Managing AFD
Jan 1, 2009 12:00 PM, By Clarence Hau
Keep image format under your control.
Figure 1. Example of 16:9 and 4:3 shot selection
The 2009 analog shutdown is here. As programming content will continue to originate in both SD and HD for many years, broadcasters have been forced to find ways to maintain the presentation quality for home viewers after the Feb. 17, 2009 deadline.
Active format description (AFD) has become the cornerstone of a number of broadcasters’ strategies for management of aspect ratio throughout the delivery path to the home. With Managing Keep image format under your control. By Clarence Hau thousands of different touch points, end-to-end deployment of AFD can be a challenge, but significant progress has been made. This article will look at how broadcasters plan to use AFD in the post DTV transition landscape, as well as what steps have been taken to deploy AFD throughout the end-toend delivery path to the home.
Dual aspect ratio challenges
The amount of HD programming available to home viewers has grown considerably over the past few years. In this environment, TV broadcasters are faced with challenges on how to best format and deliver their content to both HD and SD audiences.
Figure 2. Letterbox and center cut downconversion illustrations
When HD programming was introduced by broadcasters, a parallel production and distribution process was most commonplace. For scripted programming, two unique versions of a show were normally provided by the content providers, i.e., an HD version and an SD version. For live programJanuary 2009 | broadcastengineering.com 53 Feature Managing AFD ming, 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 on page 54. 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 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.
Figure 3. Example of adversely affected center cut downconversion
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.
Figure 4. Example of adversely affected letterbox downconversion
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 56 shows the most commonly used AFD codes along with their usage for HD to SD downconversion.
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