DDF4220HDV Picodome
®
www.dallmeier.com
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GoP-size
The H.264 coding is carried out by dividing the video stream into so-called GOPs (Group
of Pictures) of a defined length (defined GOP-size).
A GOP sequence always starts with an Intra-Frame (I-Frame), which contains all image
data and serves as a reference for the subsequent images within a GOP.
The I-Frame is compressed with a low compression rate, similar to the JPEG compres-
sion method.
Depending on the defined GOP-size, an I-Frame is followed by one or more Predicted
Frames (P-Frames), which only contain the motion predictions and difference information
of the preceding images (I-Frame or P-Frames) (Long-term prediction).
The compression rate with P-Frames is much higher than with I-Frames since changes in
relation to reference images only need to be coded as motion vectors. Thus, the required
bit rate decreases so that, with a given total encoding bit rate, more bits are available for
the I-Frame. Consequently, the quality of the I-Frame can be increased (e.g. the detail
resolution) due to a larger GOP-size.
However, if there are scenes with many motion changes, a high number of P-Frames
can have a negative effect on the image quality, because the motion predictions become
increasingly inaccurate.
Additionally, a larger GOP-size always leads to an increase in delays regarding process-
ing or accessing a stream.
The GOP sequence ends before the next I-Frame.
At a later stage, the visible single frames are generated at the decoder, using the indi-
vidual GOPs.
The GOP-size 1 (I-Frames only) indicates a low compression factor and should only be
used with specific applications, because the bandwidth requirements increase signifi
-
cantly.
Generally, a GOP-size of between 6 and 15 will provide a good image quality with suf-
ficient compression.
If large GOP-sizes are defined, reverse playback can result in frame drops with some
decoders.
Hybrid cam
In addition to video over IP, the hybrid camera supports simultaneous analogue video
output via the built-in BNC connector (CVBS).
The “Hybrid Cam” function defines the maximum available frame rates for Encoder 1 and
the analogue video output:
•
On (checkbox activated)
Resolution – Encoder 1
Frame rate – Encoder 1
Frame rate – Analogue Output (4CIF)
SXGA−
max. 12.5/15 fps
max. 25/30 fps
720p
max. 25/30 fps
max. 25/30 fps
4CIF
max. 25/30 fps
max. 25/30 fps
•
Off (checkbox deactivated)
Resolution – Encoder 1
Frame rate – Encoder 1
Frame rate – Analogue Output (4CIF)
SXGA−
max. 25/30 fps
max. 12.5/15 fps
720p
max. 25/30 fps
max. 25/30 fps
4CIF
max. 25/30 fps
max. 25/30 fps
