Some digital cinema projection best common practices

Some digital cinema projection best common practices

The best cinemas comply with the current DCI Specifications and SMPTE 431 standards.

DCI/SMPTE 431 compliant
The first versions of the DCI specifications described all aspects of digital cinema projection like the minimum contrast ratio and the white illumination target. The latest version says that cinemas must follow the projector environment described in these two SMPTE 431 documents:

ST 431-1:2006
D-Cinema Quality – Screen Luminance Level, Chromaticity and Uniformity

RP 431-2:2011
D-Cinema Quality, Reference Projector and Environment

RP 431-2:2011 replaced ST 431-2:2007 and introduced a new projector color gamut / projector configuration (PCF) called virtual white.

Different kind of cinemas:
SMPTE 431 Review room 14 fL, 100:1 Intra frame contrast, 1500:1 Sequential contrast
SMPTE 431 Cinema Theatre 14 fL, 100:1 Intra frame contrast, 1200:1 Sequential contrast-
Studio Projection Guidelines calibrated. 14 fL.
Uncalibrated uncalibrated. 8-35 fL.

The best cinemas are the common height cinemas where scope films are projected wider than flat films.

The best cinemas recalibrate their lamps at least once per week to get a flicker free image and the recommended light level of 14 fL for 2D and 7 (5-9) fL for 3D.

If the image still flickers or is too dim after a cold start and auto adjustment, the lamp should be changed.

The best cinemas keep their port glass, 3D systems and optics clean to avoid a washed out picture.
This must be done according to instructions in the projector manual. Link:Christie CP-2220 manual

The best cinemas keep up with cinema technology improvements like Atmos and better silver screens like Harkness CLARUS XC and REALD ULTIMATE SCREEN

The best cinemas have good acoustics with a mix of reflective and damping materials on the walls.

The best cinemas are adjusted and calibrated to sound good.

The best cinemas are powerful enough to be able to play content at reference levels.

The best cinemas do not turn down the volume faders to low levels for all features because some are mixed too loud.

See this post for more info.

Example: In a Uncalibrated cinema with 35 fL and low contrast ratio:
-The image will be too bright.
-Black will be gray.
-Dark scenes will look brighter.
-Muddy and washed out picture.

In the Studio Projection guidelines “The Pixar Projection guidelines” you find some tests that will insure better projection.

These are similar to other studio projection guidelines that film studios deliver with their films:

The tests mentioned in the studio guidelines are projection best common practices in digital cinemas:

-adjust the white illumination to 14 fL (using a DCI White test image, not a projector white test image which is 15-16 fL)
-check and adjust the framing with a framing chart,
-check the sound for 85 dBc per channel and play the movie at reference level (7 on dolby, 0 on other equipment)

This is how I do the standard tests from the projection guidelines:

1. Check for 14 fL

“Lamp Levels

Projection lamp levels that are set too low have a disastrous effect on the picture. When projected at the SMPTE standard of 14fL full white, the image is sharp and colorful. At lower levels, the colors become muddy and gray. The picture has less snap to it and feels lifeless. Important details in the darker areas will disappear if the light levels are too low. “

-Check that light path is clean.
-Turn the lamp on the projector and wait 10 minutes.
-Auto adjust the lamp.
-Go to the middle of the auditorium
-Point the spot meter on the Sekonic L-758-C-U at the DCI white square inside the Disney 2D framing Chart test DCP.
-Check for 14 fL.
-Then adjust the power until it is 14 fL.( If the projector is set to to auto adjust the power, check that the projectors auto power adjustment is really 14 fL.)
-Repeat at least once a week.

 

For 3D movies you want to hit the fL target for that specific movie. (DCI says it should 7 fL, their acceptable luminance levels are now between 5 and 9 fL.)

The light level specification for _________ is 6 foot Lamberts, measuring white light through the 3D display system. The acceptable range is between 4.5 fL to 7 fL.

RealD ZScreen System and other polarized systems: Remove filters from the projector. Set the light level between 30 and 48 fL.

With the Sekonic L-758Cine I check that the the white is betweeen 30 and 48 fL illuminated from the middle of the screen which will become 4.5 fL to 7 fL when glasses are used and the Masterimage/RealD 3D system is on.

Another test that should be included is a basic intra frame contrast test.

To read the intra frame contrast accurately you need the Minolta LS-110 or similar and the correct Intra Frame test image DCP.

But with a Sekonic L-758Cine or L-758-C-U spotmeter and a projector checkerboard test image you could measure the light from the black and white squares and get some idea of what the intra frame contrast is. The intra frame contrast should be 100:1 for Theaters according to SMPTE/DCI. If you get around 0,5 fL in the black squares, the intra frame contrast is around 32:1.

If the intra frame contrast is bad you could try to:
– Clean the port glass and optics.
– Avoid spill light from projection booth.
– Use matte black seats and furnishing.
– Install a iris in the projector.
– Upgrade to a RGB laser projector.

2. Check the framing chart.
If the movie has a framing chart you can adjust a preset on the projector to fit the framing chart. It is important to avoid cropping 3D movies because it could break the 3D illusion on movies with floating windows.

