Quadrox QGuard Installation Manual - Manual (Page 55)

GuardNVR Installation Manual 55
Version 4.4 Series
For a fixed bit rate, a higher resolution means that information has to be removed in other
ways, reducing the image quality.
Keep in mind that choosing a higher resolution also means putting a higher load on
your GuardNVR and may reduce its performance. Bigger images need more internal
resources like memory and processing time.
5.2.3 Frame rate
When choosing a resolution, we reduce the amount of data by reducing the size of an image.
Similarly, we reduce the amount of data by simply storing or streaming fewer images. Frame
rate is a third big factor in compression. This factor is potentially big, since the difference in
frame rates can range from 30 fps to one image every 3 seconds, a factor of 100!
When reducing the frame rate, the loss of quality creates video that is not “smooth”. The
human eye needs a certain frame rate to perceive a sequence of images as smooth motion.
When reducing the frame rate to a number below this amount, the image “shocks”. However,
while being less pleasant to look at, the quality of the individual images is not affected, so all
detail is preserved.
The threshold of the human eye for perceiving a sequence of images as smooth motion lies at
about 15 images per second, depending on the person. Although the broadcast industry
(television, DVDs, film, etc) take a substantial margin on this with streams at about 25 fps, this
is not absolutely necessary for smooth motion.
Because of this, streaming at more than 15 fps is almost never useful and mostly serves to
increase your bandwidth requirements. In most cases, even 10 or 12 fps suffice for a
satisfactory viewing experience.
For recording, frame rate is usually reduced further, since nothing much occurs in the time
period of 1/15
th
of a second. A storage frame rate of 3 to 6 fps is enough for most practical
applications.
Similar remarks about the relation between compression technology and frame rate hold as
they did for resolution. When using (M)JPEG, frame rate has a direct influence on bit rate
since each image is compressed separately. When using MPEG, H.264 or WMV, the bit rate is
set, so frame rate potentially has an influence on quality rather then on data size.