Information Technology Reference
In-Depth Information
5.93(b) shows that non-identical concatenation can result from a large number of real-world effects.
An intermediate processing step such as a fade will change the pixel values and thereby the coefficients. A DVE
resize or shift will move pixels from one DCT block to another. Even if there is no processing step, this effect will
also occur if the two codecs disagree on where the MPEG picture boundaries are within the picture. If the
boundaries are correct there will still be concatenation loss if the two codecs use different weighting.
One problem with MPEG is that the compressor design is unspecified. Whilst this has advantages, it does mean
that the chances of finding identical coders is minute because each manufacturer will have his own views on the
best compression algorithm. In a large system it may be worth obtaining the coders from a single supplier.
It is now increasingly accepted that concatenation of compression techniques is potentially damaging, and results
are worse if the codecs are different. Clearly feeding a digital coder such as MPEG-2 with a signal which has been
subject to analog compression comes into the category of worse. Using interlaced video as a source for MPEG
coding is sub- optimal and using decoded composite video is even worse.
One way of avoiding concatenation is to stay in the compressed data domain. If the goal is just to move pictures
from one place to another, decoding to traditional video so that an existing router can be used is not ideal, although
it is substantially better than going through the analog domain.
Figure 5.94 shows some possibilities for picture transport. Clearly if the pictures exist as a compressed file on a
server, a file transfer is the right way to do it as there is no possibility of loss because there has been no
concatenation. File transfer is also quite indifferent to the picture format. It doesn't care about the frame rate,
whether the pictures are interlaced or not or whether the colour is 4:2:0 or 4:2:2
Figure 5.94: Compressed picture transport mechanisms contrasted.
Decoding to SDI (serial digital interface) standard is sometimes done so that existing serial digital routing can be
used. This is concatenation and has to be done carefully. The compressed video can only use interlace with non-
square pixels and the colour coding has to be 4:2:2 because SDI only allows that. If a compressed file has 4:2:0
the chroma has to be interpolated up to 4:2:2 for SDI transfer and then subsampled back to 4:2:0 at the second
coder and this will cause generation loss. An SDI transfer also can only be performed in real time, thus negating
one of the advantages of compression. In short traditional SDI is not really at home with compression.
As 4:2:0 progressive scan gains popularity and video production moves steadily towards non-format-specific
hardware using computers and data networks, use of the serial digital interface will eventually decline. In the short
term, if an existing SDI router has to be used, one solution is to produce a bitstream which is sufficiently similar to
SDI that a router will pass it. One example of this is known as SDTI. The signal level, frequency and impedance of
SDTI is pure SDI, but the data protocol is different so that a bit accurate file transfer can be performed. This has
two advantages over SDI. First, the compressed data format can be anything appropriate and non-interlaced
and/or 4:2:0 can be handled in any picture size, aspect ratio or frame rate. Second, a faster than real-time transfer
can be used depending on the compression factor of the file.
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