DTS vs Digital Surround vs. THX

there is less compression with dts than dolby digtal surround. either format(by title)can buy a thx license.

I guess that the DTS format is a better choice to watch videos, than dolby digital. THX adds a depth to audio within the video. How can they get multiple formats onto a DVD, I have some that have both or even all 3 formats. Is there such a place to purchase original DTS format DVDs?

The three formats in your thread are about sound. Any improvements in picture would be unrelated to the surround format.
THX is a format pioneered by Tomlinson Holman under George Lucas. In fact the film THX 1138 was sort of an inside joke in this regard. Lucas wanted the story to be on the screen with accurate steering logic and the surround chanels being diffuse and not really distracting from the story on the screen. Instead, enveloping the audience in an "atmosphere". Also limitations at the time did not permit discrete sides and rears. Left and Right with a discrete Center elevated surround to prologic status with center channel steering a significant improvement over previous surround sound.
Dolby Digital Surround became an evolutionary step beyond prologic when it became possible to encode a digital soundtrack on the space between the sprocket holes in movie film. An optical reader read the information on the film. Previously, soundtracks were linear strips next to the film frames and had limited fidelity and physical space.
DTS took digital a step further by removing the soundtrack from film and placing it on a CD disc that synced with a signal on the film. With a lot more space to place a soundtrack compression became less important.
THX film soundtracks were mixed with the idea in mind that the action and the audience's attention were on the screen. Emphasis was on the placement of characters and steering of dialogue on that 2D surface. Surrounds were not full range. Dolby Digital soundtracks have a lot more going on since the mixing engineers have discrete sides to play with. The sides are usually mixed with the understanding that dipole side channels will be used. Film being different in this way over DVD Audio or SACD. These formats can be point source in each channel and are mixed accordingly.
So what's best? It can depend on how you have your theater set up. In real world conditions, you may hear very little difference between DD and DTS. Certainly a film mixed in THX won't be consistant with the films intended performance unless you play it using a THX processor. And early DVD machines cannot play a DTS encoded disc since the tracks aren't read the same way by the laser. Most if not all newer machines are compatible with all three formats.
So digital made film soundtracks tremendously more realistic and much more like the theater experience.
One thing to note, film soundtracks usually are not full range in the surround channels, while DVD-A and SACD can be. And bass management in digital formats is far superior to early THX films.

Thank You. I have a 7.1 setup, Denon 3910 DVD having the capabilities of all 3 types but do lack not having a THX processor. I was more curious when I select the formats types during playback either DTS or DD.

Alp, some basics not already covered:
1. A Dolby Digital soundtrack is required on DVD-Videos by terms of the license. It can contain from 1 thru 6 channels.
2. DTS is about the same as Dolby Digital 5.1 (= 6 channels) but the sound data are compressed much less, and generally it will sound better than the DD track. Understand that the DTS track is mastered differently, so that's another reason it sounds different. Also, DTS tracks are always 5.1; they simply don't do anything less (that I've seen).
3. The master recordings are done, usually, at CD rates, that is 44.1KHz and 16 bits, so all of this can't ever sound better than what one gets from a CD.
4. DTS soundtracks appear on lots of DVD-Vs; you just have to look for them. I have an Excel spreadsheet of my about-400 DVDs that identifies those with DTS soundtracks (about 50). I'll sort it by that field and send it to you if you'll e-mail me at jeffreybehr(at)cox(dot)net.
.