DCI/SMPTE 431 compliant cinemas
The best cinemas comply with the current DCI Specifications and SMPTE 431 standards.
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:
RP 431-2:2011 replaced ST 431-2:2007 and introduced a new projector color gamut/projector configuration (PCF) called virtual white.
Different kinds of cinemas in RP 431-2:2011:
–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
Digital Cinema projection best practices:
-Recalibrate and check the lamps at least once per week to get a flicker-free image and the recommended light level of 14 fL.
If the image still flickers or is too dim after a cold start and auto adjustment, the lamp should be changed.
-Keep the port glass and optics clean to avoid a washed-out picture.
This must be done according to instructions in the projector manual.
-Have good acoustics with good acoustic materials on the walls. The sound should be calibrated to sound good.
-Check the sound on all films and set the fader level to a fitting level. Feature films should be mixed to the 85 dBc standard, but the mixes from different countries often sound different. DCPs from India are louder than French DCPs. Films mixed in mixing stages with dry acoustics sound different in a cinema with wet acoustics compared to a cinema with dry acoustics.
If it is too much light in a cinema:
-The image will be too bright.
-Black will be gray.
-Dark scenes will look brighter.
-Muddy and washed-out picture.
Some 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
1. Check for 14 fL
-Check that the 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/Sekonic L-858D-U at the DCI white square inside the test DCP.
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-Check for 14 fL.
-Then adjust the power until it is 14 fL. (If the projector is set to auto adjust the power, check that the projector’s auto power adjustment is really 14 fL.)
2. Check the framing charts.
Check the framing chart DCPs for Flat and Scope. You can adjust the presets on the projector to fit the framing chart or do a quick reset.
You can also use the projector framing test images to check the 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 other systems, the correct master volume setting is 0.
Use an 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 Db RMS, a cinema system should also be able to play louder material without clipping.
Alternatively, you could use a Real-Time Analyzer (RTA) to check the dBC level when playing the pink noise.
– With an RTA/SPL you would see both the pink noise frequencies and the pink noise dBC level.
RP 2096-1:2017 – SMPTE Recommended Practice – Cinema Sound System Baseline Setup and Calibration describe how you can measure the loudness with an RTA.
– If there are some frequencies missing on the display when playing the pink noise an amplifier, speaker or other equipment could be faulty.
– With an RTA/SPL You will notice which frequencies are peaking.
The DCI/SMPTE standards
Some cinemas may need to change port glass, change audio equipment, change screen, change to 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.
More info: Calibrating Cinema Sound Systems
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
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.
The B-Chain Frequency and Temporal Response Analysis of Theatres and Dubbing Stages report resulted in
RP 2096-1:2017 – SMPTE Recommended Practice – Cinema Sound System Baseline Setup and Calibration
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
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.
The problem with turning down the fader level
-The dynamic range of the sound will be less, and some sounds may not be audible anymore.
-Different frequencies will sound louder.
-The surround sound could sound lower compared to the main channels.
Applying one calibration level for all cinema rooms has been shown to lead to an unpleasant, overpowering sensation in smaller rooms.
You often have to turn down the fader in small auditoriums to get a pleasant sound.
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 equalized.”
Newell et. al. Cinema Standardization V.2
-Listen to the material at the standard level and check if the sound system sounds good and can handle it.
-Better acoustics (example: Baffle wall, good acoustic materials on the auditorium walls )
-Modern amplifiers with enough power that are close to the loudspeakers
-Digital transmission of sound to the amplifiers. Digital transmission of the sound from the server to the sound processor.
Intra Frame Tests
Another test that is important is the intra frame contrast test.
“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
To read the intra-frame contrast accurately you need the Minolta LS-110 or similar and the correct Intra Frame test image DCP.
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But with a Sekonic L-758Cine or Sekonic/L-858D-U 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.
The LS-110 also gives accurate readings of sequential contrast. The sequential contrast should be 1200:1 in a cinema theatre.
If the intra frame contrast is bad you could try to:
– Clean the port glass and optics.
– Avoid spill light from the projection booth.
– Use matte black seats and furnishing.
– Install an iris in the projector.
– Upgrade to an RGB laser projector.
Lamp best practices
-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 use the auto-adjustment of lamp power so it is always 14 fL
-At the end of the lamp’s life, it should be at 100 percent, not at the start.
All silver screens not in use for 3D should be changed to matte screens to meet DCI/SMPTE 2D standards.
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 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.