| CES : Technical Guidelines for Dolby Stereo Theatres November 1994 Page 9 |
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Picture projection quality should conform to existing published standards.
Needless to say, installation of a Dolby SR decoder to achieve improved secund quality in a theatre will not affect the quality of the picture image. But as a large-screen high- definition picture presentation can be damaged by inadequate sound, the full fidelity of Dolby SR sound can only be achieved with the complement of a wellprojected picture. The movie-going experience can only be fully realised when picture quality and sound quality are matched.
Selection of state-of-the-art lenses, non-reflective angled port glass, and the installation of three-bladed shutters can provide radical improvements in perceived picture quality.
There are obvious trade-offs with selection of too small or too large a screen. As the picture gets larger, the audience becomes more involved in the story, begins to feel they are participating and not just witnessing. But as the picture gets larger, the visibility of picture flaws -- grain, jump and weave, become more apparent. Back in 1953, Twentieth Century Fox did some fundamental research into screen size, during the development of Cinemascope (with its 2.35:1 aspect ratio). While granularity has been improved since the fifties with modern film stocks, jump and weave of the picture image remain essentially unchanged, even with modern projectors. The conclusion then was that the ideal picture size should be that which subtends a horizontal angle of 45 degrees at the prime seat. Given an empty theatre, a movie-goer by-and-large selects a seat two-thirds of the way back, on the centre-line of the house, and as result the ideal geometry is set as shown in Figure 6.1. Obviously, discussion of screen size in terms of feet or metres becomes a secondary dimensional issue, depending on the size of the theatre. The key dimension is the angle subtended at a prime seat.
This is one area where selection of a picture parameter in a new theatre design directly affects the sound. Assuming the loudspeakers are mounted at the left and right extremes of a Cinemascope screen image, this 45 degree angle makes possible a fairly smooth sound field, with reasonable stereo imaging throughout the theatre (assuming a sensibly short reverberation time, as discussed in Section 4.1.2 above). Screens much wider than this aspect ratio will not only exhibit excessive picture flaws at seats in the front of the theatre, but will exhibit subjectively excessive stereo "ping-pong". Screens with an aspect ratio narrower than this ideal 45 degree figure will lead to a sound essentially mono in the back of the house, especially if there is excessive reverberation. This effect can easily be detected in old theatres badly twinned a few years ago, with a dividing wall built straight down the middle of the house.
No layout of surround speakers, mono or stereo, can compensate for an incorrect stereo balance, left to right, across the screen.
If the ideal Cinemascope screen subtends an angle of 45 degrees at the prime seat, and granularity, jump and weave are the controlling parameters, the ideal screen ratios for other formats can be derived. In the USA, 1.85:1 is the most popular shooting format, but it is apparent that this ratio is very inefficient in terms of use of the film frame. Blown up too large, a 1.85:1 film will reveal excessive granularity. For this reason, a best compromise with 'scope and 1.85 screen sizes is to set the picture height the same, and only to adjust the horizontal masking, as shown in Figure 6.2. Maintaining the same width and adjusting vertical masking, will degrade the quality of the 1.85 image, showing up every picture flaw and excessive granularity.
In Europe, 1.66:1 is a popular shooting format. This is clearly a much more efficient use of the film frame, and here the screen height can be increased without excessive picture problems. Figure 6.3 shows how a theatre would equip for an ideal presentation of 'scope, 1.85:1 and l.66:1 aspect ratios. Obviously this requires both vertical and horizontal adjustable masking.
In the US, where normally only 2.35:1 scope and 1.85:1 films are presented, only adjustable horizontal masking is needed to ensure optimum picture quality. The speaker location should be set at the extremes of the 'scope screen, so care should be taken that neither the masking nor the masking edge-mounting damage the high-frequency response when a 1.85:1 movie is presented.
Over the last few years, a few theatres in the USA have installed screens with a fixed 2:1 aspect ratio, chopping off the sides of a 'scope picture, and the top and bottom of a 1.85:1 picture. This bad practice should be strongly discouraged, as with a 1.85:1 picture, in particular, significant picture action is likely to be clipped.
Many of the decisions taken in theatre design are based on cost -- not surprisingly when considering the building and operation expense of a modern multiple-screen complex. But when one considers that the purpose behind movie-going is to see a picture, compromises in the quality of the image on the screen seem inappropriate, The cost of a larger lamphouse may seem significant during theatre building, but will soon seem trivial on an ongoing basis, especially with improved patron recognition of superior presentation.
