Retinal Prosthesis Dissertation - Student Home Pages

would be 12.78Mbit and is defined for the Society of Motion Pictures and Television
Engineers (SMPTE). Essentially these levels refer to the source format and selfevidently
the “high” level will be used for this discussion. Along with these four
levels referring to source format being defined, five profiles are described; the inputs
to all profiles are YUV component video. The “High Profile’, to be used in our
discussion, includes tools for improving signal-to-noise-ration (SNR), or spatial
resolution, plus the ability to code line-simultaneous colour-difference signals.
Encoding of video information is accomplished by spatial compression occurring on
` (I) ntra’ frames and temporal compression occurring on both ` (P) redictive’ frames
and ` (B) i-directional’ frames. I, P and B frames are grouped into a specified
sequence known as a group of pictures (GOP). The `I-frame’ is a compressed version
of a single uncompressed (raw) frame. The MPEG - I frame is a variation of the
Joint Photographic Expert Group (JPEG) format which uses the discrete cosine
transform (DCT), quantising, run length codes and Huffman coding.{(JPEGs are not
suitable for graphs, charts and explanatory explanations because the text appears
fuzzy, especially at low resolutions). P frames require an `I frame’ or `P frame’ to
forward predict motion estimation, B frames are encoded using motion compensated
prediction on the previous and next pictures, which must be either a B or P frame. B
frames are not used in subsequent predictions. It is better to use a shorter group
length for fast moving scenes otherwise pixellated frames occur. For this reason of
quality affected by rapid scene changes and excessive motion the sequence of I to B
frames must often be altered, this is done by I frame “forcing” i.e. adding I frames.
In this way each new scene can start with a fresh I frame for B and P frame
calculations. GOPs are optional in an MPEG-2 bitstream but are compulsory in DVD
video.
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