Fundamentals - The MPEG - page 20

Figure 2.1: The major types of analog video. Red, green and blue signals emerge from the camera sensors,
needing full bandwidth. If a luminance signal is obtained by a weighted sum of R , G and B , it will need full
bandwidth, but the colour difference signals R - Y and B - Y need less bandwidth. Combining R - Y and B - Y into a
subcarrier modulation scheme allows colour transmission in the same bandwidth as monochrome.
Some compression can be obtained by using colour difference working. The human eye relies on brightness to
convey detail, and much less resolution is needed in the colour information. R , G and B are matrixed together to
form a luminance (and monochrome compatible) signal Y which has full bandwidth. The matrix also produces two
colour difference signals, R-Y and B-Y , but these do not need the same bandwidth as Y , one half or one quarter
will do depending on the application. Colour difference signals represent an early application of perceptive coding;
a saving in bandwidth is obtained by expressing the signals according to the way the eye operates.
Analog colour difference recorders such as Betacam and M II record these signals separately. The D-1 and D-5
formats record 525/60 or 625/50 colour difference signals digitally and Digital Betacam does so using compression.
In casual parlance, colour difference formats are often called component formats to distinguish them from
composite formats.
For colour television broadcast in a single channel, the PAL, SECAM and NTSC systems interleave into the
spectrum of a monochrome signal a subcarrier which carries two colour difference signals of restricted bandwidth.
As the bandwidth required for composite video is no greater than that of luminance, it can be regarded as a form of
compression performed in the analog domain. The artifacts which composite video introduces and the inflexibility in
editing resulting from the need to respect colour framing serve as a warning that compression is not without its
penalties. The subcarrier is intended to be invisible on the screen of a monochrome television set. A subcarrier-
based colour system is generally referred to as composite video, and the modulated subcarrier is called chroma.
It is not advantageous to compress composite video using modern transform-based coders as the transform
process cannot identify redundancy in a subcarrier. Composite video compression is restricted to differential coding
systems. Transform-based compression must use RGB or colour difference signals. As RGB requires excessive
bandwidth it makes no sense to use it with compression and so in practice only colour difference signals, which
have been bandwidth reduced by perceptive coding, are used in MPEG. Where signals to be compressed originate
in composite form, they must be decoded first. The decoding must be performed as accurately as possible, with
particular attention being given to the quality of the Y / C separation. The chroma in composite signals is deliberately
designed to invert from frame to frame in order to lessen its visibility. Unfortunately any residual chroma in
luminance will be interpreted by inter-field compression systems as temporal luminance changes which need to be
reproduced. This eats up data which should be used to render the picture. Residual chroma also results in high
horizontal and vertical spatial frequencies in each field which appear to be wanted detail to the compressor.
2.4 What is a digital signal?
One of the vital concepts to grasp is that digital audio and video are simply alternative means of carrying the same
information as their analog counterparts. An ideal digital system has the same characteristics as an ideal analog
system: both of them are totally transparent and reproduce the original applied waveform without error. Needless to
say, in the real world ideal conditions seldom prevail, so analog and digital equipment both fall short of the ideal.
Digital equipment simply falls short of the ideal to a smaller extent than does analog and at lower cost, or, if the