What is webcasting?
Different from Video Conferencing?
What are the drivers?
Key market technologies
ProLearn Live Trails
ProLearn Summer School 2006
Basic audio-visual equipment for webcasting
Audio-Visual Webcasting Tips
Delivery of the webcast from the venue requires that the Audio-Visual
feed be encoded into a streaming format that can be broadcast over the
internet, normally to a streaming server, where it is distributed to webcast
clients. The webcast clients need some way of requesting this stream from
the server and publishing this information is most commonly done via a
Encoding an audio-visual feed for webcasting involves digitizing the
feed and then encoding it into a live stream of data. An uncompressed
video stream of “Standard Definition” video requires around
30 Mbytes/s, nearly 100 Mbit/s of bandwidth which is not practical to
stream to end users. To solve this problem the encoders use codecs to
compress the data stream. How much compression is applied depends on desired
quality, size (dimensions) and perhaps most importantly available bandwidth
both to the server, from the server and at the clients end of the network.
Good quality quarter screen webcast feeds of between 150 and 300kbit/s
can be created with modern codecs when combined with techniques such as
reducing resolution and frame rate.
Appendix 2 gives a detailed example of using a software based encoder
called Wirecast (Varasoft, 2006) to create a QuickTime webcast stream
on a PC, as used during the recent Prolearn Summer School 2006.
Serving the live webcast stream is the responsibility of the streaming
server. When a client requests a “live streaming file” from
the server, the server replicates the incoming stream and forwards it
onto the webcast clients. Each client’s request generates a single
Unicast feed. Modern streaming servers can handle thousands of such simultaneous
feeds. However bandwidth requirements are almost always the limiting factor.
For example, 100 clients viewing a 200kbit/s stream would consume 20Mbit/s
of the available bandwith from the streaming servers immediate network.
Appendix 3 outlines available server technologies and some alternative
broadcast methods that can be used to minimize bandwidth.
Publishing a live feed can be similar to providing end users with a URL.
In the case of RTSP (Real Time Streaming Protocol, 2006) for example,
the URL might look like;
The handling of webcasting streams is highly dependant on the codecs
used to encode the data, and the formatting of the files. It is quite
easy for the receiving client to become confused, eg. for the wrong media
player to launch and be unable to decode the stream. With this in mind
most streaming media solutions have ways of creating a ‘reference’ or ‘meta’ file
that refers to the above URL. This file can then be embedded in a web
page like a normal video file, allowing information about an event to
be combined with the a streaming video file. Specialized content management
systems (CMS) such as the XO Backlot (a KMi Stadium Technology) used for
Prolearn TV are ideally suited to solving the administrative issues associated
with handling many events and associated streaming video files. The Stadium
system and commercial products such as Datmedia (Datmedia, 2006) and Starbak
(Starbak, 2006) include content management within their streaming solutions.
Content management solutions also have the advantage of automatically
handling availability of the streaming media, such as limiting access
to the streaming file to only when the event is live. Appendix 4 gives
an example of using XO Backlot CMS to create an event and publish a live