Extron Electronics MS9400 HD FrEND - Manual (Page 55)

MS9400 HD FrEND User Guide Appendix D: Specifications
55 Rev. 1.54
• A profile is a subset of algorithmic tools
• A level identifies a set of constraints on parameter values (such as picture size and bit rate).
A Decoder that supports a particular profile and level is only required to support the corresponding subset of the
full standard and parameter constraints.
The MPEG compression standard is now in use throughout the television broadcast community.
A full description of the specific compression techniques employed by MPEG is beyond the scope of this
introduction. The rest of this section introduces and explains some of the terms commonly used in relation to
MPEG video.
I, B and P frames.
In order to achieve significant compression, MPEG implements an “encode only changes” mechanism. For
example in a typical video sequence, much of the image stays the same from frame to frame (except during a cut
of course). In that case why bother storing the static information with every frame?
MPEG uses this feature of video to only encode the changes from frame to frame. A problem with using this
technique is that random access becomes very difficult, because displaying a frame requires knowledge of all the
previous frames that the required frame has changed from. To overcome this problem, MPEG periodically
encodes a complete frame. With this in mind the following definitions describe the different frame types that may
be found in an MPEG stream.
I frames. These are frames that contain a complete Image. I frames act as the random access points into the video
stream.
P Frames. These frames contain only the differences from the previous frame.
B Frames. These frames contain the differences between the previous frame and the next frame. This may seem
like a strange concept. How can a video frame use information from a frame in the future? The answer is that an
MPEG encoder re-orders the frames in the stream, so that the decoder has all the necessary data to generate the B
image.
Of these frame types, B frames give the best compression, typically six times more than I frames.
Groups Of Pictures - GOP
As discussed above MPEG Video frames are encoded as three different picture types, I, B and P. Of these, I
frames are complete images and can be used as random access points into the video stream. Encoders will usually
insert an I frame into the stream at fixed intervals, every 15 frames for example. The arrangement of B and P
frames following each I frame usually follows a fixed pattern. These repeating patterns are referred to as groups of
pictures (GOP). A GOP always begins with an I frame, and can contain an embedded timecode.
A GOP can be either closed or open. In a closed GOP the B-pictures following an I frame have been generated
using only backwards prediction (i.e. using previous frames). This means that the B frames can be decoded
following a random access. Closed GOPs are generally preferred over Open GOPs for this very reason.