SMPTE Industry Survey of Vertical Ancillary Space Usage

The Society of Motion Picture and Television Engineers has published today a survey seeking information from users and manufacturers about the use of the Vertical Ancillary Data Space (VANC) for carriage of supplementary data. The background to this activity is as follows.

VANC Data
Most television broadcast engineers are familiar with the capability provided by uncompressed serial digital video in accordance with SMPTE standards SMPTE 125M (standard definition) and SMPTE 292M (high definition) to carry program-related data in addition to the video essence. This powerful feature enables large amounts of data to be carried through the same baseband distribution path as the video without consideration of separate routing systems and timing errors. Unlike analog video, horizontal and vertical blanking are not required when video is transported as a digital signal because the start and end of lines and frames are denoted by special codes for start and end of active video. However, the period when video blanking would have occurred can be used for carrying other data, known as Ancillary Data, and the spaces where this is carried are referred to as the Horizontal Ancillary Data Space (HANC) and the Vertical Ancillary Data Space (VANC). HANC Is widely used for carrying embedded audio, while VANC is used for numerous types of data including time code, closed captions, AFD/Bar Data, program description, audio metadata, film transfer data, and many others.

The underlying SMPTE standard that defines the ANC data system is SMPTE 291M Ancillary Data Packet and Space Formatting. ANC data is carried as packets and each data packet type is assigned a Data ID (DID) and Secondary Data ID (SDID) that uniquely identify the type of data. Separate SMPTE standards and recommended practices (RPs) define the payloads for each data type. These individual standards indicate the main location where each data type shall or may be carried, such as VANC field 1 or field 2 and Y (luma) or C (color difference) channels. However neither SMPTE 291M nor (with a few exceptions) the individual data type standards define specific VANC lines that shall be used for each particular type of data. This line number parameter was intentionally left undefined by SMPTE to allow maximum flexibility in implementation. The expectation was that downstream equipment would scan the whole VANC to find relevant VANC data packets, identified exclusively by their DID and SDID.

Issues
In practice, over the years, some types of data have become associated with particular lines, and some (but not all) types of downstream, equipment expects to find them there without necessarily searching the VANC for the DID/SDID. Furthermore, there is a tendency, for various reasons, for manufacturers to default to placing data early in the VANC period, e.g., on lines 9, 10, or 11 of an HD 1080i signal. The SMPTE standards allow multiple data types to be placed on an individual line, so this can create an issue with ensuring that there is sufficient space for all the data to be placed on the line, and also that particular data packets can be identified. In some cases, certain equipment may insert VANC data of one type by overwriting existing VANC data of a different type, which is then lost.

Another significant issue is that many recording devices, including VTRs and servers, do not record the whole of the vertical interval but only a selected small number of lines that must be predefined. This also creates a potential point in the distribution chain where VANC data may be lost or corrupted.

Several of these issues and problems have been known for some time but they have been highlighted recently when broadcasters have started to implement new VANC data types, including AFD, which has required further guidance on implementation issues. SMPTE has therefore started an investigation to determine the extent of the problems for VANC data, and how they may best be resolved for all data types. The first step in this activity is an industry survey to determine what VANC data types are in use in industry facilities and in which lines they are being placed by different users, with equipment from different manufacturers. The survey was prepared by a small group of SMPTE engineers led by Graham Jones of NAB Science and Technology, and the SMPTE announcement is as follows.

Survey
"Users of vertical ancillary space (VANC) data (TV stations, networks, studios, other facilities) and also manufacturers of associated equipment are requested to take part in an industry survey regarding the location of data packets carried in the VANC space of serial digital video signals. This involves data such as AFD/Bar Data, closed captions, time code, program description, audio metadata, film transfer data, and many others.

SMPTE has an initiative to harmonize the use of VANC data, to reduce problems with implementation. Please contribute to this important activity by completing the on-line survey at: http://www.smpte.org/public/SMPTE_VANC/. The survey will remain open until December 6, 2009."

NAB encourages all readers of TV TechCheck who use VANC data to respond to this survey. If you have any questions, they may be addressed to the email address on the form or to Graham Jones at: gjones@nab.org

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