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Dealing with Different Aspect Ratios

This is my first post since the updated Projector Reviews web site has gone 'live'.  A number of previous blog posts have missing photos and illustrations.  Hopefully, all of these web site transition issues will be corrected over the next few weeks. This blog post includes a discussion of dealing with video material with different aspect ratios and also provides a few updates to information posted in my previous blogs.

Constant Image Width Animation


I'm working on a review, hopefully to be posted later in December or early January, for the DarbeeVision  Darblet DVP 5000 image enhancer.  This relatively inexpensive gadget has become popular among home theater owners and based on my initial evaluation there is a good reason for this. JVC's new (i.e., 2015 models) projectors should start showing up at dealers in the first half of December.  These new models still use JVC's eShift technique to provide a pseudo 4K image and these new models can accept a native 4K signal input.  However, it is unknown if they will be compatible with future Blu-ray 4K players when they become available, perhaps late in 2014.  The main issue is these new JVC models use HDMI 1.4 enhanced with some features from HDMI 2.0 but do not support the latest version of HDCP (High Definition Copy Protection) which may be required by the upcoming Blu-ray 4K standard. Sony is also expected to begin shipping their new, lower priced, 4K Ultra HD projector (model VPL-VW600ES) during December.  The VW600ES is a model for the USA while the similar VPL-VW500ES is already starting to show up at dealers outside of the USA.  First reports from international owners of the VPL-VW500ES indicated it has nearly the lumens output of Sony's earlier (and superb) VPL-VW1000ES 4K projector, but falls short of that flagship model's black level performance.  The VW1000ES is being replaced by the soon-to-be-released VPL-VW1100ES, while Sony is offering a upgrade package to existing VW1000ES owners that will effectively make those existing projectors into the new VW1100ES model.  This upgrade, among other things, changes out the HDMI hardware to provide HDMI version 2.0 inputs equipped with the latest HDCP copy protection version so they should be compatible with Blu-ray 4K players/discs when they become available, perhaps in about a year. There has been no recent news related to the progress being made toward the the new Blu-ray 4K specification.  The next official announcement will probably come at the Consumer Electronics Show in early Jan. 2014.  The new Blu-ray 4K system is expected to use the HEVC, defined by the h.265  standard, for the encoding of the 4K video.  That standard was first released nearly a year ago and the first amendment to h.265 is expected to be released soon.  However, there has been several reports of contention among the developers of HEVC over patent licensing issues.  It appears several different companies hold patents that cover portions of the technologies employed by HEVC.   According to published reports, some dating back back perhaps 6 months, these companies have not been able to agree on either a single license authority and the amount of royalties that will be distributed to each patent holder.  Hopefully this will get resolved soon so as  to not delay the introduction of Blu-ray 4K.  


Most of you are probably already  familiar with video in different aspect ratios.  Older movies and standard definition TV used an aspect ratio of 1.33:1 (sometimes called 4x3) where the image is 1.33 times as wide as it is high.  The standard aspect ratio for HDTV, and the new Ultra HDTV, is 1.78:1 (also called 16 x 9).  However, the motion picture industry has used a wide range of aspect ratios for their theatrical movie releases with most modern movies having aspect ratios between 1.85:1 and 2.39:1.  In a theater, be it a commercial cinema or a home theater, there are two different techniques used to display video with differing aspect ratios. Constant Image Width (CIW) approach -   The first approach is called Constant Image Width and, as the name implies, the width the image remains fixed and the vertical height of the image become taller or shorter depending on the aspect ratio of the source video material.  This CIW approach  is illustrated with the following animation (click on image to view full size animation).
Constant Image Width Animation

Click to show Constant Image Width Animation

  Note that the above illustration would only fully apply for the case where the display has a 1.33:1 aspect ratio and the actual image would appear as being "letterboxed", with black bars displayed at the top and bottom for any widescreen image (i.e., with an aspect ratio greater than 1.33:1).  However, the more common case today is where the display, or projector and screen, have the standard HD aspect ratio of 1.78:1 (i.e., 16 x 9) in which case only video source material with an aspect ratio of 1.78:1 or greater (i.e., wider) can be displayed using the constant image width approach illustrated above, while older 1.33:1 video material would be displayed with black vertical bars on each side (referred to as a 'Pillarbox") to frame the more squarish image (unless video processing is used to stretch the image to fill the screen).   This approach is essentially the technique that is inherently built into all HDTVs and most modern home theater projection systems.  See below for CIW use in commercial cinemas. Constant Image Height (CIH) approach - The second approach for dealing with different aspect ratio source material is called Constant Image Height.  As this name implies, the vertical height of the image remains fixed while the width of the image becomes wider or narrower depending on the aspect ratio of the source video material.  The CIH approach is illustrated with the following animation (click on image to view full size animation).
Click to show Constant Image Height animation