Gain screens provide improved illumination, but only on a reflected axis from the projector lens. Someone sitting on the centre-line of the theatre will see a brighter image on the centre of the screen, but the illumination will fall off towards the edges. Viewers seated to one side of the theatre will see a brighter image towards the side of the screen on which they are seated, but the illumination will fall off significantly on the opposite side of the screen.
Only matt-white, non-gain screens can achieve uniform illumination across the entire screen width, regardless of the viewer's location. Gain screens only make sense for special applications where the viewer is seated on axis (such as slide or video one-on-one AV presentations). Depending on theatre shape, matt-white screens may place an additional requirement that the wall surfaces by the screen are dark, and do not allow excessive reflection. See section 6.5 below.
Next, modern good quality lenses have been developed over many years to achieve an in-focus image over a flat screen. Curved screens in a single dimension tie a horizontal wrap) would require virtually impossible lens geometry to achieve optimum focus throughout the screen area. And while lenses could be developed to achieve approximate focus throughout the area of a two-dimensionally curved screen, such a shape may well have several sound and picture problems; first, the illumination will not be uniform throughout the seating area, and, more pertinent for this document, such a screen has to be solid Iwithout perforations) requiring separation of high and low-frequency loudspeaker units, with consequent dispersion and damage to the sound image.
For many years, the nominal screen luminance figure in the USA has been defined as 16 foot-lamberts. Many theatres fall short of this figure, by design or accident -- with inadequate lamp-house capability, worn-out lamps, or mis-aligned equipment. A few films have been released "timed" assuming theatres projecting at 12 foot lamberts. A very simplistic analysis of this trend shows a most obvious flaw --16 leads to 12, then 8, 6 etc, and sooner or later no light on the screen at all! But more seriously, 16 foot-lamberts was not a random selection. The nature of current film stocks is such that a projection luminance of 16 fL ideally balances between black and white saturation conditions.
For example, films timed for 10 fL and projected at 10 fL will look correct at mid-density, but the film is so light that all bright shots will loose graduation, and will tend towards clear film.
Not only should the centre-screen luminance be 16 fL, but the luminance across the screen should be uniform, as described in the relevant standards. A fall-off of 20% at the screen edges is the maximum acceptable. (2)
The optimum screen color temperature is 54000K, with a good theatre tolerance of +/-2000K. See SMPTE 196M, attached.
6.5 Reflected and Ambient Light
Reflected light should not exceed 0.25%.
Porthole design affects both picture and sound. First, good design practice makes the size of the porthole and viewport the very minimum necessary. Large port apertures act as acoustic reflectors bouncing sound back towards the screen, and can also result in a large amount of ambient light leakage from the booth worklights.
Good acoustic practice requires a double-stud isolation wall between auditorium and projection booth. With such a wall design, there must be no hard coupling between the two walls, as typified in the design shown in Figure 4.7. Both projection and viewing ports should be double-glazed to achieve sound isolation. Regardless of the wall structure, projector noise will escape through any single sheet of projection glass. port glasses should be angled to reduce lateral reflections; as a worst case example, internal reflections from a single port glass set at 900 to the line from lens to screen will result in numerous reflections leading to a soft screen image apparently out-of-focus. A good result will be a front glass set at perhaps 70 forward from the lens angle, and a rear (auditorium side) glass set at 150 backwards.
Care should be taken over the quality of glass used, and optimum projection requires coating to further avoid internal reflections. (3)
Figure 6.4 shows a front-and-back pair of port glasses optimised to avoid internal reflection problems.
Three-bladed shutters raise the flicker frequency such that white skies on the screen will seem essentially flicker-free. While pan nicker will still occur, the stability of white areas is significant in terms of reduction of "picture fatigue". While the cost pof purchase of threebladed shutters is relatively low, increased illumination is required to compensate for the shorter "on" time. This could become really significant if the existing lamp-house is running near its maximum rating, and a larger lamphouse is required.
1 The ANSI standard for picture dimensions is PH22.145.
2 See ANSI PH22.196:Screen Luminance, included in Appendix.
3 In the US, a suitable material is "Water White Glass," available from Schott Glass, 400 York Avenue, Durvea, Penn. Both sides of the glass should be coated M'ith "Photopic HEA--Muitilayer Anti-reflection Coating #6035001" by Optical Coating Laboratory, Santa Rosa, CA, or by approved equivalents
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