H.264 Network Video Streaming Server
NVS-25
24
Main Profile
The ‘Main’ or Mainline Profile includes support for interlaced video, inter-coding using B-
slices, inter coding using weighted prediction and entropy encoding using context-based
adaptive binary arithmetic coding (CABAC). Ideal application of the Main Profile
includes long term video archival storage and standard-definition digital TV broadcasts
that use the MPEG-4 format as defined in the DVB standard.
High Profile
The High Profile is used as the preferred choice for HD broadcast and disc storage
applications, adopted by the Blu-ray Disc storage format and DVB HDTV broadcast
service.
9.5
Group of Pictures
H.264/MPEG-4 AVC (Advanced Video Coding) is a common video stream compression
method. It eliminates redundant video information within a frame known as intraframe
compression as well as over a period of time known as interframe compression. The
two compression techniques are described as follows:
•
Intraframe compression (Spatial Compression): a compression technique
applied to information contained only within the current frame, creating video
frames called I-frames
•
Interframe compression (Temporal Compression): a compression technique
applied to a sequence of video frames, rather than a single image; instead of
storing complete frames, only changes from one frame to the next are stored
in order to reduce the amount of data needed to be stored while still achieving
high-quality images.
H.264/MPEG-4 AVC uses the two above-mentioned compression techniques, whereby
three types of compressed frames described as follows are generated and organized in
a group of pictures, or GOP.
•
I-frame: Intra (I) frames, also known as reference or key frames, contain all
the necessary
data to re-create a complete image.
•
P-frames: Predicted (P) frames are encoded from a “predicted” picture based
on the closest preceding I- or P-frame.
•
B-frames: Bi-directionally (B) predicted frames are encoded based on an
interpolation from I- and P-frames that come before and after them.
The Group of Pictures (GOP) structure specifies the order in which intra- and inter-
frames are arranged. All three frame types do not have to be used in a pattern. The
typical GOP structure is IBBPBBP. The I-frame is used to predict the first P-frame and
these two frames are also used to predict the first and the second B-frame. The second
P-frame is predicted using the first P-frame and they join to predict the third and fourth
B-frames.The relationship between I-frame, P-frame, and B-frame is illustrated
diagrammatically below.
