Key to the smooth running of any broadcast operation is the delivery of the content from post-production facilities in the correct format. Most commissioning broadcasters have a detailed delivery specification that lays down the media of choice (tape format), scanning format (720P, 1080I) and audio track layout. There are many other parameters that will be listed including: caption format, AFD and loudness metadata. But, looking forward, the videotape delivery specification was pretty straightforward.
Move to delivering programs and commercials as files, however, and it gets a lot more complex. Many parameters remain the same, like AFD and scanning format. But, the use of a file adds many more variables such as which codec should be used. With tape, the codec is internal to the recording system; it’s like a black box with SDI in and out of the deck.
Then, there is the matter of the wrapper: QuickTime, GXF and MXF are all possible candidates. Dig deeper into the detail, and that is where the devil lies. Should data be multiplexed or interleaved? Should audio be one multichannel track or multiple single-channel tracks? Where should the timecode be stored in the wrapper? And, what about closed captions?
As early users of MXF found, it is easy to comply with SMPTE standards, but difficult to get equipment to interoperate. This stems from the many choices offered in the standards, and the ways each vendor interprets and uses the standard. The end result is compliant but not interoperable files.
Traditionally, each broadcaster has had their own technical specifications for program delivery. In a move that should ease the tasks for contributing post facilities, the primary UK broadcasters have agreed upon a common delivery specification. An initiative, dubbed the Digital Production Partnership (DPP), set up a small committee to draft common delivery standards. The Partnership is funded by ITV, BBC and Channel 4, with representation from Channel 5, Sky and other interested parties.
The first standard was for HD programs to be delivered on HDCAM SR tape. The next focused on file-based delivery, and the committee searched several possible format candidates. Key requirements included the AVC Intra codec with uncompressed audio for HD content and SMPTE D10 for SD video and audio. The DPP members wanted an HD video codec that supported 10-bit coding and was I-frame only. AVC-I and JPEG2000 both support such coding, but AVC-I had the edge in the lower, 100Mb/s data rate. JPEG2000 coding for a similar quality would be around twice the data rate. AVC-I was chosen as a codec that would work for the next five years or so. Although it was acknowledged that it does need high-performance computing platforms for editing, over time Moore’s Law comes to the rescue.
The AMWA Application Specification AS-03 - MXF for Delivery met many of the requirements but is limited to 50Mb/s video coding. The AMWA was approached to see if a new specification could be developed that would better match the DPP’s exact requirement. Following approval by the AMWA Business Standards Committee, work started on the new specification, titled AS-11 – MXF for Contribution.
The DPP needed the specification in a relatively short timescale as standards development goes. Luckily, the AMWA recently changed development methodology to reflect the faster pace of today’s technological development. In a period of only six months, the specification has been developed and awaits only final review and sign-off.
AS-11 is a superset of AS-03, which was originally developed for PBS to deliver content to its affiliates. AS-11 is backwards compatible with AS-03 but includes the AVC Intra 100 codec. AS-11 adds more detail on Descriptive Metadata tracks (DM as defined in SMPTE 377) and includes a scheme for a core DM set, plus UK DPP specific DM.
It is UK practice to add closed captions in master control, so there was no need for files to be delivered with embedded captions. AS-11 does, however, support CEA-608 and -708, as does AS-03. AS-11 adds explicit support for late delivery of files, which is where files can be played to air while still streaming. The operational need for this somewhat risky operation is not there yet, but could be used in the future.
Another area that has been further defined is program segmentation, where a program can be delivered in one part, in soft parts as defined by time code values, or with hard segmentation, where black interstitials are included in the file between program segments.
It is all very well having a standard specification, but experience shows that sometimes a file will not open. In cognizance of this, BBC R&D has created some sample files that manufacturers of editing equipment and servers can use to check interoperability. The DPP and AMWA are to hold a plugfest in March 2012 to iron out gremlins in equipment that purports to support AS-11.
So, with compliant files and interoperation between broadcast equipment, that just leaves the Descriptive Metadata. The DPP believes that using a standard DM should ease operations in traffic and master control, but how does the post house add relevant metadata? It is hoped that the popular NLEs will add support in future releases. However, as a stopgap, the DPP is to make a simple tool available to partners for the addition of DM to the DPP schema. As an example, this could be used by an editor to add key information about a production.
It does seem that those concerned in this fast turnaround project have taken great care to ensure that the move from videotape to files does not encounter many of the same pitfalls some early adopters have suffered. Let us hope that the familiar phrase “it was alright leaving me” does not have to be used so much in the future, as recipient and sender try to figure why a file won’t play or even open.
Although AS-11 — MXF for Contribution and AS-03 — MXF for Delivery are both designed for the delivery of program content as files, AS-11 does have a wider application than AS-03, which is intended for distribution to playout servers.
AS-11 files, with the higher bit rate AVC codec, could be edited after delivery from the post house into versions and stored as AS-02 — MXF Version files. Another use is for reversioning for the many other digital platforms. The three specifications are all constrained versions of MXF designed to make the creation of interoperable files much easier than was the case when MXF was first released.
AS-11 — MXF for Contribution is anticipated to be released before NAB 2012.
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