Published in April 2005

AV/IT Convergence and MPEG
By Ed Carabetta

Getting the right compromise in business communications.



      Convergence of AV signals in order to utilize an IT infrastructure for the delivery of content mandates using a digital protocol. In most cases, there will be an analog-to-digital conversion. As we progress from an analog to a digital world, the common perception is that performance and feature sets improve. In fact, there are compromises to be made, and it is the careful balancing of these compromises that are significant to the success of a digital project.
     In the digital video arena, the over-arching compromise is that of bitrate versus quality. As we increase the compression ratios in the conversion of analog video to digital, we see a reduction in quality of the resultant video. Taken to its extreme, low bitrate video becomes blocky and stutters, and is out of synchrony with the accompanying audio. It is at this point that the viewer is distracted by the poor quality of the experience and does not concentrate on the communicated message.
     The optimum situation is one in which the bitrate is sufficiently high in order to provide no distractions to the viewer, but low enough to ensure that the delivered video does not overload the network infrastructure. This optimum bitrate is sometimes called the “sweet spot.”
     A catalyst to the momentum in AV convergence is the availability of higher bandwidth circuits. Corporate networks are being expanded and developed continually. DSL coverage is increasing and broadband access is becoming widely available. However, experience shows that, in the corporate communications sector, there is also constant demand and competition for that additional bandwidth.
     A method of addressing this challenge is to use a complementary approach to streaming live data, which is to use “store and forward” techniques. In this way, digital material is sent over a network as a file that is played locally by the viewer once the file has downloaded. This method has the advantage of allowing higher bitrate clips to be created, and their transmission will not impact a network unduly; the disadvantage is that it is not suitable for live video events.
     Another major influence on the success of a digital video communications project is consideration of the audience and the method by which they will view the video. By understanding the viewing environment, it is possible to design a system with the optimum sweet spot bitrate, and in turn understand the network requirements prior to rollout of the service. It is vital for the survival of any new video service or channel that the audience rapidly becomes loyal and enthusiastic for the service.
     Interoperability is also an increasingly important issue, so adherence to standards is becoming crucial. MPEG is a well established and effective method of video and audio compression, and exists broadly in three forms: MPEG-1, MPEG-2 and MPEG-4. MPEG-1 was originally targeted at compression suitable for PC multimedia use. MPEG-2 was targeted at TV applications, and the more recently ratified MPEG-4 is a scalable compression suite suitable for delivery of content to devices ranging from PDAs to high definition TVs. The flexibility of MPEG-4 means that, ultimately, it probably will replace MPEG-1 and -2 in networked applications.
     The table provides a simplified overview of the key considerations of the use of MPEG formats in business communications.
     Generally, the sweet spot bitrate is also dependent on the amount of action within the video. For example, a head-and-shoulders view during an interview will require a lower bitrate than an action scene in which the camera pans over a crowd. Another compromise has to be made: high enough bitrate to accommodate the action scenes, but low enough to ensure bandwidth is not being wasted on interview scenes. More sophisticated encoders provide a variable bitrate feature whereby bandwidth is allocated according to the action within a scene.
     Convergence in the AV world to the IT infrastructure is driven primarily by business communications. The goals of business communication channels are to inform, motivate and entertain. These goals will be achieved by the creation and delivery of imaginative and well-produced content. It is vital that the delivery method be scaled correctly so it provides a seamless experience for the audience. Careful consideration must therefore be made regarding the audience, the content, and the network, display and compression technologies.

Standard Bitrates Key uses Display considerations
MPEG-1
• 500Kbps
to 2Mbps.
• Sweet spot around 1.2Mbps.
• Video on CDs.
• Video clips delivered over the internet.
• Video to PCs over
networks.
• News gathering from remote locations.
• Sweet spot may be lower if the MPEG is viewed in a window on a VGA screen.
• If lower quality audio can be tolerated, for example, if the viewer is using PC speakers to listen to speech, then more bandwidth can be allocated to the video thus increasing quality.
• PC-based software decoding of MPEG possible, obviating the need for dedicated hardware at the point of delivery.
• Suitable for small video monitors, but not for large-format devices such as plasmas or videowalls.

MPEG-2 • 1.5Mbps
to 8Mbps.
• Sweet
spot around
3Mbps.
• Broadcast applications.
• High-quality presentations in museums, exhibitions,
public spaces, reception areas, etc.
• Video on DVDs.
• Video archives.
• Software decoding of MPEG-2 is CPU intensive. Unless the PC is only being used for video display, it is advisable to use a hardware decoder to allow other applications to use the CPU.
• Not advised for display within a window on VGA devices because
MPEG-1 or -4 is more efficient for this application.
• Sweet spot will be higher for large-format applications such as display on plasmas or videowalls.
MPEG-4 • 40Kbps to
8Mbps.
• Sweet spot depends on
application.
• Live video over corporate
networks.
• Video to PCs over networks or to large-format
displays such as plasmas.
• More efficient algorithms than MPEG-1 or -2, so
ideal for restricted bandwidth or limited storage applications.
• MPEG-4 benefits from advanced algorithm development resulting in better bitrate-quality performance than predecessors. Less prone to “blockiness” in cases where bandwidth is limited.
• Suitable for a wide range of output devices ranging from handheld to large-format displays.
• Software available for PC decoding of MPEG-4. Hardware decoders becoming more widespread with growth of DSL applications.


Ed Carabetta is president of Cabletime USA, the US distributor of the Newbury, England-based Cabletime. Founded in 1984, Cabletime is a developer and manufacturer of video distribution and communications systems.

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