Axis Network video Manual page 58

58 CHAPTER 7 - VIDEO COMPRESSION
An I-frame, or intra frame, is a self-contained frame that can be independently decoded without
any reference to other images. The first image in a video sequence is always an I-frame. I-frames
are needed as starting points for new viewers or resynchronization points if the transmitted bit
stream is damaged. I-frames can be used to implement fast-forward, rewind and other random
access functions. An encoder will automatically insert I-frames at regular intervals or on
demand if new clients are expected to join in viewing a stream. The drawback of I-frames is that
they consume much more bits, but on the other hand, they do not generate many artifacts,
which are caused by missing data.
A P-frame, which stands for predictive inter frame, makes references to parts of earlier I and/or
P frame(s) to code the frame. P-frames usually require fewer bits than I-frames, but a drawback
is that they are very sensitive to transmission errors because of the complex dependency on
earlier P and/or I frames.
A B-frame, or bi-predictive inter frame, is a frame that makes references to both an earlier refer-
ence frame and a future frame. Using B-frames increases latency.
I B B P B B P B B I B B P
Figure 7.1d A typical sequence with I-, B- and P-frames. A P-frame may only reference preceding I- or P-frames,
while a B-frame may reference both preceding and succeeding I- or P-frames.
When a video decoder restores a video by decoding the bit stream frame by frame, decoding
must always start with an I-frame. P-frames and B-frames, if used, must be decoded together
with the reference frame(s).
Axis network video products allow users to set the GOV (group of video) length, which deter-
mines how many P-frames should be sent before another I-frame is sent. By decreasing the
frequency of I-frames (longer GOV), the bit rate can be reduced. To reduce latency, B-frames are
not used.