Managing AFD

Keep image format under your control.
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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.

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.

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 56 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 creative control by allowing the audience to view content as the content creator originally intended.

The most proven use of AFD is applied at the final stage of production — not on individual production elements. Within this model, the proper AFD code is recorded onto the final version of content as it will be aired, whether it be a live show, recorded program or commercial spot.

From an operational standpoint, a predefined downconversion format should be assigned to HD programs or commercial spots through application of the proper AFD code.

For live programming produced from a control room, a device can be used to insert the proper SMPTE 2016-3 AFD code into the main program output. This will help to ensure that the proper AFD code is attached to the final version for air and available to all downstream distribution systems. (See Figure 5.)

For preproduced programming, a number of options are available for AFD authoring. If the output of the production session is HD-SDI, the same SMPTE 2016-3 AFD insertion described above can be effectively used.

For finished content that is delivered via tape (e.g. commercial spots) or baseband video (e.g. from edit rooms), the appropriate AFD data can be authored during the ingest process as shown in Figure 6 on page 58. Using this model, a process should be established to allow the content providers to select how their HD content should be downconverted.

With file-based transfers from NLEs to playout servers becoming more common, AFD insertion in this domain has also been successfully implemented. This production workflow allows for finished NLE projects to be automatically transcoded and transferred to playout servers without going back to baseband video. As illustrated in Figure 7 on page 58, a completed project can be transcoded and transferred directly to a playout server with the desired AFD code attached.

Many HD servers preserve vertical ancillary data space (VANC) on ingest and playout. However, each manufacturer has a proprietary method of storing VANC information in its internal file formats. For this reason, simple file transfers in and out of playout servers may result in the loss of AFD data. Limiting these types of file transfers between the same manufacturer models will help to protect against AFD data loss. File compatibility and VANC preservation must be tested and established.

Distribution and AFD preservation

As discussed in the previous section, ensuring that AFD data is encoded on final versions of content is the first important step in establishing a successful AFD driven process. Ensuring that the AFD data is preserved through the downstream signal path within the broadcast facility is just as critical.

AFD data in accordance with SMPTE 2016 is carried in the VANC of SDI video signals. Specific attention should be paid toward ensuring all downstream video processing systems (e.g. video routers, switchers, DAs and frame syncs) will reliably pass VANC data uncorrupted. The following recommendations should also be considered:

1. Focus on HD signals.
System designs should be focused on carriage of AFD data in the HD-SDI signals for the purposes of downconversion to SD. Most legacy SD devices have been found to strip out any VANC data present in the video signal, so system designs should not rely on AFD data within SD signals.

2. Select video line location.
Select a specific video line location to carry AFD data to be followed plant-wide. This will be essential for quality control and troubleshooting throughout the signal path.

3. Avoid multiple AFD codes.
For any AFD inserting system that is implemented, specific care should be taken to avoid the introduction of multiple AFD codes in a single video stream. This can cause adverse affects in many AFD supported systems.

4. Insert AFD on upconversion.
SD originated content must go through an upconversion process before being inserted into a facility’s HD distribution path. Where possible, the proper AFD code should be inserted at the point of upconversion. Many upconversion devices allow for this and will help to ensure that the AFD code will be available throughout the facility.

Despite best efforts to encode program-specific AFD codes, mecha mechanisms should be put into place to ensure that a default AFD code is automatically inserted into video streams when authored AFD data is not present. This is best handled close the final distribution point out of the facility as shown in Figure 8.(with Figure 7 above)

Distribution to the home

After the DTV transition, most cable and satellite providers will continue to provide broadcast network signals on their analog tier (or digital SD channels), but will no longer have off-air NTSC signals available to do so. Cable and satellite providers will downconvert HD signals from local stations to provide these SD services to their subscribers. The ATSC has published RP A/79, which provides guidance on capabilities needed on these professional IRD devices including AFD usage.

Leading up to the February 2009 transition, cable and satellite providers will be installing new IRD devices within their headend/local collection facilities. These new devices will downconvert the DTV signals from local stations for delivery to their SD subscribers.

Most of these new IRD devices include support for AFD controlled downconversion. If AFD data is properly encoded into the local stations’ DTV signals, cable and satellite providers will be able to automatically create properly formatted SD signals as shown in Figure 9.

Local station implementations

Proper aspect ratio management will be critical for preserving creative control of programming after the DTV transition. Using AFD technology can improve the capability for program providers to deliver content to their audience exactly as they intend.

Some of the steps discussed in the article should be taken by a national network to insure proper AFD codes on all content delivered to local stations. If network HD programming is delivered with AFD, local station implementation can be straightforward. For simplified implementation of AFD at local stations, the following recommendations should be considered.

1. Insure preservation of AFD data through signal path.
Steps should be taken to insure that AFD is preserved through each station’s downstream HD-SDI signal path.

2. Insert fixed AFD at upconversion from SD plant.
A fixed AFD code of 16:9_9 should be inserted in all upconverted SD originated content. This will insure that all SD originated content is marked with the correct AFD code. Many upconverter devices can be configured to insert a programmable AFD code into the HD-SDI signal, or alternatively a dedicated insertion device can be used.

3. Insert proper AFD codes on HD content.
As discussed in this article, proper AFD codes must be inserted into all HD content. To simplify AFD implementation, a local station may elect to present all local HD content in center cut format for SD viewers. This would require a simple fixed AFD code inserinsertion at the appropriate place.

4. Upgrade ATSC encoder to support AFD.
AFD data must be encoded into the ATSC stream of a station’s primary DTV channel for it to be available to cable and satellite providers. Most manufacturers will provide a software upgrade for existing ATSC encoders. These encoders have the ability to read SMPTE 2016 compliant AFD from its HD-SDI input and frame accurately encode into the MPEG stream.

5. Station downconversion.
For stations that provide direct fiber delivery of the station’s SD signal to cable and satellite providers, the SD signal may be generated from the station’s primary HD signal through downconversion devices. These downconverters should respond to AFD to insure proper formatting of the SD signals.

The example in Figure 10, illustrates the common scenario of an SD only station that receives HD content from the network. A fixed AFD code is inserted into local content and the station’s DTV signal is encoded with AFD.

The example in Figure 11, illustrates a scenario where network content provided in a pre-encoded ATSC stream. AFD codes are properly inserted into HD and SD originated local content and encoded in the station’s ATSC encoder. AFD codes will follow local and network content through the MPEG splicer system.

Conclusion

Proper aspect ratio management will be critical for preserving creative control of programming after the DTV transition. Using AFD technology can improve the capability for program providers to deliver content to their audience exactly as they intend.

AFD will not solve all of the aspect ratio challenges, but existing technical solutions have been proven to be successful at providing the flexibility needed by broadcasters. As AFD usage expands and technology solutions mature, more advanced uses are sure January 2009 | broadcastengineering.com 61 Feature Managing AFD to emerge.

Clarence Hau is director of systems engineering at NBC-Universal. This article is based on a paper he presented at the SMPTE 2008 Technical Conference.