You can also use the projector framing test images to check framing.

You could also check if the picture is in focus when checking the framing.

3.Check sound levels

Your theater sound system should be tuned to 85dB (C-weighted) on all screen channels using pink noise. The correct fader setting on Dolby and DTS systems is 7. On SDDS systems, the correct master volume setting is 0.

Use a SPL meter to check the dBC level. If there is a drop in dBC in one channel you should check speakers and amplifiers for faults. You should also play a familiar DCP at the reference level to check if the sound system works OK.
Note:A cinema sound system can pass the 85 dbC test without having enough amplifier power headroom. The pink noise signal that is measured to 85 dbC is -20 Dbfs, a cinema system should also be able to play louder material without clipping.

Note: Many films these days (2015) are not meant to be played at reference levels. The dialogue can be mixed at high levels and you need to turn the fader down to a low level. These movies lack dynamics.

Notes:

RTA/SPL

Alternatively you could use a Real Time Analyzer (RTA) to check dBC level when playing the pink noise.

– With a RTA/SPL you would see both the pink noise frequencies and the pink noise dBC level.
– If there is some frequencies missing on the display when playing the pink noise an amplifier, speaker or other equipment could be faulty.
– With a RTA/SPL You will notice which frequencies are peaking.

I’ve tried the Behringer DEQ2496 with ECM8000 microphone as a SPL/RTA and the umik-1 microphone with 90 degree calibration in REW as a SPL/RTA. They will both give decent measurements .


The DCI/SMPTE standards

Some cinemas may need to change port glass, change audio equipment, change screen, upgrade to series 2 projectors, dual projectors, laser projectors and similar actions to meet the DCI/SMPTE standards.

The X-Curve/85 dbc sound standard:

Virtually all cinema playback systems today are aligned using SMPTE 202M or ISO 2969. The two documents are virtually the same and both define the measuring method and the resulting frequency response known as the “X-Curve”. This provides a uniform frequency response adjustment for all theaters throughout the world. Cinema playback is also tuned to a specific level. All standard cinema systems are optimized for 85dB SPL (2/3rds back in the center) from each front channel and 82dB SPL for each of the rear channel arrays. All channels should have 20dB of headroom. The LFE channel is set at 10dB of in band gain; that is, 10dB greater than the screen channels in each 1/3 octave frequency band. SPL ranges from 88dB to 92dB, depending upon the specific bandwidth of the LFE system in use.

“Recommendations for Surround Sound Production” -The Recording Academy’s Producers & Engineers Wing

More info here: http://duc.avid.com/showthread.php?t=87830

There is some work being done to update the SMPTE 202M and other cinema standards:

Making a movie soundtrack sound consistent in multiple venues is a big challenge

https://www.smpte.org/sites/default/files/2013-03-12-Standards-Cinema_Audio-Vessa-v2.pdf

Suggested new documents include:
• SMPTE Standard: Electroacoustic Performance of Theatre Sound Systems, In Situ
• SMPTE Recommended Practice: Measurement and Calibration of Theatre Sound Systems
• SMPTE Standard: Theatre Sound-System Performance Requirements and Verification
• SMPTE Engineering Guideline: Designing Theatre Sound Systems to Meet Performance Standards
• SMPTE Standard: Theatre Room-Acoustics and Measurements
• SMPTE Recommended Practice: Measurement and Verification of Theatre Room-Acoustics
• SMPTE Engineering Guideline: Theatre Room-Acoustics Design Considerations

– B-Chain Frequency and Temporal Response Analysis of Theatres and Dubbing Stages
https://www.smpte.org/standards/reports

The present alignment standards and recommended practices leave some room for degradation of the sound system, by using equalization to mitigate problems which should be solved in more fundamental ways.

– B-Chain Frequency and Temporal Response Analysis of Theatres and Dubbing Stages
https://www.smpte.org/standards/reports
Other tips from the report:
-Match the channels. Do not use a different rollercoaster EQ on the left and right surround channel. The phase cancellation from the different speakers in the surround array can not be removed with EQ.
-Do not EQ the subwoofer too much.
-Use a 120 hz low pass filter on the subwoofer.

You can also read more here http://www.aes.org/technical/sdctv/

The current concepts of calibration were developed at a time when reasonably good room acoustics and high output, low distortion, wide directivity loudspeaker systems were by no means as easy to find as they are today. Good systems in good rooms should automatically produce good sounds

Newell et. al. Cinema Standardization V.2

Loudspeaker and amplifier technology has also moved far ahead of where it was in 1971. Above all, however, there has been a trend towards reduced theatre sizes and drier acoustics, which reveal much more detail in the sound and render much more obvious the effects of inappropriate equalisation.

Is the X Curve Damaging Our Enjoyment of Cinema?

Many cinemas turn down the fader level because the sound does not sound good at the standard level. The problem with turning down the level is that the sound will be different at lower fader settings:
-The dynamic range of the sound will be less, some sounds may not be audible anymore.
-Different frequencies will sound louder.
-The surround sound could sound lower compared to the main channels.

