Who Controls AFD?

By now, most local TV stations have probably completed surveys from the cable and satellite provider that carry them, gathering all of the technical data necessary to continue to provide basic level service for subscribers once the local station shuts off its analog transmitter next February. Oddly, it was only after the FCC's mandate that cable systems carry a downconverted version of the primary service of a local station's digital signal that a problem that we have known about for years was brought to the surface.

That problem is putting widescreen content on a conventional display and conventional content on a widescreen display. Since there is no real difficulty with doing this, the actual problem is the black bars on the top and bottom, or sides, or both.

As I travel around Iowa doing DTV presentations, I am frequently confronted by viewers who want to know why this happens. Some of them think that we are cheating them by not filling their screens, but after explaining aspect ratios to them and a couple of demonstrations, they at least understand that they are not being cheated. But they still don't like black bars.

This really isn't a significant problem for viewers with new widescreen TVs or even people using converter boxes since all of these devices allow the viewer to modify the image to fill the screen by either cropping or anamorphic stretching the image. Our position at Iowa Public Television has always been to digitally broadcast the content in the screen format in which it was created and allow the viewer to adjust the image. Problem solved…or is it?


This is a great solution provided that the viewer has the ability to control how the screen handles various aspect ratios. But what about the majority of viewers who subscribe to cable or satellite and will continue to use their analog TVs long after the end of analog transmission? To accommodate those viewers—and in the case of cable—to meet the federal mandate, one of the questions that is asked on the technical survey is whether you want your signal center cut or letterboxed. So far I haven't seen anamorphic stretch as an option.

So here we are being asked to select between two non-ideal options on an "all or none" basis that for the foreseeable future will impact how the majority of our audiences view us. No matter which choice is made, some of our viewers will not be happy. Those with conventional big screens are going to wonder about the two talking noses in the cropped widescreen content. Meanwhile the person watching the letterboxed panorama image on the 13-inch set in the kitchen may not be able to even tell what the content is about. No kidding, I watched a center cut of "The Good, the Bad and the Ugly" and was completely confused by the disembodied voices. I also watched a letterboxed version of "Spartacus" on a 13-inch set and the scenes of the thousands of people running down the hills were indecipherable.

To deal with this, several industry standards bodies have developed "Active Format Description" that is carried in the DTV stream and provides instructions to the encoder or TV set on how to frame the image. I don't want to say that AFD is a solution to the problem because it really isn't in the case of the cable/satellite carriage of a station's primary digital feed. It does however put some dynamic control back in the hands of the station to make intelligent decisions based on the content. And when married with digital receivers in the viewer's home, it allows the station to broadcast the content as the creator intended and lets the viewer adjust the content to suit their preferences.


But even if the viewer doesn't have that control because they are receiving service from cable or satellite and using a 4:3 television, AFD can still provide some level of intelligent control to mitigate some of the problems. Here are some examples where AFD could be used to minimize viewer dissatisfaction.

Many news and variety shows are now being created in widescreen but protect the 4:3 center. When these shows are broadcast, using the 4:3 protect description allows a center cut with little detriment to the original program content.

Likewise, widescreen content that would be degraded by cropping can be sent with the 16:9 with no cropping description. Another variation for widescreen might be to send it with the 16:9 protected in 14:9 description. I must confess that I initially didn't see much value in the 14:9 option but that was because I was looking at content on 16:9 displays. I initially found the benefit of this by accident while doing a converter box test using the "Spartacus" movie on a small set. The box I was using offered 14:9 as one of the aspect ratio selections (not all of the converter boxes do this) and I noted that on the small display, the change in the aspect ratio from true letterbox to 14:9 actually made the images more viewable.

The road to using AFD is not as easy as I hoped. Properly applying it will require that the AFD data be incorporated into the content creation process and become part of the metadata used by traffic and automation systems in the ingesting and play-out of the content. In our environment we use Pro Track traffic software and Omnibus automation and I am told that both are working on integrating AFD into their respective systems. But I haven't seen anything that convinces me that they will be ready to go before the end of analog.

Couple that with the current push by satellite and cable to get the digital-to-analog downconversion for basic service users completed and there is cause for concern. During a panel discussion at the MSTV engineering breakfast at this year's NAB Show, I asked cable representatives if their receivers would recognize AFD data and I was told that they would eventually, but not immediately. I happened to be sitting with a representative from DirecTV who also confirmed that AFD was not in their initial rollout plans.

So even if a station applies the data and gets their automation and traffic systems to work with it, the majority of viewers will not see the benefit until the receivers used by cable and satellite are AFD-capable. I am not aware of any firm time commitment on when that will happen.

So what are we going to do? At IPTV, we will work with our vendors to ensure that all of our locally created content has accurate AFD data incorporated into the program. Content that comes from outside sources that includes AFD data will pass though our systems to the viewer, and content without AFD will have it applied locally based on an analysis of the images within. Beyond that we will continue to request that other systems that deliver our content also use AFD to ensure the viewer gets the best possible viewing experience.

(For a more detailed explanation of AFD, how it works and what it accomplishes, see Jerry Whitaker's ATSC Update: "Using AFD to Resolve Aspect Ratio Discrepancy," in the July 12, 2006, issue of TV Technology. NAB also provides a helpful resource at www.nab.org/AFDready).

Bill Hayes

Bill Hayes, director of engineering and technology for Iowa PBS, has been at the forefront of broadcast TV technology for 40 years, 23 of them at Iowa PBS. He’s served as president of IEEE’s Broadcast Technology Society, is a Partnership Board Member of the International Broadcasting Convention (IBC) and has contributed extensively to SMPTE and ATSC.  He is a recipient of Future's 2021 Tech Leadership Award.