Imagine we have a digitally switched video network connecting
several offices and laboratories. The video is controlled
by software running on a workstation, and is
transparently integrated into a multimedia conferences.
The conference video channel is controlled by the floor
holder through the conferencing software, and switches
automatically when there is a change of floor. This degree
of integration ensures that users view the video and
applications as being one conference, rather than separate.
The floor holder can also select quad mixing, which allows
four conference sites to be visible to everybody, which is
especially useful for small interactive conferences.
The audio channel for a conference is open-channel, and so
anyone can speak at any time. To prevent feedback, we
employ an n-1 audio mixer, which returns mixed audio to a
site from all the other sites, but not to the originator.
User trials[#sasse##1#] show that in a collaborative
conference, much of the user's attention is taken up by the
design applications. There tend to be quite long periods
when a designer is concentrating on a design task, and often
he does not explain verbally what he is trying to do. At
these times, a video link is invaluable to find out what the
other participant(s) are doing, and whether they expect you
to be doing something. For this purpose high quality video
is really not necessary. However for larger lecture style
conferences, if video is required, greater bandwidth would
These findings indicate that many users (especially designers for example)
do not wish to sacrifice screen space for a video window,
however small. For the purposes of interactive design
conferences, we also find that a frontal head and shoulders
view does not include enough contextual information. Much
better appears to be a view from one side, which allows the
remote site to see the user from the waist upwards. Ideally
it should include the workstation keyboard, so the remote
site can clearly see when the user is busy. To create an
illusion of co-presence, the camera and video display must
be situated close together. This enables a user to talk to
the <#1736#> image<#1736#> of the remote site, and feel he is being
talked to. The implication of this is that many users will
not require the video to be displayed on the workstation
screen, but rather on a separate monitor situated away to
one side of the desk.
Using analogue video for local distribution provides us with
maximum flexibility, and very high quality at relatively low
cost. However it is clearly not cost effective to use
analogue video for wide area distribution, and the problems
of scaling switch control are certainly not trivial. For
wide area distribution we employ both proprietary Picturetel
and H261 video codecs, as well as lower quality devices in
workstations for capturing video and compressing to a usable
data rate. As video codecs are currently expensive it makes
sense to use them as a shared resource, and hence connecting
them to our analogue video switch rather than to an
individual workstation is a cost-effective solution.
Although car designers value the real estate of the
workstation screen, it may prove much more cost effective to
display video there. We estimate that on workstations with
greater than 100 MIPS processing power, H261 video
compression will be possible in software without expensive
hardware assistance. This is supported by findings at INRIA[#ivs##1#]
on current top end workstations.
As a result, digital multimedia will become much
more widely available, which will have far reaching effects
for tomorrow's networks.
In a multimedia conference, events will occur that users
should be informed about. If a new conferee joins the
conference, all the existing conferees should be notified
immediately that this has occurred, for it may affect the
way they behave. This is a high priority event, and so the
user should be distracted sufficiently from whatever she is
doing to take notice of the event. The CAR system does this
by sounding an audible warning, and popping-up a new window
containing the information.
Generally though this sort of warning proves too distracting
for users if it is used for lower priority notifications
such as change of floor holder (though it will be fairly
high priority warning if you were the old floor holder).
The CAR system has an area set aside for these
notifications, however trials show that users sometimes
cover this with applications and then get confused.
Sometimes implicit notification can be used. An example of
this is the video communication channel in the CAR system
which switches to show the floor holder when the floor
changes. However this default can be overridden, and thus
confusion can easily be created.
A example of a low priority event is the resizing of a
shared window by the floor holder. Here the notification
received needs to be ignored until it becomes relevant -
i.e. when the floor holder starts drawing in an area that
can't be seen. Clearly there is a need to provide
communication between conference sites of this information,
but irrespective of whether this is done using a shared
window manager, or aware applications, the presentation of
this information to the user presents some problems. In
order to lessen the uncertainty of a user and thus her
confusion, more information needs to be given, which may
result in increasing her confusion. The question is how to
present this information in a way that will reduce the
complexity, not increase it?.
Which notification method is used for which event should
depend on the user. High priority events should always be
intrusive, but lower priority events should depend on the
user's level of expertise.