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Home Theaters and THX

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NY_EE

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Post Fri Mar 20, 2009 10:32 pm

Home Theaters and THX

http://www.avguide.com/how-to/home-theater-audio
October 30th, 2008 -- by Robert Harley
Source: The Perfect Vision

Home THX is a set of technologies and performance standards established by Lucasfilm that better translates film soundtracks mixed for the movie theater to playback in your home. THX is not a surround-sound format such as Dolby Digital or DTS. Rather, THX works in conjunction with Dolby Digital or DTS to improve the home-theater experience.

A THX Certified A/V receiver or controller incorporates four THX-developed signal-processing technologies, as well as meets a set of performance standards. (THX Certified power amplifiers and speakers don’t include this signal processing, but must conform to technical standards set by Lucasfilm.)

The four THX processing technologies are called reequalization, timbre matching, surround decorrelation, and subwoofer crossover. Let’s look at each of these individually.

Re-equalization is a treble roll-off (cut) applied in your receiver or controller when THX mode is engaged. Reducing the amount of treble in the soundtrack during home playback restores the tonal balance you’d hear in a movie theater. Films are mixed with extra treble energy to compensate for the fairly dead acoustics of a movie theater. In addition, some treble is lost in the relatively large distance between the speakers and audience (high frequencies suffer greater attenuation with distance than low frequencies). The result is that these bright soundtracks sound just right in a theater, but when reproduced in your home, are far too bright.

Determining the re-equalization circuit’s characteristics (how much to reduce the treble, and at what frequency) was solved in an ingenious way. THX inventor Tomlinson Holman (THX is an acronym for “Tom Holman’s eXperiment”) played film soundtracks on a home-theater system for the engineers who originally mixed them. The engineers were asked to adjust an equalizer in front of them until the sound they heard over the home-theater system sounded like what they remembered hearing on the dubbing stage. Holman took note of the equalizer’s settings. This process was repeated with many mixers, who made nearly identical changes to the equalizer. Holman used this information to create and patent the THX “reequalization curve,” which removes just the right amount of brightness from film soundtracks for naturalsounding home-theater playback.

The next THX technology, surround decorrelation, attempts to make Dolby Surround’s monophonic surround channel less monophonic. The process slightly changes the sound (specifically, the time and/or phase between the signals in the midrange and treble frequencies) in the left and right surround channels. This difference between left and right surrounds prevents the “inside the head” localization of surround signals that can occur between two loudspeakers reproducing the same signal.

Surround decorrelation produces greater ambience, spaciousness, diffusion, and envelopment from the surround speakers. Surround decorrelation is unnecessary with those discrete 5.1-channel Dolby Digital and DTS soundtracks that have completely independent left and right surround channels. Keep in mind, however, that many DVDs containing a 5.1-channel Dolby Digital soundtrack still have monophonic surround channels. If the original source had a monophonic surround channel (films mixed in the pre- Dolby-Digital era), the Dolby- Digital-encoded soundtrack will contain identical signals in both surround channels. In this case, the “inside the head” localization problems mentioned earlier will persist.

THX’s timbre matching circuit compensates for the differences in the way we perceive timbre between sounds arriving from the front and the sides. Try this experiment: snap your fingers in front of your head and then again at the side of your head. The finger-snap’s timbre is “sharper” when arriving from the side. THX’s timbre matching ensures that, as sounds move from in front of to behind the listener (or vice versa), their timbres remain constant.

Finally, the THX subwoofer crossover separates bass frequencies from the midrange and treble. The bass is reproduced by a subwoofer and the midrange and treble by the main loudspeakers. Although all AVRs and controllers contain a crossover, the THX crossover is standardized with regard to frequency (80Hz) and steepness of roll-off (24dB/octave low-pass and 12dB/octave high-pass).

In addition to incorporating those four core signal-processing technologies, a THX Certified receiver must meet certain performance criteria, including output power, dynamic headroom (the ability to reproduce peaks without distortion), noise levels, and the ability to drive lowimpedance loudspeakers, among other factors. (Specifically, a THX Certified receiver must be able to deliver a minimum of 211Wpc into a 3.2-ohm load.)

Products with the THX Select designation incorporate the four technologies described above, but have somewhat relaxed performance standards with regard to output power. This allows lower-priced receivers to employ THX processing. The original THX certification is now called THX Ultra to differentiate it from THX Select.

