THX option on my AVR

I understand that the THX certification on an AVR really doesn't mean much. I have a Yamaha RX-V1400 and it has a THX button on it. My understanding of this selection is that it takes the normal Dolby Digital signal and processes it a little differently. My question is does anybody know what it does differently? Does it color the sound in some way? Does it change the way the bass management works? It does sound a little different but I just can't figure out why or how it is different.

Thanks.

The main difference with the THX mode is the re-eq feature that filters down the high frequencies. This feature was created because movie soundtracks are purposely mixed with a house curve that boosts the highs as needed to fill the large space of a movie theater. If the original soundtrack got transferred to DVD directly without any re-eq to compensate for those boosted highs, then it might sound overly bright when played back at home. Apparently, newer DVD soundtracks already account for this, so the need for the re-eq feature has diminished. Unfortunately, there's no consistency in identifying which soundtracks benefit from the re-eq and which ones don't.

The THX mode also plays around with the phase relationships to try and create a more spacious and coherent soundfield, but that's not all that different in principle than Yamaha's own Cinema DSP modes. I would just go with the default 5.1 mode, and try the THX mode if a DVD soundtrack seems overly aggressive in the highs or if your speaker alignment creates gaps in the front-to-back soundfield.

Jeff,
Unless you have a THX certified speaker package this circuit is pretty useless. Here is a few reasons why it doesn't work on a non THX certified speaker system.

1. All main speakers in a THX certified speaker systems have certain roll off characteristic to comform with a THX certified subwoofer system. The crossover system is design to work with speakers that are down 3db of so at 80hz, and fall at a rapid rate below that. If you speakers do not conform, then it is very likely you will not get a smooth frequency response around the crossover point.

2. THX requires that the front three speakers are vertically oriented(or at least the tweeter and midrange should be in the new certification), have a tight frequency response over a specified horizontal and vertical window, certain directional characteristics, and a 80hz cutoff at 24dbpo. It requires that the surrrounds be of dipolar nature, but maintain a flat power response over all of its range(requires a certain type of driver alignment in the bass) and down to 80hz. The decorrolation curcuits work with the nature of the dipole speaker to break up whatever phase relationships that may cause the rear channels to become directional. If you speakers are not dipole, then you will hear the decorrolation curcuits at work, and the sound will be very phasey sounding.

3. The timbre matching curcuits are designed to mate two dissimular speaker dispersion patterns for an even sound from front to back. If your speakers are not dipoles and are monopoles, there will be a dulling of the sound as it moves from front to back. The back speakers have a slightly different frequency response than the fronts, and the curcuits compensate for that. Considering that a entire THX system must be equalized as an overall package, you are appling two layers of equalization on the rear channels. This can't be good for sound, even on a THX speaker system.

The very premise of trying to make 5 speakers and a subwoofer to sound like a dubbing stage is impossible from its inception. There is no way to recreate the space of a large room to the space of a small room. The different room volume makes this impossible. There is no way to make two speakers sound like twelve or sixteen no matter how much decorrolation curcuitry you try.

This is why that little button is pretty useless to non THX spreaker owners.

Thanks to both of you. That makes sense. I am sure my speakers are not certified and I know my subwoofer isn't because I made it.