Quadrox QGuard Installation Manual - Manual (Page 53)

GuardNVR Installation Manual 53
Version 4.4 Series
5.2 Factors that influence bit rate and video
quality
There are several factors that have an influence on the amount of data (bit rate), the quality of
the video and the performance of the video recorder. The most important factors are explained
in this section.
5.2.1 Compression technique (codec)
The biggest influence on quality and data reduction comes from the algorithm that is used to
compress the video. Over the years, many algorithms have been conceived, but they can be
roughly categorized into two groups:
1. Intra-fame coding: each frame is compressed independently of the other frames (e.g.
MJPEG).
2. Inter-frame coding: since frames are usually very similar, this information is used to
further reduce the video size (e.g. WMV, MPEG, H.264)
Inter-frame MPEG-like coding can lead to a bigger compression (typically up to 10 times more
video for the same size), resulting in more storage or less bandwidth requirements for the same
quality. However, the calculations are more complex, so the performance of the encoding
machine will be lower (less streams, more CPU usage).
In general, there is a big difference in performance between doing the compression on the
recording device and doing it on the camera or any other specific compression hardware (e.g. a
network video server). The latter is highly recommended.
When choosing a particular compression technique, there may be other considerations. For
instance, WMV video can be played on any PC running the Windows operating system and on
most other devices with video capabilities (e.g. PDA, phone, etc).
Quadrox recommends:
1. For analogue cameras connected to an internal capture board: use WMV7.
2. For network cameras and network video servers: use the native format in
which the device streams (no recompression on the recorder).
The chosen compression technique also has an influence on the following discussion.
1. For intra-frame codecs (MJPEG), the quality is usually set by a parameter. As a
consequence, frame rate and resolution have a direct influence on the resulting bit rate.
2. For inter-frame codecs, the bit rate is usually a parameter. As a consequence, frame rate
and resolution don’t have a direct influence on storage and bandwidth requirements,
although they might have an influence on the quality of the resulting video.
This means that when the bit rate increases, the resolution will decrease as well as the frame
rate. All these parameters together produce a better image quality.