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SMPTE/ISO 2696 X-curve de-emphasis


JohnA
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Toole has a very good history of the X curve in his book, particularly the third edition.  Apparently (and according to Toole's sources) the X-curve is bogus--not supported by the data.

 

Chris

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I've seen that paper and I still don't know what to make of it.  Old film and cinema systems had poor HF response.  So? 

 

KPT-200s are advertised as having the X-curve built in.  But the response curves Bob Crites (I think) published showed no real improvement in response when the "X-curve" elements were removed from the circuit. 

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7 hours ago, Chris A said:

Apparently (and according to Toole's sources) the X-curve is bogus--not supported by the data.

  • The consensus I've seen is that the X curve is inappropriate in commercial cinemas and horrendously inappropriate in home theater
  • In some of the articles I've read, it sounds like serious attention to soundtracks and theatrical acoustics started in the 1970s; earlier magnetic tracks without Dolby, with 4 and 6 channel "acoustovoiced" theaters are largely ignored.   In the article Bruce provided for us, it says, "Even though a few films were being released with magnetic stripe, both 35mm and more rarely 70mm, the ubiquitous format in the late 60s and early 70s was 35mm with a mono optical ..."    In those days San Francisco had 8 theaters equipped for 70 mm.  Almost all medium or big budget films at least had magnetic tracks, and many were 70 mm.  Theaters that featured magnetic sound were pretty much standard in the big cities and we would phone ahead to make sure the actual print received was "mag."  Any theater (except Art Houses) that was not equipped for "mag" was on our s**t list. 
  • Magnetic soundtracks really sparkled.  That's with ears that were then young and unabused.  On a few occasions, I saw a given film in optical one time and in magnetic another, and there was a huge difference.
  • It is true that some of the theater speaker systems had response that dropped like a rock at 11KHz.  I think, during the magnetic era, on magnetic tracks, the response was boosted above 11KHz, either in the A or B chains, to make up for the roll-off. 
Edited by garyrc
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47 minutes ago, garyrc said:

I'm assuming you meant 16 dB down at 16 kHz?

 

Didn't read the paper? I was typing on my phone, but here is a bit of the text:

 

The early 1970s saw the introduction of the third-octave realtime analyzer for audio analysis.
This made measurement of the theatres’ B-chains much easier, both in terms of time taken and
increased detail. Dolby made extensive measurements of electro-acoustic responses in theatres,
both in the U.K. and the U.S. Figure 5 shows the average U.K. B-chain response of 45 theatres.
There are several interesting issues. First, the crossover dip at 500 Hz is clearly evident,
suggesting that in some cases there was a very deep notch. Secondly, the response is about 16 dB
down at 8 kHz
—very close to the 14 dB suggested in the 1969 ISO draft document referred to
earlier.

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3 hours ago, Marvel said:

 

Didn't read the paper? I was typing on my phone, but here is a bit of the text:

 

The early 1970s saw the introduction of the third-octave realtime analyzer for audio analysis.
This made measurement of the theatres’ B-chains much easier, both in terms of time taken and
increased detail. Dolby made extensive measurements of electro-acoustic responses in theatres,
both in the U.K. and the U.S. Figure 5 shows the average U.K. B-chain response of 45 theatres.
There are several interesting issues. First, the crossover dip at 500 Hz is clearly evident,
suggesting that in some cases there was a very deep notch. Secondly, the response is about 16 dB
down at 8 kHz
—very close to the 14 dB suggested in the 1969 ISO draft document referred to
earlier.

 

Oh!  Sorry.  I scanned the article quickly; I had read it before, but did not retain that detail.  So people won't be misinformed I will remove my incorrect guess.

 

Wow, 16 dB down at 8 kHz is really extreme! 

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4 hours ago, JohnA said:

Old film and cinema systems had poor HF response.  So? 

 

Optical soundtracks had poor HF response, and a good deal of distortion.  Magnetic sound tracks were usually quite good in both those respects.  Theaters showing 70 mm before Dolby entered the cinema sound business had a black box (actually, it was blue, and usually had the word Ampex on it) that provided custom EQ.  70mm soundtracks moved at 22.5 ips, and had extremely good HF response. 

