AFD - Aspect Ratio Management using AFD - June 2007
Images — Letterboxed. Pillarboxed. Postage-stamped. Horizontally stretched or squeezed. We’ve all seen them, and wished we hadn’t. The migration from 4:3 to 16:9, plus the rapid introduction of widescreen cameras and displays, has raised the issue of managing aspect ratios to optimize the image for the display’s shape. More than 30 attended the Washington, DC, section’s June 2007 meeting, held at WETA’s Arlington, VA, facility, at which our featured speakers addressed that subject.
Graham Jones (NAB’s Director of Communications Engineering; Chair — SMPTE S22 Image Formatting Group), Michel Proulx (Miranda’s CTO), and Brian Markwalter (CEA’s VP for Technology & Standards) provided a succinct background and explained how SMPTE, ATSC, and CEA standards for Active Format Description (AFD) & Bar Data (in place after only two years intense effort) will help solve the problem in the broadcast facility, for transmission, and in the receiver/display.
Broadcasters will continue to use new and old material produced in 4:3, plus programs created in 16:9. Meanwhile, in addition to the rapidly growing numbers of 16:9 displays being bought, millions of perfectly working 4:3 sets will be used by people who won’t discard them. Thus, at both ends of the broadcast chain, multiple aspect ratios will coexist for many years.
SMPTE’s S22 Image Formatting Group’s goals have been to:
· Optimize the image display for the viewer, taking into account multiple production aspect ratios and at least two display aspect ratios.
· Provide guidance for the format conversion process in post production, pre-broadcast, and in the receiver/display.
· Minimize changes to the originally created image.
· Retain critical image content.
Manual adjustment and data entry must be avoided as much as possible. To solve the aspect ratio problem, metadata that is carried from program creation through the entire production and broadcast chain to the home display is the only rational approach. AFD & Bar Data is intended to guide downstream format converters, image scalers and display processors to show the optimum picture.
The metadata needed has been undergoing development in three parts:
· AFD & Bar Data was added to ATSC A/53B DTV standard in 2002. It is carried in the MPEG-2 transport stream video user bits — the vertical ancillary data space. The codes are based on European DVB/ETSI standards. However, up till now, they have not been implemented by broadcasters or in receivers in the US.
· In June 2004, the ATSC Implementation Subcommittee issued IS-312 “Optimization of Image Formatting for Transmission and Display.”
· During 2005-6, a joint initiative involving SMPTE, the ATSC and the CEA produced an end-to-end standardized system solution for image formatting using AFD & Bar Data.
· This resulted in a three-part system solution:
· SMPTE 2016, which includes AFD & Bar Data generation and entry, plus handling of that data throughout production and post-production, on into the broadcast chain up through editing/switching. SMPTE 2016 has five parts:
1. SMPTE 2016-1: “Format for AFD & Bar Data” specifies the raster format and the active image area.
2. SMPTE 2016-2: “Format for Pan-Scan Information.”
3. SMPTE 2016-3: “VANC Data Mapping of AFD & Bar Data” defines carriage in SMPTE 259M and SMPTE 292M serial bitstreams.
4. SMPTE 2016-4: “VANC Data Mapping of Pan-Scan Information.”
5. SMPTE 2016-5: “KLV Coding for AFD, Bar Data & Pan-Scan Information,” needed for MXF. This section is in preparation.
In addition, a SMPTE Engineering Guideline on system implementation is in preparation.
· ATSC A/53 Part 4, which includes the ATSC encode/MUX and transmission, added clarifications to the semantics to match CEB16.
· CEA’s CEB16 “Active Format Description & Bar Data Recommended Practice” (published July 2006) reviews methods for representing AFD, aspect ratio information and bar data, and provides guidance for consumer devices (receivers, STBs, and displays) and recommendations for output signaling and display appearance.
Displays are also affected by Aspect Ratio Control (ARC; SMPTE RP-186; carried in the VBI; used within the broadcast plant), the widescreen signaling (WSS; carried in the VBI in transmission to the viewer), CEA 861 (for HDMI uncompressed video), and CEA 805 (for component analog video).
AFD, which is independent of image resolution, uses a four-bit code that defines active image area and protected image areas for various combinations of aspect ratios. In other words, it guides downstream devices in automatically adjust the image’s shape, optimizing the image aspect ratio (including cropping and framing) to the display’s aspect ratio. It has been designed to eliminate postage stamped images.
While AFD & Bar Data will most often be used with 16:9 and 4:3 images and displays, it has been designed to accommodate other aspect ratios, and includes codes for 14:9. The Bar Data will accommodate non-standard aspect ratios for which no codes have been established, such as for Cinemascope movies, by delineating the thickness of the bars in number of lines or pixels for the aspect ratio being processed or transmitted.
The Pan-Scan information uses vertical and horizontal image size, as well as vertical and horizontal offsets to control processing and display.
AFD & Bar Data has been implemented for NBC’s HD/SD operation in New York, and for HD/SD simulcasting at the Canadian Broadcasting Corporation.
AFD & Bar Data offers a way for content creators to optimize the image aspect ratio and protected image area seen by viewers whether they have a 4:3 or a 16:9 display. It still depends on someone setting the parameters during program creation, plus equipment at every stage in production, post-production, broadcast and reception/display that properly handles, and where necessary passes or appropriately modifies, the AFD & Bar Data.
Thanks to Ed Kennedy, Pat Williamson, and Chris Lane at WETA-26 for their hospitality, and Miranda for providing excellent refreshments.
David J. Weinberg (Manager, SMPTE-DC)
PowerPoint from the SMPTE DC presentation on AFD & Bar Data Graham Jones, NAB
Michel Proulx, Miranda; Christopher Lane, WETA; Brian Markwalter, Consumer Electronics Assoc.