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Managing AFD

Jan 1, 2009 12:00 PM, By Clarence Hau

Keep image format under your control.

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.

Figure 9. Headend signal path with AFD
Click to enlarge

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.

an illustration of
a common scenario of an SD
only station that receives HD content
from the network.

Figure 10: Local station implementation scenario A
Click image to enlarge

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.

An illustration of
a scenario where network
content provided in a pre-encoded
ATSC stream.

figure 11: Local station implementation scenario B
Click image to enlarge

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

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.