Originally Posted by bsoko2
Why then is THX spec'd at 80 hz for the LFE?
THX is not "...spec'd at 80 Hz for the LFE". You might be confusing this with the THX crossover, which is spec'd at 80 Hz. However, this is completely different than the LPF of LFE.
The LPF of LFE and the THX crossover, (or any Bass Management crossover), are essentially unrelated. The Bass Management *crossover* is the combination of the HPF that is applied to the main channels that are set to "Small", (or in some Bass Management nomenclatures "have a crossover invoked"), and the LPF that is applied to the re-directed bass from those channels. The re-directed bass is then combined with the LFE channel, and the combined signal is sent to the subwoofer output. The LFE channel has been separately filtered by the LPF of LFE before being combined with the re-directed bass from the main channels. The LPF of LFE is not a "crossover" per se
. It is *just* a Low Pass Filter applied *only* to the LFE channel. It should always be set to 120 Hz.
Only newer receivers even have a LPF of LFE. Before the advent of BluRay with Dolby TrueHD and DTS-MA, the LFE channel was filtered during the recording process at 120 Hz. There was no need to re-filter it on playback. However, TrueHD and DTS-MA are "full-range" in all channels, including the LFE channel. Therefore, there is the possibility of some content in the LFE channel above 120 Hz. Hence, the need for an LPF of LFE on receivers/pre/pro's capable od decoding TrueHD and DTS-MA.
Most recording engineers don't place any content above 120 Hz in the LFE channel, (and , in fact, not much above 80 Hz), so it's not a huge issue either way. However, to ensure you don't "lose" any LFE content, the LPF of LFE should always be set to 120 Hz.
Some thoughts on the THX crossover:
The THX crossover is a very unique crossover designed to work specifically with THX speakers and sub. The HPF applied to the speakers is a 2nd Order, (12 dB/Octave roll-off) high pass. THX speakers are spec'd to have a -3 dB point of 80 Hz with a complimentary 2nd Order roll off. Add the two roll-offs together and you get a 4th Order, (24 dB/Octave) combined high pass roll off, with both starting at 80 Hz.
The THX LPF, (not the LPF of LFE, but the LPF applied to the re-directed bass), is spec'd at 24 dB/Octave, staring at 80 Hz. The combination of a 4th Order HPF and 4th Order LPF result in a perfect "Linkwitz-Riley" crossover with minimal phase issues.
The THX crossover is always included in THX receivers and pre/pro's, and it is always set at 80 Hz, with the HPF and LPF slopes pre-determined as above. However, in non-THX receivers and pre/pro's, unless the manufacturer specifies the crossover slopes, (and virtually none do), the slopes are unknown. They could be anything from 1st to 4th Oder, and they could be the same or different above or below the crossover point. In addition, the speakers being used with the crossover can vary in their low frequency roll off, as can the HF roll off of the sub. Whether or not the built-in crossover works "correctly* with the given speakers and subs is a total crap shoot.
The THX crossover plus THX speakers and subs is the *only* setup I'm aware of that is designed as a "system", where the components are designed to work together to yield a specific result. Say what you will about THX, their crossover is the best designed crossover "system" available, IMO.