SMPTE TUTORIAL Searching for the Perfect Aspect Ratio By Mark Schubin A debate is currently taking place over the appropriate aspect ratio for some degradation of the imagery advanced television displays. Any selected aspect ratio is inherently involved.' There are only three basic incompatible with any other and will require the use of some form of methods of accommodating existing accommodation technique. The derivation ofthe 16:9 (1.78:1) aspect ratio material shot in a fixed aspect ratio on a display of a different fixed aspect ratio, from accommodation techniques and display modes is explained, as is the though the techniques may be com­ relationship between aspect ratio and display memory. Research into the bined. These three basic techniques are history ofaspect ratios indicates that the 1.78:1 aspect ratio was adopted shown in Fig. 1. by the Standards Committee of the Society of Motion Picture Engineers Figure I a shows the truncation (SMPE) in 1930. It also indicates that the factors that may initially have method, a variant of which is some­ led to widescreen motion picture systems may no longer be applicable. times referred to as "pan and scan." The research for this paper found no clear indication ofa preference for When going from a wider aspect ratio any particular aspect ratio for moving images nor any physiological rea­ to a narrower one in this method, the son to favor one over another. The research did show that cinematogra­ heights of the two images are matched, phers have not always favored the same aspect ratio. and any excess width in the wider image is removed from the display. The A 1988 paper entitled "Another That incompatibility became most position of the displayed rectangle in ./'"'\..Method of Aspect-Ratio Conver­ noticeable in 1961, when the 1953 the pan-and-scan mode may vary either sion For Use In Receiver-Compatible CinemaScope movie, How to Marry a by gradual panning (or tilting, in the EDTV Systems" begins: ''Two systems Millionaire, was broadcast on the NBC case of accommodation of a narrower with different aspect ratios are inherently television network.' Intended to be seen aspect ratio) or by rapid repositioning incompatible."' (EDTV is extended-def­ at an aspect ratio of 2.55: 1 (and with (cutting) between frames. inition television.) The statement bears image composition intentionally filling Figure 1b shows the shrinking looking into. the wide frame), the movie was truncat­ method, referred to as "letterbox," due For the purposes of this paper, aspect ed to television's 4:3 (1.33:1). Almost to the shape of the shrunken image ratio will be defined as the ratio of an immediately, technical publications window when a wider aspect ratio is image's width to its height. Ever since began to carry information about how being accommodated on a narrower there have been rectangular images, best to deal with the "conversion" of display. The black bands need not be there have been aspect ratios (and it one aspect ratio to another.' evenly spaced. It is often the case that may be argued that even elliptical the lower band (when a wider aspect images have aspect ratios). Aspect Ratio Accommodation ratio is being accommodated) is made We are surrounded daily by multiple In fact, imagery is not "converted" larger for the purpose of carrying subti­ aspect ratios not seeming to cause any from one aspect ratio to another; one tles, and, as will be discussed later in incompatibility problems. Images in aspect ratio is merely accommodated this paper, when a narrower ratio is newspapers and magazines have a vari­ by another, almost invariably with being accommodated, the elimination ety of aspect ratios both greater and less than one; the same is true of paintings and photographs. Even some computer display screens may be rotated from a horizontal aspect ratio (landscape) to a vertical one (portrait). When theatrical motion picture and television screens are !IJJJ OJ considered together for the purpose of displaying the same imagery, however, the inherent incompatibility becomes ..r---···-------------, more clear. Presented at the 137th SMPTE Technical Conference in New Orleans (paper no. 137·61) on September 8. , J0 1995. Mark Schubin is a technological consultant in New York. NY 10036. An unedited version of this paper appeared in Moving Images: Meeting the B c Challenges, SMPTE. 1995. Copyright © 1996 by the A Society of Motion Picture and Television Engineers. Inc. Figure 1. Aspect ratio accommodation: (a) truncation; (b) shrinking; and (c) distortion. 