Broadcast Engineering - February 2009 - (Page 35)

D content providers, i.e., an HD version and an SD version. For live programming, two separate control rooms and production groups were often used. One of the main benefits of this approach was being able to maintain full control on how programming was viewed by both HD and SD audiences. (See Figure 1.) With a changing distribution landscape and increasing cost pressures, broadcasters have been forced to move toward a more efficient production and delivery approach. In this model, a single version of content is produced and delivered in HD. The SD version is derived from the HD version through a downconversion process. The most common methods of downconversion, center cut and letterbox, are illustrated in Figure 2. The use of downconversion to generate SD from HD introduces certain restrictions on how each version can be presented to viewers. As the aspect ratio of HD (16:9) and SD (4:3) formats differ, certain decisions must be made on how to reformat the original HD Feature Managing aFD HD 16:9 original 4:3 center cut version Figure 3. Example of adversely affected center cut downconversion HD 16:9 original (pillarbox) Letterbox version (postage stamp) Figure 4. Example of adversely affected letterbox downconversion signal into SD. HD content produced for center cut SD delivery must be center cut safe. Center cut safe video content has no important details on the left and right sides of the screen. This content can be center cut to generate a properly displayed full screen 4:3 image. However, for HD content that takes full advantage of the entire 16:9 frame, a center cut downconversion can result in 4:3 content that is missing important details. Figure 3 illustrates how full frame HD content can be adversely affected when center cut. The preferred downconversion method by many HD content providers is letterbox. Letterbox downconversion is the best way to ensure that all image details from the original HD program are preserved for the 4:3 SD audience. However, much of the content in HD distribution paths is upconverted from SD sources. When upconverting 4:3 SD signal to HD, the result is a pillarbox representation (with black bars on the right and left sides of the 16:9 frame. Letterbox downconversion of this material will result in a postage stamp representation. (See Figure 4.) Most HD broadcasters will continue to have a mixture of both HD and SD originated content for many years. Locking into a specific downconversion format (center cut or letterbox) will force broadcasters to make compromises on the presentation quality for HD and SD viewers. Dynamic aspect ratio control with AFD AFD is a method of describing aspect ratio and picture characteristics of video signals. It has been used to control how television sets optimally display pictures transmitted with varying aspect ratios. In 2007, SMPTE released SMPTE 2016-1 and 2016-3. With these new standards, AFD codes were updated, and a method of carrying AFD within baseband SDI video signals was defined. One of the first implementations of the new standard was dynamic control of aspect ratio on baseband video format converters. These AFDsupported downconverters opened the door for new production and distribution methods optimized for dual format delivery. Table 1 on page 36 shows the most commonly used AFD codes along with their usage for HD to SD downconversion. Production scenarios One of the main benefits of AFD is the ability for program providers to maintain creative control of their content. With proper use of AFD, broadcasters can maintain this February 2009 | broadcastengineeringworld.com 35 http://www.broadcastengineeringworld.com

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Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

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