Finally, the relatively new THX Ultra2 designation is a combination of new performance criteria for the video-switching circuits, as well as new signal-processing algorithms for creating 7.1-channel playback from 5.1-channel sources. THX Ultra2 Certified receivers contain sevenchannels of amplification, as well as a Boundary Gain Compensation circuit to reduce boominess caused by speakers being placed close to walls. In my experience, THX improves the sound quality of films and is worth having in a receiver or controller
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NY_EE

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Post Fri Mar 20, 2009 10:48 pm

Re: Home Theaters and THX

From the manual for my Audio Design Associates "Cinema Rhapsody" Preamp/Surround Processor. This tells me that the only crossover point for output to the subwoofer is the THX standard, which is 80hz. Am I missing something?

When setting the Speaker System Configuration, you are in effect, determining how the bass is managed through your speaker system. The Cinema Rhapsody is very flexible when it comes to this setup. For those systems that utilize a THX certified speaker system, there will be no need to go through the elaborate Speaker System Configuration setup. However, if you are using speakers that are capable of full range operation or wish to have different bass options for different inputs, the Cinema Rhapsody is very flexible. The Cinema Rhapsody supports two options for Speaker System Configuration. The easiest option (SPKR SYS ALL) permits you to setup the speaker configuration for all inputs in one easy step. The second option (SPKR SYS IND), permits you setup a different
speaker configuration for each input. It is worth noting at this time, if are planning to set individual inputs to their own speaker configuration and then choose to access the SPKR SYS ALL configuration option, you will immediately reset all previous individual speaker settings to what ever configuration first comes up when entering the more automatic option. As such, do not enter the SPKR SYS ALL option if you have already adjusted the settings for particular inputs, unless of course, you wish to reset the previously selected individual settings. The chart below details the various options available. Please note, that the “0” option is the THX standard. It is also the Cinema Rhapsody’s default speaker setup for all inputs. To change the Speaker System Configuration for all inputs:

1 While in the PRO SETUP mode, turn the MODE knob until the display reads SPKR SYS ALL, then press the MODE knob. The display will read BASS CONFIG on the top line and the second line will read the ALL CONFIG #, where the number indicates the current setting for all inputs.

2 Turn the MODE knob until the desired Speaker System Configuration is displayed. To exit this setting, press the MODE knob (to return to the PRO SETUP mode) or turn the VOLUME knob (to exit the SETUP mode entirely). Pro Setup - Speaker System Setup - Bass Management

The Cinema Rhapsody’s “Out Of The Box” Setup is defaulted to the THX standard where the High Pass Filter is active on all speakers with their subwoofer information summed to with the LFE channel (Setting 0). If you are using full range speakers in your home theater system, you can alter the speaker system configuration aspect of the Cinema Rhapsody. Please note, that you have two options when setting up the Cinema Rhapsody for your speaker system. The first
option permits you to setup the Bass Configuration setting for all inputs in one easy step. The other option permits you to set the speaker system configuration, individually for each input. If you are using full range speakers (speakers capable of handling bass information) either for all five main speakers or just some of the five main speakers, you will need to use the chart to determine which Bass Configuration # best applies to your speaker array.
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NY_EE

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Post Fri Mar 20, 2009 10:52 pm

Re: Home Theaters and THX

More from the Cinema Rhapsody Manual - basically the same as seen above.

THX is an exclusive set of standards and technologies established to make your experience of the film soundtrack as faithful as possible to what the director intended. Movie soundtracks are mixed in special movie theaters called dubbing stages and are designed to be played back in movie theaters with similar equipment and conditions. The soundtrack created for the movie theater is then transferred directly onto Laserdisc, VHS tape, DVD, etc., and is not changed for playback in a small home theater environment. THX engineers developed patented technologies to accurately translate the sound from the movie theater environment into the home, correcting
the tonal and spatial errors that occur. When the THX mode is on, the following three THX technologies are automatically added after the decoded signal:

Re-Equalization - The tonal balance of a film soundtrack will be excessively bright and harsh when played back over audio equipment in the home because the film soundtracks were designed to
be played back in large movie theatres using very different professional equipment. Re-Equalization restores the correct tonal balance for watching a movie soundtrack in a small home environment.

Timbre Matching - The human ear changes our perception of a sound depending on the direction from which the sound is coming. In a movie theater, there is an array of surround speakers so that the surround information is all around you. In a home theater, you use only two speakers located to the side of your head. The Timbre Matching feature filters the information going to the surround speakers so that they more closely match the tonal characteristics of the sound coming from the front and surround speakers.

