Fundamentals - The MPEG - page 32

In order to sample 625/50 luminance signals without quality loss, the lowest multiple possible is 4 which represents
a sampling rate of 13.5 MHz. This frequency line-locks to give 858 samples per line period in 525/59.94 and 864
samples per line period in 625/50.
In the component analog domain, the colour difference signals used for production purposes typically have one half
the bandwidth of the luminance signal. Thus a sampling rate multiple of 2 is used and results in 6.75 MHz. This
sampling rate allows respectively 429 and 432 samples per line. Component video sampled in this way has a 4:2:2
format. Whilst other combinations are possible, 4:2:2 is the format for which the majority of digital component
production equipment is constructed. The D-1, D-5, D-9, SX and Digital Betacam DVTRs operate with 4:2:2 format
data. Figure 2.20 shows the spatial arrangement given by 4:2:2 sampling.
Figure 2.20: In CCIR-601 sampling mode 4:2:2, the line synchronous sampling rate of 13.5 MHz results in samples
having the same position in successive lines, so that vertical columns are generated. The sampling rates of the
colour difference signals C R, C B are one-half of that of luminance, i.e. 6.75 MHz, so that there are alternate Y only
samples and co-sited samples which describe Y , C R and C B. In a run of four samples, there will be four Y
samples, two C R samples and two C B samples, hence 4:2:2.
Luminance samples appear at half the spacing of colour difference samples, and every other luminance sample is
co-sited with a pair of colour difference samples. Co-siting is important because it allows all attributes of one picture
point to be conveyed with a three-sample vector quantity. Modification of the three samples allows such techniques
as colour correction to be performed. This would be difficult without cosited information. Co-siting is achieved by
clocking the three ADCs simultaneously.
For lower bandwidths, particularly in prefiltering operations prior to compression, the sampling rate of the colour
difference signal can be halved. 4:1:1 delivers colour bandwidth in excess of that required by the composite
formats and is used in 60 Hz DV camcorder formats.
In 4:2:2 the colour difference signals are sampled horizontally at half the luminance sampling rate, yet the vertical
colour difference sampling rates are the same as for luma. Whilst this is not a problem in a production application,
this disparity of sampling rates represents a data rate overhead which is undesirable in a compression
environment. In this case it is possible to halve the vertical sampling rate of the colour difference signals as well,
producing a format known as 4:2:0.
This topic is the source of considerable confusion. In MPEG-1, which was designed for a low bit rate, the single
ideal 4:2:0 subsampling strategy of ( Figure 2.21 ( a) was used. The colour data are vertically low- pass filtered to the
same bandwidth in two dimensions and interpolated so that the remaining colour pixels are equidistant from the
source pixels. This has the effect of symmetrically disposing the colour information with respect to the luma. When
MPEG-2 was developed, it was a requirement to support 4:4:4, 4:2:2 and 4:2:0 colour structures. Figure 2.21 (b)
shows that in MPEG-2, the colour difference samples require no horizontal interpolation in 4:4:4 or 4:2:2 and so for
consistency they don't get it in 4:2:0 either. As a result there are two different 4:2:0 structures, one for MPEG-1 and
one for MPEG-2 and MPEG-4. In vertical subsampling a new virtual raster has been created for the chroma
samples. At the decoder a further interpolation will be required to put the decoded chroma data back onto the
display raster. This causes a small generation loss which is acceptable for a single-generation codec as used in
broadcasting, but not in multi-generation codecs needed for production. This problem was one of the reasons for
the development of the 4:2:2 Profile of MPEG-2 which avoids the problem by retaining the colour information on
every line.