Skip to main content

Active format descriptor

Are you ready for active format descriptor (AFD)? Or maybe you're already sending it with your content. You should be. AFD codes will play a significantly important role in how content appears to your viewers.

Simpler times

Some of us remember simpler times. Signals were analog, there was only one aspect ratio, and viewers received signals over-the-air or via cable, which at worse, frequency-converted a signal to a different channel slot. Back in those days, the viewer essentially saw what broadcasters transmitted — subject to the vagaries of the transmission path or the cable operator's trunk amplifier gain settings.

Remember those third-order distortion products that manifested themselves by appearing as faint video lacking horizontal and vertical sync, seeming to float through the background? I won't get into the signal processing characteristics of those early television receivers and what they did to your signal! But certainly one constant was aspect ratio — 4:3 format was 4:3 format was 4:3 format. Oh, some of those old receivers would underscan or even overscan the raster, but fundamentally it was all 4:3. Even those early, fully round CRTs were masked to 4:3.

A cornucopia of technology

Let's look at what viewers have today with regard to video display and device technology. In addition, let's discuss what they use to view that signal you take such painstaking care to deliver in its richest, full frame video fidelity. Come February, many viewers will be using a new device called an analog converter box. Current statistical surveys vary, but most indicate that about one-third of total TV households have HDTV, and that number will continue to grow at double-digit rates. Unfortunately, slightly less than half of those households are actually viewing HDTV signals on their expensive HD sets, but that's another matter.

DTV receivers represent a virtual cornucopia of digital processing techniques and display options for handling digital signals. Thanks to the continuing price drop of true HDTV receivers, 4:3 DTV receivers and enhanced-definition TV (EDTV) receivers have gone by the wayside, but there is still a generation of them being used. 16:9 HDTV receivers offer the viewer a variety of options for handling 4:3 formatted content, from pillarbox to stretch to cropping. 4:3 DTV receivers for 16:9 content typically offer letterbox, a 14:9 or even a full crop. Are you confused yet? Imagine the poor consumer who is viewing 4:3 letterboxed content on a 16:9 display and winds up with a double-boxed picture — black bars on all four sides!

Then there are many viewers who do not rely on off-air reception. Instead they subscribe to a cable, satellite or telco fiber system. Those carriers take a broadcaster's precious signal and demodulate it, remodulate it and statistically multiplex it. In other words, they do anything they can to squeeze it into the smallest possible, bandwidth-stingy bit stream. Oh, and along the way they may decide to do an extra format conversion for you.

AFD to the rescue

Enter AFD. AFD is a set of standardized codes adopted by both the ATSC and SMPTE. They can be embedded in an MPEG stream or in a baseband SDI signal. These codes define information about aspect ratio and active picture characteristics. As most consumer manufacturers of receivers and set-top boxes support AFD, it is now an effective tool for broadcasters. It communicates to appropriately enabled receivers and set-top boxes and lets them know how to display the attached content. The codes enable the best display on both 4:3 and 16:9 television sets of content transmitted in either format and can dynamically control downconversion circuitry, which formats widescreen 16:9 pictures for display on older 4:3 receivers.

In addition to its comprehensive format control capabilities, AFD also provides signaling controls for active picture information. Active picture codes identify the active video in the coded picture as well as the protected area that must be shown. A significant amount of program material is shot today in a “shoot 16:9, protect 4:3” capture mode. As such, information outside of the protected area at the sides or the top can be lost in 4:3 crop without impairing the intent or meaning of that particular scene.

What's in store

We will be living in a world of mixed display and resolution formats for many years, with AFD being a powerful tool during this period. Content provided to broadcasters has or will soon have embedded AFD codes. Broadcasters need to ensure they insert appropriate AFD codes in their own locally produced content.

Yep, things used to be a lot simpler, but would you really want to go back to watching next Sunday's football game in plain old analog 4:3 NTSC?

Anthony R. Gargano is a consultant and former industry executive.

Send questions and comments to:anthony.gargano@penton,com