The fundamental elements of media workflows
Mar 1, 2009 12:00 PM, By Al Kovalick
Another graphical modeling tool is BPMN. Don't be put off by the business term in the name. The tools translate to media workflows as proven by their usage by some systems integrators. The modeling in BPMN is made by simple diagrams with a small set of graphical elements. The four basic categories of elements enable designers to describe media flows:
- Flow objects: events, activities and gateways.
- Connecting objects: sequence flow, message flow and association.
- Swim-lanes: pool and lane.
- Artifacts: data objects, group and annotation.
Fortunately, there are reasonably inexpensive layout tools for both UML and BPMN.
Process orchestration
Figure 5. Fundamental media transfer modes
Click to enlarge
There are three primary media flows using networked media techniques. (See Figure 5.) Physical videotape methods are not included.
Digital networking has enabled these three modes of media transfer. Mode 1 is the ubiquitous file transfer at rates less than, approximately equal to and greater than video real time. The second mode is streaming over IP or some other link type. The mother of this type is video over SDI — the workhorse of the modern facility. Video over SDI is not considered networked media in that SDI is not networkable but exists as a switched circuit.
Video over IP is common for distribution to the end user, including Web media and IPTV related services. In the professional facility, video over IP has yet to replace SDI. Until Ethernet has an equivalent synchronous version with guaranteed low latency (QoS specs like SDI), SDI will remain the ruling incumbent. The IEEE is developing such technology. The 802.1 Audio/Video Bridging Task Group is developing a comprehensive set of standards to enable high-quality, low-latency streaming of time-sensitive applications. These standards will specify a means for time synchronization (IEEE 802.IAS), a resource reservation protocol (IEEE 802.1Qat), and a set of forwarding and queuing rules that bound the variability of delay in an AVB network (IEEE 802.1Qav). These are new standards, and it will require time for the industry to embrace them if it ever does.
Figure 6. Three common media fl ow models
Click to enlarge
The third method of media transfer is storage access. This is distinctly different from the other two methods. Storage access supports the read/write of data in random access style using storage area network (SAN) or network attached storage (NAS). There are storage systems specifically designed to support the real-time (in a video sense) access to hundreds of simultaneous Ethernet-connected HD media clients with no-excuse data delivery.
These three methods are the building blocks for the modern media facility. Designers must use wisdom when selecting one method over another. A big mistake when doing a new design is to mimic a videotape workflow using networked media. Videotape flows are limited in many ways, and networked media allows for many dimensions not permitted using only tape.
Comparing flow types
Next, let's compare three flow types — one using pure streaming and two using file transfer. (See Figure 6 above.) The general idea is to process an incoming video signal program as follows: ingest/record, apply an aspect ratio convert and add side panels, add a lower-third graphic and finally output the result. The top flow is most commonly seen using SDI (or AES/EBU links for audio-related applications) connectivity. Of course, a process may be any transform or human-assisted means. For live events demanding a minimal in/out latency (few video frames), SDI streaming connectivity is often used.
The middle flow uses the bucket brigade method. First, the entire program is ingested and saved to storage. Then, either by a file transfer between processes or via an intermediate “parking place” storage device, the program file is moved to the aspect ratio converter (ARC) process then to the graphic composite overlay process, and finally is output. In each step, the entire file is imported to a process and then exported to the next process in the chain. Hence, the bucket brigade nickname.
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