watch native TrueHd/DTS-MSTR or matrix?

If you have a 5.1 setup, can you even apply THX Ultra2 to 5.1 source material? What would it do?

Also, you wouldn't be using it instead of the native bitstream. The bitstream must be decoded back into PCM. Then, the DSP is applied.

Back to your original question - there's no sound degradation using a DSP like THX Ultra 2, PLIIx, or Logic 7. But, the audio does get changed, of course, to reroute some of the surround audio to your rear speakers. I don't have a 7.1 system myself, but from what I've read, 7.1 matrixing is pretty much a hit with those who do.

If you are talking about the THX Ultra 2 surround mode to create 7.1 channels from a 5.1 source (and not just Re-EQ, decorrelation, etc.), keep in mind that the Ultra 2 surround mode creates only a decorrelated mono channel in the back surrounds. THX recommends that the back surrounds be close together when using this mode.

DPLIIX, on the other hand, will create stereo back surround channels and the speakers should be spaced apart.

I have an Onkyo 706 with THX Select 2 Surround. Same thing as the Ultra 2 Surround. When I compared it to DPLIIX, DPLIIX was the clear winner. Better separation, better ambience, clearer sound.

Yes, you can apply THX post-processing to 5.1-channel material being played back on a 5.1-speaker set-up. It will add the usual THX enhancements: Re-EQ, Timbre Matching, Adaptive Decorrelation, Boundary Gain Compensation, etc. On newer receivers, there's also the option to apply THX Loudness Plus (similar to Dolby Volume and Audyssey Dynamic EQ/Volume).

THX Ultra 2 only available when you have 7 speakers hooked up. So the answer is no, you can't apply Ultra 2. You could use like "THX movie".


When you're using THX post processing the signal is already decoded and converted to PCM to have your BM and TA to work, so there isn't much of a difference besides what THX post processing will bring to the table.

One approach is to just pick the software you like best!
On the other hand, it makes sense to stick with the software that best matches your setup. There's definitely some variation between Dolby and THX. If you have a full THX system, with compliant speakers in the specified spots, then it makes sense to always apply THX processing.