Quadrox QGuard Installation Manual - Manual (Page 51)

GuardNVR Installation Manual 51
Version 4.4 Series
5 Storage / Bandwidth considerations
Digital video that captures several days or weeks comprises a massive amount of data. If you
want to store this data or stream the video over a network, there is a need to reduce the size,
because storage devices (hard disks) and networks are limited in capacity.
The size reduction of digital video data is called compression: it is a mathematical algorithm
(called a codec) that is applied to the data. The algorithm carefully removes information that is
less important for a human viewer. Because information is lost, inevitably the quality of the
videosdf is affected.
There is a trade off between the quality of the video and performance of the system on the one
hand, and data size, the compression technique and its parameter on the other hand. The
explanation below will help you to make the best choices depending on your specific needs
and situation.
5.1 Terminology and basic video technology
This section provides an overview of the terms used in the following sections and the basic
underlying technology. It is important that you understand these concepts in order to
understand the influence of different factors on quality and size.
A digital image consists of an array of image points, called pixels. The amount of points in
horizontal and vertical directions is the image resolution. Each image point has a certain
colour and brightness attached to it, and is represented inside the computer as a number.
If there are more points in an image (higher resolution), more detail will be preserved in the
image, but also the necessary space or bandwidth increases.
Digital video consists of series of digital images displayed one after the other. Each image is
called a frame, and the speed at which images are displayed is called the frame rate. This
number is indicated in frames per second (fps).
If more frames are displayed (higher frame rate), objects in the video will move smoother, but
also the necessary space or bandwidth increases.
Each number in a computer is represented by a number of 1’s or 0’s, called bits. The amount
of data that a digital video contains per second is called the bit rate. This is usually expressed
in kilobits (1024 bits) per second (kbps).
For uncompressed video, the bit rate is calculated as:
Resolution x frame rate x colour depth (amount of bits per pixel)
Example: 5 fps of full D1 PAL video:
Bit rate = 768 x 576 pixels/frame x 5 frames/second x 24 bits/pixel