THX Tech Pages

II. Technical Aspects of Presentation

Viewing Conditions and Image Quality: continued

9. Screen image size and vierving angle distortion

A motion-picture image should be as large as possible within the dimensions of the auditorium and be presented to an audience at certain angles to maximize impact and minimize discomfort. The image angle (the horizontal field of vision) from the most distant seat should be no less than 26⁰, with 36⁰ recommended.

Seating should be arranged so that it provides all patrons a comfortable field of view. The viewing angle distortion should be less than 45⁰. Viewing angle distortion is defined by iso-deformation lines described in the reference.

Reference: SMPTE Engineering Guideline 18

10. Sight lines and floor pitch

Sight lines, floor pitch, seat back tilt, and viewer comfort all interact. Auditorium floors should be constructed so that all seats have unobstructed sight lines to the screen and provide comfortable fields of view. (See #9, Screen image size and viewing angle distortion.)

Reference: SMPTE Engineering Guideline 18

II. Screen image

The desired condition is to project precisely the following image dimensions:

35mm Flat image (1.85:1):	0.447 x 0.825"
35mm Scope image (2.35:1):	0.700 x 0.839"
70mm image (2.2:1):	0.870 x 1.912"

The screen image should have sharp edges, and the corners should be square (90 degree right angles). Cropping more than 5% of the projectable image is undesirable.

12. Image geometry (skewing geometric distortion, key stoning, etc.)

Geometry of the projected image is most noticeable on titles, especially moving titles, on architectural shots having straight parallel lines, or on the horizon. Too great a distortion in display is undesirable.

Reference: SMPTE Engineering Guideline 18

13. Framing

Framing must be highly accurate in the 35mm Scope (2.35:1) and 70mm (2.2:1) formats because white lines caused by negative splices may sometimes be visible if the framing is off more than a few percent. Framing errors on non-black-masked prints in the 35mm Flat [1.85:1) format cause composition errors. The image should be framed so that the top and bottom masking are within 3% of the picture height from the frame lines.

14. Splice quality/changeover quality between reels

Splices and changeovers between reels of a print may be noticeable, but should not interrupt continuity or be distracting to audiences. Distractions include interruptions to the image such as jumping, loss of continuity, or the brief appearance of opaque splices (black horizontal flashes). Interruptions to the sound may include silence, a loud pop, clicking, or thumping.

(c) 1993 Lucasfilm Ltd. _TM TAP and Theatre Alignment Program are service marks of Lucasfilm Ltd. _TM

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15. Interruptions in feature

Features should be presented without interruption. Theatre management has an obligation to respond quickly to interruptions in a feature. This should include a method or procedure for quickly determining, addressing, and informing audiences about the source of interruptions. In the event of a prolonged interruption Imore than five minutes), theatre management should make an announcement to the audience about the nature of the problem and give an estimate as to how long it will take to resume the show.

If there is an intermission in a feature, this should be indicated at the boxoffice along with the duration of the intermission. When appropriate during the feature, the word intermission should appear on the screen along with the duration of the intermission.

Sound Quality:

I. Sound format

All motion picture theatres should be equipped to properly reproduce standard 35mm stereo variable-area optical sound tracks.

2. Level (loudness)

Each channel of a theatre's sound system should play at a sound pressure level of 85 dBC at a standard fader setting. This is measured for each channel using a Sound Pressure tevel Meter when pink noise is injected into the sound system. Operators should play features at the standard fader setting (as set forth on the sound system manufacturers' speciiications). Overly loud trailers may need compensation downwards, but every effort should be made to play features at standard fader settings.

Reference: The standardized dubbing stage sound pressure level (SPL) of 85 dBC

3. Frequency range and balance

Frequency range is the limit from low bass to high treble over which the sound system works. 8alance over the range is the adjustment of the sound system so that it reproduces all frequencies from bass to treble according to the reference standards.

The A-chain (projector and preamplifier) and B-chain (equalizers, power amplifiers, loudspeakers, and room acoustics) frequency response shou id meet the reference standards.

Reference: 35mm: SMPTE 214, 70mm: SMPTE 217

4. Wow and flutter

Wow and flutter are speed variations in sound reproduction and are audible as a wavering pitch. Wow and flutter should not be audible to audiences.

Reference: Audio Engineering Society Standard AES 5

5. Stereo coverage

If the conditions under screen image (see #9, Screen image size and viewing angle distortion) are met and the screen loudspeakers are mounted lust inside the masking for the format in use, stereo should be perceived throughout the auditorium.

(c) 1993 Lucasfilm Ltd. _TM TAP and Theatre Alignment Program are service marks of Lucasfilm Ltd. _TM

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6. Maximum undistorted sound pressure level

The maximum sound pressure level in the 70mm format should be greater than 105 dB in any one channel without power amplifier overload, called clipping. In the 35mm format, maximum unclipped sound pressure level should be greater than 97 dB.

7. Surround system

The surround loudspeaker array coverage should be uniform within +/-2dB over the seating area. The requirements for screen loudspeakers (see #3, Frequency range and balance) should apply to the array of surround speakers as much as possible. The maximum undistorted sound pressure level (see #6, Maximum undistorted sound pressure level) should also apply to the array of surround loudspeakers.

8. Acoustics (reverberation and echos)

Reverberation and echos are destructive to dialogue intelligibility and should be minimized in theatres. Reverberation will vary depending upon the room volume, and is naturally longer in larger room volumes. The acceptable range for reverberation should be from 0.5 to 2 seconds, never exceeding 2 seconds.

9. Background noise (HVAC, lobby, employees, sound system)

Background noise such as noise from the HVAC system or sound system should not interfere with the sound quality in auditoriums. No sound system problems such as hum, hiss, crackle, pops, etc., should be audible to audiences. Hinges on auditorium doors and seats should not create noise. Maximum background noise should not exceed NC-30 or be lower than NC-25.

Reference: American Society of Heating, Refrigeration, and Air Conditioning Engineers Guide, SMPTE RP-141

10. Sound "bleed-through"

Sound from adjacent auditoriums is expected to be the most common intrusive noise, and it should be minimized so that it does not interfere with the sound quality. All intrusive noises should be less than NC-30, and no pure tones from adjacent auditoriums should be audible. (See #9, Background noise.)

III. Theatre Maintenance & Operations

Viewing Conditions and Image Quality:

I. Marquees/Attraction panels and Exterior signs

Theatres should have the means to identify the feature(s) currently showing (marquees/attraction panels, boxoffice signs, etc.). Where applicable, the marqueeslattraction panels should be lit at night so that titles are legible in the dark. It is recommended that the rating(s) of the featureIs) be indicated.

For all exterior signs (including marquees/attraction panels, boxoffice signs, one-sheet frames, etc.) all words and titles should be spelled correctly. The letters of all words, titles, and phrases should be the same color, size and style.

2. Parking facility

A parking facility is desirable for all theatres. Where parking is mandated by state and local zoning and building codes, adhere to the codes.

(c) 1993 Lucasfilm Ltd. _TM TAP and Theatre Alignment Program are service marks of Lucasfilm Ltd. _TM

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