Adaptive Decorrelation - In a movie theater, a large number of surround speakers help create an enveloping surround sound experience, but in a home theater there are usually, only two speakers. This can make the surround speakers sound like headphones that lack spaciousness and envelopment. The surround sounds collapse into the closest speaker as you move away from the middle seating position. Adaptive Decorrelation slightly changes on surround channel’s time and phase relationship with respect to the other surround channel. This expands the listening position and creates--with only two speakers--the same spacious surround experience as in a movie theater.

When you select the AC-3 THX, DTS THX, or PROLOGIC THX modes, all three enhancements are active (On). When you select the AC-3 RE-EQ, DTS RE-EQ, or PROLOG RE-EQ modes, the reequalizer is active (On) and the Timbre Match and Decorrelation filters are off.
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Rainman

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Post Fri Mar 27, 2009 6:36 pm

Re: Home Theaters and THX

I recall the THX subject being brought up at Carverfest. As I remember Mr. Carver told us the only difference in his amplifers such as the TFM35 and the TFM35x is only one simple little resistor was added to the X series amps.

Rainman
Question: Do you know why turds are tapered on the ends? Answer: So your asshole doesn't slam shut..

Carver- SDA490t, SDA450, 2 TFM45's, TFM15, CT23, CT28v, CT27v, Linn Kaber speakers, Sunfire crm2's, PS Audio Statement speaker cables and I/C's, A.P.C. power
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OBI56

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Post Fri Mar 27, 2009 7:43 pm

Re: Home Theaters and THX

Rainman wrote:I recall the THX subject being brought up at Carverfest. As I remember Mr. Carver told us the only difference in his amplifers such as the TFM35 and the TFM35x is only one simple little resistor was added to the X series amps.

Rainman


Rainman, I also read that but with a slight twist. The variable line input level pot was replaced by that one fixed resistor because the THX spec required that a specific input level was required to drive the amp to a specified output level (100W I think), but since the variable pot allowed that input level to deviate from the THX standard, it was removed.
Why let facts or common sense get in the way of your opinions.
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Rainman

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Post Fri Mar 27, 2009 8:07 pm

Re: Home Theaters and THX

OBI56 wrote:
Rainman wrote:I recall the THX subject being brought up at Carverfest. As I remember Mr. Carver told us the only difference in his amplifers such as the TFM35 and the TFM35x is only one simple little resistor was added to the X series amps.

Rainman


Rainman, I also read that but with a slight twist. The variable line input level pot was replaced by that one fixed resistor because the THX spec required that a specific input level was required to drive the amp to a specified output level (100W I think), but since the variable pot allowed that input level to deviate from the THX standard, it was removed.



OBI, once again you continue to amaze me! =D> You know details on this stuff like I know how to make Super Latemodels and "Taxicabs" turn better. :D

Rainman
Question: Do you know why turds are tapered on the ends? Answer: So your asshole doesn't slam shut..

Carver- SDA490t, SDA450, 2 TFM45's, TFM15, CT23, CT28v, CT27v, Linn Kaber speakers, Sunfire crm2's, PS Audio Statement speaker cables and I/C's, A.P.C. power
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BillD

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Post Fri Mar 27, 2009 9:12 pm

Re: Home Theaters and THX

Well, my A-500x has variable pots on the back, that need to be set to max for THX compliance. The requirement for amplifiers was 1v @ 1kHz input = 100 watts out to be THX compliant.

I think the treble rolloff is something that should not be done. I wouldn't buy a THX rated processor for just that reason.
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
_________________
Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer
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OBI56

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Post Fri Mar 27, 2009 9:58 pm

Re: Home Theaters and THX

Yes Bill, they did go back to adjustable pots with a calibration mark on later models, but for those 2 first THX models, the 35x and 55X, that was they way they did it as far as I could find out. Maybe the THX standard loosened up a bit as it got more and more popular. To be absolutely certain we would need ot find someone who was around back then or some sort of written documentation on this specific case.
Why let facts or common sense get in the way of your opinions.
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BillD

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Post Fri Mar 27, 2009 10:08 pm

Re: Home Theaters and THX

Well, I figured that with the pots all the way, they are essentially "removed" from the circuit when you are running THX.
It should sound like it isn't there!
There is a difference between hearing and listening...
Making life enjoyable through expensive electronics.
_________________
Carver: C-4000 & C-1 preamps, PSC-60 preamp/tuner, TX-11a tuner, M-400 (2), C-500, M-500, M-500t, M-500t Mk.II, A-500x, AL-III loudspeakers (2 pr.)
Sunfire:Theater Grand III processor, Ultimate Receiver, Cinema Grand Signature 400 ~ seven, True Subwoofer Mk. II, D-10 Subwoofer

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