central tweeter. This is commonly known as the D'appolito configuration or also an MTM (midrange - tweeter - midrange)
The idea is that, in such a configuration - the speaker acts as a point source and all the sound appears to come from the centre of the cabinet viz the tweeter. This configuration also provides vertical directivity by increasing the energy in the frontal plane of the radiation, thus reducing floor and ceiling reflections.
While this confiuration is not bad in itself, the issues that they throw up are tremendous. For the configuration to work correctly, the speakers have to be centred at the ear level of the listener and all the 3 speakers have to be placed at the same height. these 2 conditions can never be met in a typical home - for the simple reason that the centre speaker has to be placed above or below the tv. In a theatre, this is possible because the 3 speakers are placed behind an acoustically transparent screen.
If the speakers are not placed at ear level, severe phase cancelations occur, due to 2 radiators (the 2 mids) giving the same sound but from asymettrical distances. In addition, all centre speakers in homes i've seen are placed horizontally - again a no-no for clean sound, but a standard placement constraint in all practical rooms.
These two factors itself kill the thx format in homes. also, these factors are never highlighted by thx themselves - because then their system becomes impractical.
How many people with thx speakers place the left right centre at the same level, at ear level, and then place them all vertically? No one! Then, bye bye thx, hello phase cancellations and bad sound!
Dipole surrounds suffer the same fate - to be used accurately, the listener has to be placed optimally in the null radiating field of the dipole. For more than 1 listener, this is impossible unless all listeners are seated in the same horizontal line! That's why all dipole speakers today come with a dipole off switch...so that they can act as monopole or point sources for 'practical' placement.
The concept of 7.1 thx takes these absurdities to a higher plane!
Remember, creating a sonic illusion is what good sound should be all about. Visually, to see an illusion, it means that an image appears to be floating around in the air - not created by anything practical that we know of, like a tv etc. Seeing a 3 dimensional image from a 2 dimensional plane is also an illusion - because it is beyond the practical.
The same with sound - if we hear sound from a position where a speaker is not present - that's a sonic illusion. if sound comes from the same spot that the speaker is in - that's electronics. So, the more the speakers, the less the possibility of a sonic illusion.
Besides -with all the practical constraints of home listening environments, getting 5 speakers to be placed correctly is a far cry, let alone 7 speakers being optimised for placement in terms of position, phase, delay, level etc - it's almost impossible. a sonic illusion is even more impossible with a 7.1 system than with a 5.1 system.
At the end, good sound is about an immersive iilusion. If a scene in a movie has the sound of rain, obviously, the engineers at dolby will send the sound to all the speakers and the rain will come appear to be all around you - because the speakers are placed all around you. But when you hear the rain coming from the ceiling, you feel the movement of the clouds overhead and you almost see the spots of wetness in your room, when you shove your arms together and huddle up because of this rain sequence, that's an illusion.
If a scene like this makes you want to open up your umbrella as you continue to sit on your sofa - that's what good sound is all about - if not, you're missing out on the real 'theatre experience'!
Any more coments about the accuracy of the thx format (even when all speakers are actually placed and configured correctly) is a topic for another day!
As it stands today - stereo (2 speakers) is the ultimate illusion, because it effectively takes into account (perhaps by fluke even), the psychology of human hearing and the psycho-acoustic aspect of a human beings (2) ears!
cheers