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This might make more sense of it... from another consultant and from another SMPTE paper "X-Curve is Not AN EQ Curve" :. Only pasting part of it here.

 

Michael Karagosian is president of MKPE Consulting, a cinema industry consultancy. He led the development of Dolby cinema products in the late 1970s and early 80s. His monthly publication mkpeReport.com provides in-depth analysis and technology coverage for digital cinema.

 

X-Curve is a measurement response window, based on measurements made in the reverberant environment of actual cinema auditoriums. It allows a cinema technician having off-the-shelf measurement tools to obtain a close approximation of the sound experienced in the mixing room, without the need to employ a more complex measurement method, based on first arrival sound. X-Curve generalizes the impact of room reverberation on frequency response as displayed by a realtime analyzer. SMPTE ST 202 manages this generalization by providing a range of measurement windows for use with differently sized rooms. By using the minimal equalization necessary to produce a response within the X-Curve window, the impact of room reverberation on the measurement is mitigated, and the equalization more accurately reflects that which is needed to overcome acoustical coupling problems associated with speaker, screen, and room. While the use of X-Curve as a measurement technique has been successful in establishing a high degree of uniformity of sound in cinemas around the world, it has also received criticism. Users that do not understand the proper application of X-Curve may sometimes treat it as an equalization curve, attempting to equalize the system until an exact replication of the curve as published in ST 202 is achieved, ignoring the response window described by the text in the standard. This generally leads to excessive equalization, and not necessarily the best possible sound. In addition, the X-Curve measurement method is standardized in SMPTE ST 202 and ISO-2969, and methods for equalizing a room, based on first arrival sound are not standardized, causing temptation to discount the use of first arrival methods in cinema. But, in fact, many skilled technicians prefer to use first arrival methods, and proper use of such methods should lead to equivalent results. First arrival measurement methods tend to be based on proprietary equipment, so caution must be taken if pursuing a first arrival standard so as not to disfavor a manufacturer of valid measurement tools.

 

SMPTE established a new Technology Committee, TC-25CSS Cinema Sound Systems. As the name suggests, members of the committee will be evaluating other measurement methods for cinema, based on first arrival of sound. If one or more alternative cinema sound measurement standards should emerge, one should keep in mind that they are an alternative, and not a replacement, for ST 202. X-Curve, the method that takes into account room reflections, and newer methods based on first arrival of sound, both strive for the same thing: a measurement method that allows the cinema technician to best approximate the sound heard in the mixing room.

 

2013

 

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1 hour ago, JohnA said:

So, perhaps a time delay/alignment? 

 

Closer, I think. A way to adjust for your theater to play back and sound the way it did for the mix engineers? Not your home theater, but the multiplex, etc.

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On 1/27/2020 at 6:20 PM, JohnA said:

KPT-200s are advertised as having the X-curve built in.  But the response curves Bob Crites (I think) published showed no real improvement in response when the "X-curve" elements were removed from the circuit. 

I thought the mod brought the top end up to 17K from the original 14k but it's been several years since I did mine.  I might be misremembering but those numbers stuck in my head for some reason.

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10 hours ago, glens said:

I love it when makers of proprietary schemes try to get them "standardized"... (not!)   Mandated money in the bank for some entity ought to be disallowed.

 

Totally disagree on this. The industry as a whole wanted to make sure theaters could all have consistent playback of content for the viewers. Whether you have an AMC or mom and pop, you could get the some quality.

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3 hours ago, Marvel said:

 

Totally disagree on this. The industry as a whole wanted to make sure theaters could all have consistent playback of content for the viewers. Whether you have an AMC or mom and pop, you could get the some quality.

 

I had in mind this part from the article:

 

On 1/28/2020 at 9:38 AM, Marvel said:

"First arrival measurement methods tend to be based on proprietary equipment, so caution must be taken if pursuing a first arrival standard so as not to disfavor a manufacturer of valid measurement tools."

 

And was rather seconding his motion.

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