Click to show Constant Image Height animation

The CIH approach essentially requires the screen to have an aspect ratio that matches that of the greatest aspect ratio you wish to accommodate without the necessity of displaying black bars on the screen.  That generally means using a projection screen with an aspect ratio of between 2.35:1  and 2.40:1, which matches the widescreen aspect ratio used my the movie industry for movies presented CinemaScope(TM) (i.e., 1.35;1 for older films and 2.39:1 for newer films) or Panavision(TM) (i.e., 2.39:1) formats (sometimes referred to as 'scope format).  Most, but not all, projectors have no built-in facilities for directly projecting the video in a CIH format.  Traditionally the film projection systems used by commercial movie theaters supported CIH for the two aspect ratios most commonly used for theatrical films.  Since the 1950's an aspect ratio of 1.85:1 as been the industry standard for films while films distributed with the wider aspect ratios used by CinemaScope and Panavision were accommodated by using an anamorphic lens placed in front of the projector's lens to horizontally expand (stretch)  the image by approximately 30%. For home theater system there are two approaches commonly used to provide a CIH capability.  Again this requires a wide aspect ratio projection screen, typically with 2.35:1 or 2.40:1 aspect ratio. Zoom Method for CIH - The simplest, and least expensive, method for CIH is the so called 'zoom method".  With this approach the projector's zoom lens is set to project an image that fills the full width of the screen when projecting a movie provided in the wide 'scope format (there are many movies offered in Blu-ray Discs in this format).  In this case the black bars present on the Blu-ray recording will now  fall above and below visible area of the projection screen and the widescreen 'scope images from movie will just fill the screen's height and width.   Then when projecting a program with a normal HD aspect ratio of 1.78:1 the projector's zoom is adjusted to reduce the image size to match the vertical size of the screen.  This will produce unused space on the right and left of the screen, essentially with the image appearing in what is called a  "pillar box" with vertical black bars on both the right and left sides of the screen visible area.  In order to use the zoom method for CIH the zoom, focus and (in many cases) lens shift will need to be adjusted to correctly size, focus and center the image.  This is not a very practical approach for CIH when using projectors with only manual adjustments for these items.  While it is more practical to create a CIH system using projectors with remote control power adjustments for zoom, focus and lens shift.  Since the 'scope image needs to be one third wider than a standard HDTV image that means this approach needs a projector equipped with a lens with at least a zoom ratio of at least 1.33:1 or more ideally 1.5:1 or greater.  Also the projector will be need to be placeed with a projector-to-screen throw distance that allows the projected image size to be zoomed between the required maximum width (for viewing 'scope movies) with down to the required minimum width (for viewing standard HD video).  There are now several Home Theater oriented projectors available that include a lens memory function to automate going between different aspect ratios.  Panasonic was the first manufacturer to include the lens memory feature in their flagship home theater models (the most recent model being the PT-AE8000) while JVC and Sony are now including the lens memory feature in several of their models.  The Sony VPL-VW95 and the newer 4K models (e.g.., VPL-VW1000) also offer a lens memory function.  JVC offered a lens memory in last years models, such as the DLA-X95, as well as the new 2015 models.  Anamorphic Lens Method for CIH - The second approach for home theater owner's seeking to set a CIH system is based the traditional approach used in commercial cinemas.  There are three key ingredients for implementing this approach.  First the projector must be able to optically and mechanically accommodate an external anamorphic lens placed just front of the projector.  Second video processing must be provided to vertically stretch the image (either built into the projector or provided with an external video processor).  Third a compatible add-on external anamorphic lens must the mounted in just in front of the projector.  Such anamorphic lenses work best when the projector's lens has a relatively long throw ratio (i.e., providing lower magnification).  This latter point means that you would typically need to set the projector's lens toward minimum zoom and mount the projector toward the maximum supported projector-to-screen throw distance.  The advantage of this approach to CIH is you can use all of the pixels on the projector's display panels rather than three forth of them (for a typical 1.78 aspect ratio HD projector).   This is done by electronically vertically stretching the image by 33% then optically (with the add-on anamorphic lens) horizontally stretching the image by an equal 33%.  This approach in many cases will results in a somewhat brighter image, but has the potential of introducing distortions due to the additional optics involved. Does 4K Made a Difference - A number on owners of new 4K projectors have expressed an interest in using the traditional anamorphic lens method for implementing CIH.  However, the Sony 4K projectors and the JVC 4K-lite models have lens memory features built-in.  I did recently note, when viewing a couple recent movies at the local Regal Cinema multiplex, that is equipped with Sony 4K digital cinema projectors, they were using a CIW approach and they did not appear to be using an anamorphic lens for presenting widescreen 'scope' movies in 2.39:1 format (as contrasted to the the use of an anamorphic lens that was typical with film projectors). Screen Masking -  Some home theater owners add masking systems to change the aspect ratio of the visible portion of the projection screen to match that of the video content.  For a CIW system, masking systems are added to the screen to place movable horizontal masking panels across the top and across the bottom of the screen that are adjusted up or down to reveal only the portion of the screen that corresponds to the actual aspect ratio of the image that is being projected.  For a CIH system,  vertical masking panels are placed along the two sides of the screen and move horizontal to reveal only the portion of the screen that corresponds to the actual aspect ratio of the image that is being projected.  There are also a few 4-way masking systems available that allow the user to apply horizontal and/or vertical masking as needed.  Several of the screen manufacturers, such as Stewart and Carada offer their own masking systems while there are also some third party 'universal' masking systems available for use with fixed frame screens. ____________________________

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