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[John Roberts {JR}]

Jaska Saarinen wrote on Wed, 08 April 2009 15:48

... forget to mention... THX don't have it's own levels... that's silliest thing what I have heard... THX is a quality standard, including how you should do your acoustics, sound isolation , viewing angles, etc....THX is not black box or a special system, it's just very well done system.... Antone wrote: "Your channel levels for THX are incorrect, as is your reason for the x-curve. Jaska below has things correct."

Please work on keeping your quotes straight.. actually it was Phillip Graham who wrote to Antone saying you were correct.

Yup THX was somehow involved in equipment blessing/approval for THX theater use. I recall dealing with them related to getting some amps approved for cinema use. Some of their requirements seems unrelated to the task at hand, but I have much respect for Tomlinson Holman's  earlier work at APT.  

He seems to be up to a similar scheme with TMH approvals for home cinema.. I may start offering JR approvals for homebrew beer.

JR

[Antone Atmarama Bajor]

     I know that Tomlinson Holman started THX at skywalker, as far as I understand it he started it because most cinema speakers and hardware were inadequate to perform to spec.  I am not aware of them creating any standards, but their role was more to certify what hardware was able to perform adequately.  Be it A chain B chain, room treatment, and dimensions.  Their office is still on Lucas Valley Road near San Rafael, But Tomlinson no longer is involved.  As far as I know their current function is still to test and certify that equipment meets predetermined standards, and charge a lot of money for the certification.

   I have actually had a discussion with one of THX's testers, in their test room about the nature of screen attenuation, is it by any small coincidence that Perfscreen attenuation starts at about 2kHz and rolls off at 3dB per octave???  From the conversation I learned that some of the micro perf screens have a lighter roll off slope but they are not used as often.

   I don't know all the details I worked with the man who took over at Skywalker after Tomlinson left.  I don't know all the details but from what I gather, the x-curve is a result of the effects of room reflection and screen attenuation even if they didn't identify screen attenuation as part of the equation back then.

   Can you imagine, how much power one needs to put in the top octaves to try and get flat response, from behind a perf screen.  Most compression drivers can handle that level of power, and many that can suffer from terrible breakup modes, at those Levels.

    I never had a discussion of tuning small and large rooms, but what Jerry used for tuning the B-Chain was a D2-unit (The successor of the R2-Unit)  Which is a bunch of 1/3rd octave filters that sum, and uses a diamond pattern array of 4 countryman Mics with their own calibration files.

    The 4 microphones are averaged together using a slow averaging method.  That ensures that the reverberant characterstic in the room is picked up, and is part of the equalization process, and the room is equalized to the X-curve, with the reverb averaged in I don't think there is a big need for alternative slopes for larger rooms, but I could be wrong.

    I've met a few of the Dolby guys but haven't really discussed cinema with them much.

   I suppose my conclusion is although X-curve doesn't specifically mention or address screen attenuation, it is a result of behavior that seems consistent with the exhibition house environment.  Which are all effected by things, such as room acoustics, and dimensions.  As well as B chain devices, and to think that putting speakers behind a screen had no part in the equation seems like one is ignoring the elephant in the room, the screen is by no means acoustically transparent in the high frequencies.

    If you removed the screen from the equation the X-curve would be quite different.

    I also think that it can be argued that much of THX certified rooms and rooms since have much more aggressive, sound treatment then in the past so room reverberation is much less a problem to a point.

    I don't think I've misinformed anyone thus far.

Except that I was 3dB off for Surround calibration settings.

    The Dolby X-curve paper confirms the extra head room past calibration.  

    Can one argue that screen attenuation has noting to do with the characteristic roll off of xcurve?

[Jens Brewer]

John Roberts  {JR} wrote on Wed, 08 April 2009 17:17

I may start offering JR approvals for homebrew beer.

Preliminary specs?  Measurement hardware/software?  More details JR.  

[John Roberts {JR}]

Jens Brewer wrote on Wed, 08 April 2009 17:17

John Roberts  {JR} wrote on Wed, 08 April 2009 17:17 I may start offering JR approvals for homebrew beer. Preliminary specs?  Measurement hardware/software?  More details JR.

Actually I was just kidding...  I don't encourage anybody to send or carry their inferior homebrew to Hickory.

In the words of Emily Litella, never mind...

