in April 2005
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
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.
• Sweet spot around 1.2Mbps.
|• Video on CDs.
• Video clips delivered over the internet.
Video to PCs over
• News gathering from remote locations.
|• Sweet spot
may be lower if the MPEG is viewed in a window on a VGA
• 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
• 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.
|| • 1.5Mbps
| • Broadcast applications.
High-quality presentations in museums, exhibitions,
public spaces, reception areas, etc.
• 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
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.
|| • 40Kbps to
• Sweet spot depends on
|• Live video over corporate
• Video to PCs over networks or to
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.
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.