The Art of Accurately Estimating Bandwidth Usage
One of the first things to consider when determining the perception of quality by a user is image rate. The higher the image rate, the more fluid is the video. The more fluid the video, the more digital images created per second, and the more bandwidth used. There’s that compromise again. Traditionally, the security industry has defined “real-time” as around 15 images per second (ips). This image rate pretty much eliminates the problem of large movements of an object being missed between frames. For most standard applications, 15ips gives you all the information you would need.
There are some applications, though, such as casinos, where nothing less than 30ips will do. Obviously, the bandwidth used in these types of projects is far above what most people deal with on a regular basis.
The next component of quality that needs to be considered is compression, or the shrinking of digital video images to a manageable size. This component has the largest impact on how good or bad the video actually looks to the eye.
Raw video at the point it is encoded is far too large to be streamed and stored as-is, so that video must be reduced in size to f t within today’s technology. This is the job of compression.
THE INCREDIBLE SHRINKING IMAGE
There are several methods of compressing video, known by their common working group names such as MPEG4 (Motion Picture Experts Group), JPEG (Joint Photographic Experts Group) or H.264 (a variation of MPEG4). These names, which were restricted to the world of network geeks in years past, have become commonplace in today’s society.
Again, as with image rate above, there is compromise. The more compression that is applied to an image, the worse that image looks, but the less space in that network pipe is used up. The less compression, the more crisp and clear the image, but far more bandwidth is consumed.
All of these compression methods work a little differently to accomplish the same goal, and specifically how they do it is the subject of another day’s column, but they are all affected by one thing, the amount of activity, or movement, in a scene.
The more movement, the less compression can be applied and the larger the resulting video file. Again, there’s a compromise.
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