File-based workflows
Jan 1, 2008 12:00 PM, BY DAVID AUSTERBERRY
The technology opens entirely new and efficient ways to produce and air content.
Attracted by the promises of lower costs, broadcasters are eagerly adopting file-based workflows.
There is nothing new about processing video as files. They have long been the video format of choice for editing, and more recently for playout. In the process of editing files on disks, there is the great advantage of random access, which lies at the heart of the nonlinear editor. In playout, playing files from disks saves wear and tear on tapes, decks and robotics.
The challenge today is to link these two islands into a seamless workflow from acquisition to transmission. It is conventional to store and transfer material as videotape. To handle a production as files all the way to the consumer requires cost-effective and secure distribution and storage.
Over the years, broadcasters have developed tried and trusted procedures to ensure the safe transport of tape from one department to another. The issues for long-term storage of tape are no different from the storage of books or documents.
To join together many file-based islands into a connected workflow is not a trivial task. Once the issues of distribution and storage are resolved, then the real issues of a workflow emerge. In the world of videotape, a new facility can be added without affecting other technical areas. The transport of tape decouples each area, neatly avoiding many issues of integration. A file workflow cannot be simply overlaid onto existing processes if the true benefits are to be realized.
Security of transmission
It is commonplace to find a lone VTR in a playout area. This can be used to play a tape that arrived too late for ingest, or it can be used as a fallback to play a tape to air if the server or automation systems were to fail. Before a broadcaster will part with that tape deck, a file-based system must offer a higher level of reliability.
Acquisition
In the last few years, manufacturers have released video cameras that can shoot as files and store on a variety of media, including solid-state memory, optical disk and hard drive. Some manufacturers have gone as far as removing the tape transport altogether from the camcorder.
These original camera files can be delivered to the post house and ingested by the NLE. The editor software developers have kept pace with the acquisition formats, so in most cases, the ingest process is pain free. The post house is where the issues start. Post involves offline, online and numerous review and approval processes. Tapes are checked in and out of the vault, and handling media as files demands a similar form of control. Digital or media asset management offers such control, plus it can handle all the processes of reformatting and managing transfers and file storage.
Tapeless acquisition has several advantages, one being the speed with which the rushes can be loaded to a laptop editor for the process of logging and shot selection to start. But again, there can be problems. Files can be accidentally deleted or become corrupt. Much of this is a lack of established procedures and a consequence of the use of smaller crews.
On a film shoot, the DP could have a camera operator, a grip to look after the camera and a clapper loader to look after the precious negative. On a video shoot, one person may have to cover all these roles, plus record the sound. It's not surprising that a flash memory card goes missing.
Film crews have developed a methodology to get the negative to the lab without mishap, and video crews have worked out a logical way to handle tapes — set the record lock, make notes for the editor. File shooters must do the same by figuring out a way to back up files as they shoot, log metadata and create a routine that is as safe as legacy workflows.
File formats
The media business had to construct a set of file formats from the ground up. The many formats spawned by the multimedia business were soon found to be lacking in basic features considered essential for broadcast applications. Time code is a good example of this.
A broadcast format must work with existing videotape formats and ideally support streaming and conventional file transfer. A media file is usually a container of video, audio and metadata essence files, or at least referencing atomic files. The files in the container must be synchronized so that the audio plays in a correct time relationship to the video track.
International standard or proprietary?
Until the development of DV and IMX tape formats, the only way to get video off a tape was to play it out at real time as analog or SDI, and then ingest and convert back to a file format. To get beyond this, the Digital Video Digital Interface Format (DV-DIF) used the IEEE 1394 protocol as a transfer format. This enables data to be transferred from the videotape to a hard drive without the need to use an intermediate video connection. It removed the need to digitize video into an NLE. Instead, the files can be ingested over FireWire, making laptop editing a possibility.
Sony soon followed with the e-VTR, based on 4:2:2 I-frame MPEG-2 recording at 50Mb/s (IMX); contrast with the closed recording format used by Digital Betacam (DB). The IMX tape format is now standardized as SMPTE D10.
GXF
As playout servers became the favored format to broadcast commercials and interstitials, broadcasters found that they were locked into one vendor because the internal file formats were proprietary. Copying a file from one server to another from a different vendor required a decompress to SDI and then a re-encode, with the attendant quality issues of concatenation of codecs. Since most servers used variants of motion JPEG or MPEG-2 for storage, why not transfer content as files?
The answer might be that tape dubbing uses baseband SDI, but server-to-server file transfer avoids the decode/re-encode step. Plus files are supposed to bring efficiencies, including in-media transfer over IP, vendor independence and faster-than-real-time transfer. The answer was the development of the General Exchange Format (GXF) later standardized as SMPTE 360M.
MXF
Figure 1. An end-to-end broadcast chain can now be built using files throughout the workflow processes. MXF greatly eases file exchange between platforms.
The Material eXchange Format (MXF) extends the principles established by GXF by adding a great deal of flexibility so that it can be used throughout the broadcast workflow. (See Figure 1.) MXF addresses the requirements of a file-based replacement for videotape, and adds many more features that tape could never offer, like multiple video tracks.
For some, it is too flexible, leading to complexity in the implementation. In response, some user groups are developing recommended practices that constrain the number of options for a given application. A good example is the MXF Mastering Format, which aims to simplify file handling and content repurposing in playout centers.
The standardization processes have sensibly limited the number of codecs to around 10 of the popular SD and HD formats. This avoids all the issues that have existed in the world of graphics where hundreds of formats have been spawned, predominately proprietary.
Limiting the number of codecs not only simplifies the design of file readers, but will also earn thanks from future generations as they mine our program archives.
| Want to use this article? Click here for options! |  |
