MPEG video compression - The MPEG - page 205

Equipment which allows this is becoming available and its use can mean that the full economic life of a SDI routing
installation can be obtained.
An improved way of reducing concatenation loss has emerged from the ATLANTIC research project. [ 8 ] Figure 5.95
shows that the second encoder in a concatenated scheme does not make its own decisions from the incoming
video, but is instead steered by information from the first bitstream. As the second encoder has less intelligence, it
is known as a dim encoder.
Figure 5.95: In an ATLANTIC system, the second encoder is steered by information from the decoder.
The information bus carries all the structure of the original MPEG-2 bitstream which would be lost in a conventional
decoder. The ATLANTIC decoder does more than decode the pictures. It also places on the information bus all
parameters needed to make the dim encoder re-enact what the initial MPEG-2 encode did as closely as possible.
The GOP structure is passed on so that pictures are re-encoded as the same type. Positions of macroblock
boundaries become identical so that DCT blocks contain the same pixels and motion vectors relate to the same
screen data. The weighting and quantizing tables are passed so that coefficient truncation is identical. Motion
vectors from the original bitstream are passed on so that the dim encoder does not need to perform motion
estimation. In this way predicted pictures will be identical to the original prediction and the prediction error data will
be the same.
One application of this approach is in recompression, where an MPEG- 2 bitstream has to have its bit rate reduced.
This has to be done by heavier requantizing of coefficients, but if as many other parameters as possible can be
kept the same, such as motion vectors, the degradation will be minimized. In a simple recompressor just
requantizing the coefficients means that the predictive coding will be impaired. In a proper encode, the quantizing
error due to coding say an I picture is removed from the P picture by the prediction process. The prediction error of
P is obtained by subtracting the decoded I picture rather than the original I picture.
In simple recompression this does not happen and there may be a tolerance buildup known as drift. [ 9 ] A more
sophisticated recompressor will need to repeat the prediction process using the decoded output pictures as the
prediction reference.
MPEG-2 bitstreams will often be decoded for the purpose of switching. Local insertion of commercial breaks into a
centrally originated bitstream is one obvious requirement. If the decoded video signal is switched, the information
bus must also be switched. At the switch point identical reencoding becomes impossible because prior pictures
required for predictive coding will have disappeared. At this point the dim encoder has to become bright again
because it has to create an MPEG-2 bitstream without assistance.
It is possible to encode the information bus into a form which allows it to be invisibly carried in the serial digital
interface. Where a production process such as a vision mixer or DVE performs no manipulation, i.e. becomes bit
transparent, the subsequent encoder can extract the information bus and operate in 'dim' mode. Where a
manipulation is performed, the information bus signal will be corrupted and the encoder has to work in 'bright'
mode. The encoded information signal is known as a 'mole' [ 10 ] because it burrows through the processing
equipment!
There will be a generation loss at the switch point because the reencode will be making different decisions in bright
mode. This may be difficult to detect because the human visual system is slow to react to a vision cut and defects
in the first few pictures after a cut are masked.
In addition to the video computation required to perform a cut, the process has to consider the buffer occupancy of
the decoder. A downstream decoder has finite buffer memory, and individual encoders model the decoder buffer
occupancy to ensure that it neither overflows nor underflows. At any instant the decoder buffer can be nearly full or
nearly empty without a problem provided there is a subsequent correction. An encoder which is approaching a
complex I picture may run down the buffer so it can send a lot of data to describe that picture. Figure 5.96(a) shows
that if a decoder with a nearly full buffer is suddenly switched to an encoder which has been running down its buffer
occupancy, the decoder buffer will overflow when the second encoder sends a lot of data.