An Introduction to Digital Video for Video Conferencing

In order to understand the relationship between video quality and bandwidth requirements, we need to look briefly at some of the basic concepts of video display.

Video Quality and Framerates

What the human eye perceives as a continuously moving video is actually a series of still images (referred to as frames) being displayed one after another so rapidly as to appear to be in motion.  The rate at which the still images are displayed is referred to as the "framerate".  The North American standard for a "full motion video" framerate is 30 frames displayed per second (30fps). At lower frame rates, the "motion video" can begin to appear jerky and and can become undesirable to view. Most VCUs have the capability for you to set what framerate you want it to use.  While it may seem reasonable to choose the highest framerate possible in order to produce the best quality of video for your participants, that is often NOT the best course of action depending on the internet connection.

Internt Bandwidth and Video Transmission

In order to transmit video, the sending VCU must convert ("encode") each video frame into data to be transferred over the internet.  Each receiving VCU must convert that data back into a video frame ("decode") and display it to the viewer.  In order for the data to be transmitted quickly enough to be converted properly by your VCU, your internet connection needs to be capable of maintaining a certain "bandwidth" or data transfer rate. Bandwidth is measured in Kilobits Per Second (Kbps) or Megabits Per Second (Mbps). The higher the frame rate of the video, the greater the amount of bandwidth required to transmit it.

The minimum requirement for full motion video (30 fps) and audio between two points is 768 Kbps.  This requirement is arrived at as follows:  an encoding rate of 384 Kbps (typically used) is selected. This is broken down into two parts: 

• 64Kbps is for the audio
• 320 Kbps is for the video.

The resulting 384 Kbps stream is compressed and sent (from you - the source) to the remote point (the destination). Similarly a 384 Kbps stream is received from the remote point to you. Thus twice 384 Kbps in bandwidth is required.

Does it REALLY take this much bandwidth?

This 768 Kbps requirement assumes that EVERY frame of video sent must be encoded and converted.  In reality, the requirement is often significantly lower due to the ability of the VCU to "compress" the video data being sent. Put simply, the VCU monitors how much change is occurring in the video frames that it is sending, and tries to ONLY encode and transmit the changes.  If there is a lot of motion in the video, very little compression is achieved. If there is little or no motion in the video, the savings can approach 50%. It would, however, be foolish to design a system which relied on the participants being forced to hold still.

But I HAVE a high speed internet connection, and the video STILL looks bad!  Why?

Unfortunately, one of the biggest problems for videoconferencing is that the VCUs are dependent on having a high speed internet connection existing THE ENTIRE distance between the two videoconfernecing units.  This is something that, due to the complexity of the network that might exist between the two videoconferencing units, cannot be guaranteed 100% of the time to 100% of the locations you might be conferencing with.  

In some cases, the U of S can reach agreements or establish partnerships with other organizations to provide "dedicated network links" between the two organizations in order to improve the quality of videoconferencing connections.  However, these types of arrangements can take months or years to put into place and often require substantial financial commitments.  They will not usually be of benefit to you if you are planning a videoconference in the short term and want to guarantee a 100% success rate.  

Your best options in cases like this is to try to arrange to test your videoconferencing connections ahead of time AND to make sure you have a "Plan B"  available in the event of problems.