JR

[Scott Raymond]

Jaska Saarinen wrote on Wed, 08 April 2009 15:10

I use  WinMLS, Smaart, Room-Capture... also Ivie IE-45 for some quick checks. When using  Smaart type analyzing system I normally go in 6-ch external analog input (you need to have cable=  6 female XLR's and other end 25pin D connector) where I can play eighter Smaart (or similar analyzer) own generator or like NTI minirator... and after that you need to do just channel levels with using cinema processor internal pink noise generator: Left- Center- Right= 85dBC / each channel, left and right surround = 82dBC / each channel, analog (for optical sound) subwoofer same as center channel and when measured together (center+ sub) it looks +3dB LF area (summing)... and from that analog sub level raise digital subwoofer channel +10dB... practically this means in SLP meter C-weighting  analog sub 79-81dBC and digital sub 89-91 dBC of course depending of subwoofer overall response...

Thanks Jaska.  Using the analog input is a good point that I hadn't considered.  They'd gotten a DTS 6D with the purchase and the CAT 685 card was with it so wiring up a DB25 could have been an option and I'll keep that in mind if I need to do any more tweaking on it in the future.  On a side note I find it somewhat interesting how the DTS audio comes off the CD(s) gets converted to analog, hits the 685, converts to digital, goes through the processor and then finally gets converted back to analog for the last time.  But as the DTS process came later I'm assuming that was the only option as it had to be backward compatible with the older processors.

[Antone Atmarama Bajor]

     I'm not sure that there is much difference between iso 2969 curve and x-curve as they both seem ~the same.

Perhaps some one here knows the difference.

    But read page 4 here on this JBL paper.

http://www.jblpro.com/pub/technote/tn_v3n03.pdf

[Scott Raymond]

Antone Atmarama Bajor wrote on Thu, 09 April 2009 15:10

I'm not sure that there is much difference between iso 2969 curve and x-curve as they both seem ~the same. Perhaps some one here knows the difference.     But read page 4 here on this JBL paper. http://www.jblpro.com/pub/technote/tn_v3n03.pdf

Antone,

In the Loan Allen link I posted above a fairly detailed description and history of the two starts at around page 16.

[Antone Atmarama Bajor]

     Yes I read it, it is a good paper and confirms much of what I've said.  It does also give a vague nod to screen playing a roll in HF roll of even though there is nothing specific said about screen attenuation.

    I figured I should find a document that helped support my statements about the significance of screen attenuation, and the existing target response curves for cinema today.

    I know that when a lot of the original tests and investigations were performed, that a majority of the test equipment, as well as reproduction equipment, was limited and not able to perform very well.

    I think the technology finally started catching up to the standards in the 90's.

  At least thats my opinion, not that the men and women at Dolby and the ISO and SMPTE teams didn't do great work with what they had available to them over the previous 50 or 60 years.  

    In modern cinema with some of the slow averaged room response tuning methods, I wonder how relevant some of the room size dependent response curves are.  Those curves are from the early 80's when much test equipment was pretty questionable.  It seems like they may have been going for a best guess with what the room reverberation will be like.  I'll have to ask Jerry what he does, I've been around him when he's tuning rooms but I haven't done the deed.

     I wonder if the Large room EQ curve is supposed to be as a result of more air attenuation of HF, or because room resonances will tend to be lower frequency in a larger room, so it is thought less HF energy is needed to feel balanced with the LF since the reverberant field gain isn't encroaching on the HF range?

    Its all fun stuff.  Theirs a lot of details involved in Cinema, and I'm only a grunt trying to learn as much as I can.

I want to go to a cinema with some Danley Super Subs, I'm tired of hearing the LFE Subs crap out on some of the rumble effects, that have been mastered into the film.

Antone-

[John Roberts {JR}]

They have been putting speakers behind the screen far longer than they had speakers that could out run the screen's LPF.

I find Ioan's paper interesting since he calls the '70s the old days..  

Here's a take from another Dolby Engr..   http://www.hometheaterhifi.com/volume_9_2/feature-article-cu rves-6-2002.html

In the early days frequency response was just one of the sharp knives they were juggling, as (HF) speakers got better, they started hearing  wow and flutter and noise floor,,, Things we take for granted today.

JR

[Scott Raymond]

I'd big interested in experiencing Cinema with the Matterhorn sub.  

I'd have to admit to a growing fascination with the Cinema environment.  The history behind everything is good reading as well as some of the methods and studies they undertook to advance the realism of Cinema sound.  It really reinforces how complex the interaction of audio and visual is in our noggens, even more so than the effects we run into in live audio.  I haven't had time to read all the info Brian sent me, but have read some of the pertinent items several times and seem to pick new things out of it each time.

[ProSoundWeb Community]