460 SMPTE Journal, August 1996 Authorized licensed use limited to: Mark Schubin. Downloaded on August 22,2016 at 17:03:04 UTC from IEEE Xplore. Restrictions apply. SMPTE TUTORIAL of one of the side bands offers the pos­ preserves the original image composi­ oped for excitation of the unused areas sibility of stacking additional images in tion but reduces the visual angle avail­ with signals that vary to match average the other side band, a technique that has able to the viewer and, often, resolution picture level, however, and those tech­ been referred to as multiple picture-in­ as well. Detail that is just perceptible in niques appear to eliminate image strip­ picture (MPIP). an image when it is viewed at a particu­ ing." No investigation of viewer accep­ Figure lc shows the distortion lar display resolution will be lost if the tance of display stripes with varying method, whereby the linearity of the same image is shrunk on the same dis­ brightness was found in the research for geometry of the image is changed to play. this paper. squeeze it into a different display In most cases when the viewing The differential phosphor luminance shape. In a recent variation on this tech­ screen is video-based, this shrinking decay issue is related only to displays nique, a nonlinear distortion is used, results in noticeably empty portions of using phosphors, such as those based affecting the edges of the image more the display device, a condition that has on typical direct-view or projection than the center (e.g., in two of JYC's been considered objectionable to audi­ cathode-ray tubes. Some video projec­ consumer widescreen projection ences by some television program­ tors, such as the Schlieren-optics-based receivers). mers." One television set manufacturer Eidophor,!" have never used phosphors, As can be seen from the two rows of (lVC) has introduced a widescreen and advanced television displays may Fig. 1, the same basic methods apply model with a mechanical masking sys­ be able to take advantage of other phos­ whether the original image is wider tem that covers the unused portions of phor-free technologies. 11.12 than the display or narrower. In fact, the display much as drapes mask There are two other techniques asso­ the same techniques apply whether the unused portions of some motion picture ciated with aspect ratio accommoda­ two aspect ratios are both film, both theater screens, possibly resulting in the tion, but they require that either the video, or one of each. From 1961 to reduction or elimination of such objec­ image or the display be effectively non­ date, however, generally only the tech­ tions.' fixed in shape. One of these techniques niques of the upper row have been A potentially more serious problem is sometimes used in video walls. As seen, as widescreen movies have been related to the shrinking technique is shown in Fig. 2, a video walI com­ shown on narrower video screens in differential phosphor luminance prised of 4:3 image modules can create homes, aircraft, or other venues. Unless decay, a reduction in the light output a 4:3 image when stacked in a 3 x 3 or otherwise specified, the word wide­ of the cathode ray tube phosphors in 4 x 4 module configuration, but the screen, for the purposes of this paper, the active picture section relative to same modules can create a 16:9 will be used as defined by the British that in the blank section, often affect­ (1.78:1) image when stacked in a 4 x 3 Kinematograph Sound and Television ing blue phosphors more than red or configuration. Society (BKSTS): "in general, pictures green." As a result, when the full dis­ When the goal has been not aspect presented with an aspect ratio greater play area is viewed, the shrunken ratio accommodation but the creation than 1.4:1."5 image area can become visible as a of a different aspect ratio than is com­ AlI of the accommodation tech­ stripe somewhat yellower than the rest monly used in a particular medium, niques of Fig. 1 are problematic. of the display. The effect is greater in similar modular-screen techniques have Sometimes aspect ratio accommodation projection displays than in direct-view been used in many film and video pro­ is demonstrated with so-called "neu­ displays due to the higher beam cur­ jection systems. These range from the tral" imagery: pictures that appear no rents of the former. 19th-century Cineorama system (using less desirable when cropped. Motion It has been suggested that the differ­ ten interlocked motion picture film pro­ pictures and television shows are not ential phosphor luminance decay prob­ jectors)" to the current Geographica shot to be neutral, however. The trunca­ lem may be eliminated by making the theater (using three synchronized video tion technique clearly causes portions inactive sections of the display gray sources) at the National Geographic of the image to be lost, and the variants instead of black, but in one experiment, Society's Explorers Hall in Washing­ associated with pan and scan introduce the outline of an inactive section of a ton, D.C.