A better solution would be to get the sound mix to sound good at the standard fader level.

This is difficult.

Applying one calibration level for all cinema rooms has been shown to lead to an unpleasant, overpowering sensation in smaller rooms.

Newell et. al. Cinema Standardization V.2

You have to turn down the fader in small auditoriums to get a pleasant sound.

Here are some possible ways to improve the sound:

-The sound X-curve has some +/- tolerances, so the X-Curve EQ could be adjusted by ear to sound better. The X-Curve was made for big rooms, for small rooms you need to use a modified X-Curve.
-“Floor dips cannot, and should not, be equalised.”
Newell et. al. Cinema Standardization V.2
-Listen to material at the standard level and check if the sound system sounds good and can handle it.
-Better acoustics (example: more damping material around the speakers, mix of reflective and damping materials on teh auditorium walls )
-Modern loudspeakers like QSC, Luis Wassmann Krix)
-Modern amplifiers with more power that is closer to the loudspeakers like QSC
-Digital transmission of sound to the amplifiers (Example: QSC Q-SYS) . Digital transmission of the sound from server to the sound processor.

Is the 1 degree Sekonic L-758cine spotmeter good enough to measure fL levels?

“A spot photometer is used to measure screen luminance, with a spectral response of the CIE standard observer (photopic vision), as defined in CIE S002. The photometer should have a collection angle of 2 degrees or less. For white field measurements, an accuracy of +/-0.5 cd/m2 (+/- 0.2 fL) is required. For black field measurements, an accuracy of +/-0.007 cd/m2 (+/-0.002 fL) is specified. In order to provide stable readings that are insensitive to flicker, the photometer must integrate over a period of time sufficient to remove all frequencies above 24 Hz, displaying the arithmetic mean value. The Konica Minolta LS-110 is a commonly used photometer in the motion picture industry”

-Color and Mastering for Digital Cinema
by Glenn Kennel

When I compared the Sekonic L-758Cine to more expensive measurement equipment it gave the same fL levels measuring a white test image.

Intra Frame Tests

“Intra-frame (or ANSI contrast) is measured with the Checkerboard target shown earlier in Figure 5.3. The luminance of each of the white patches and each of the black patches is measured with a spot photometer. Intra-frame contrast is then computed by summing the white patches and dividing by the sum of the black patches. In its operating environment, the intra-frame contrast is reduced by many factors including projection lens flare, portal glass flare, ambient light spilling onto the screen and back reflections from the room itself.”

-Color and Mastering for Digital Cinema
by Glenn Kennel

The Sekonic L-758cine has less accurate fL readings than a Minolta LS-110, but it will give you a good idea of what the intra frame contrast is and gives you a accurate white illumination fL reading. Flicker is not a problem.

A LS-110 costs around USD 2000 on ebay
ls-110 minolta | eBay

An L-758cine costs around USD 600 on ebay.
L-758cine | eBay

The L-758Cine is a good tool to adjust the projector settings as the lamps age.

The LS-110 gives accurate readings of intra frame contrast and sequential contrast. The sequential contrast should be 1200:1 in a cinema theatre.

Alternatives to using a L-758cine:
– You could use the built-in fL system in the projector and match the fL values the various presets had when they were calibrated. But having a L-758cine to check the projector’s fL system for accuracy is great.

Lamp best practices
http://www.christiedigital.co.uk/TechPapers/Christie-Xenolite-Lamp-Best-Practices-Guide-English.pdf

-You should be at the target illumination 14 fL (2D)/ 4,5 fL (3D) when a new lamp is at 65-75 percent power in the Scope preset.
-You should regularly auto adjust the lamp and then increase it’s power to get 14 fL
-At the end of the lamp’s life it should be at 100 percent, not at the start.

Silver screens

All silver screens used for 2D films should be upgraded to the new Harkness CLARUS XC  and REALD ULTIMATE SCREEN screens to try to meet DCI/SMPTE 2D standards.

3D silver screens/high gain screens can cause some difficulties in conforming to the 2D 14 fL standards.
In a common height cinema you often have to use a more powerful lamp to get 4.5 fL for 3D, and another less powerful lamp to get 14 fL for 2D. If you use the same lamp for 2D as you did for the 3D feature, you could get too much light in 2D.

Old silver screens have difficulties conforming to the SMPTE 431-1-06 standard that says:

Luminance sides – Theater – 75-90% of center.

-Even with a curved screen, 75 percent luminance on the sides of center is not likely.
-You are allowed to have 14 fL +/-3 fL in the center so you could use up to 17 fL (or more) in the middle to compensate for the lower total luminance because of the low luminance uniformity.
-The checkerboard test image will give you some idea of the luminance uniformity.

2 Replies to “Some digital cinema projection best common practices”

  1. What I’m concerned about is theater owners or managers
    who intentionally deviate from standard sound and light
    levels in their auditoriums, in attempts to be brighter than
    the competition.

    This could lead to a “brightness” war between theaters and
    chains of theaters.

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