Information Technology Reference
In-Depth Information
Figure 5.92:
In composite video the subcarrier frequency is arranged so that inversions occur between adjacent
lines and pictures to help reduce the visibility of the chroma.
Whilst this makes the subcarrier invisible to the eye, it is not invisible to an MPEG decoder. The subcarrier
waveform is interpreted as a horizontal frequency, the vertical phase reversals are interpreted as a vertical spatial
frequency and the picture-to-picture reversals increase the magnitude of the prediction errors. The subcarrier level
may be low but it can be present over the whole screen and require an excess of coefficients to describe it.
Composite video should not in general be used as a source for MPEG encoding, but where this is inevitable the
standard of the decoder must be much higher than average, especially in the residual subcarrier specification.
Some MPEG preprocessors support high-grade composite decoding options.
Judder from conventional linear standards convertors degrades the performance of MPEG. The optic flow axis is
corrupted and linear filtering causes multiple images which confuse motion estimators and result in larger
prediction errors. If standards conversion is necessary, the MPEG system must be used to encode the signal in its
original format and the standards convertor should be installed after the decoder. If a standards convertor has to be
used before the encoder, then it must be a type which has effective motion compensation.
Film weave causes movement of one picture with respect to the next and this results in more vector activity and
larger prediction errors. Movement of the centre of the film frame along the optical axis causes magnification
changes which also result in excess prediction error data. Film grain has the same effect as noise: it is random and
so cannot be compressed.
Perhaps because it is relatively uncommon, MPEG-2 cannot handle image rotation well because the motion-
compensation system is only designed for translational motion. Where a rotating object is highly detailed, such as
in certain fairground rides, the motion compensation failure requires a significant amount of prediction error data
and if a suitable bit rate is not available the level of artifacts will rise. The additional coding tools of MPEG-4 allow
rotation be handled more effectively. Meshes can be rotated by repositioning their vertices with a few vectors and
this improves the prediction of rotations dramatically.
Flash guns used by still photographers are a serious hazard to MPEG especially when long GOPs are used. At a
press conference where a series of flashes may occur, the resultant video contains intermittent white frames.
These are easy enough to code, but a huge prediction error is required to return the white frame to the next picture.
The output buffer fills and heavy requantizing is employed. After a few flashes the picture has generally gone to
tiles.
5.32 MPEG and concatenation
Concatenation loss occurs when the losses introduced by one codec are compounded by a second codec. All
practical compressers, MPEG included, are lossy because what comes out of the decoder is not bit- identical to
what went into the encoder. The bit differences are controlled so that they have minimum visibility to a human
viewer.
MPEG is a toolbox which allows a variety of manipulations to be performed both in the spatial and the temporal
domain. MPEG-2 has more tools than MPEG-1 and MPEG-4 has more still. There is a limit to the compression
which can be used on a single picture/object, and if higher compression factors are needed, temporal coding will
have to be used. The longer the run of time considered, the lower the bit rate needed, but the harder it becomes to
edit.
