MPEG video compression - The MPEG - page 143

These requirements are met by sending two colour difference signals along with Y . There are three possible colour
difference signals, R - Y , B - Y and G - Y . As the green signal makes the greatest contribution to Y , then the
amplitude of G - Y would be the smallest and would be most susceptible to noise. Thus R - Y and B - Y are used in
practice as Figure 5.13 shows.
R and B are readily obtained by adding Y to the two colour difference signals. G is obtained by rearranging the
expression for Y above such that:
G = Y - 0.3 R - 0.11 B
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If a colour CRT is being driven, it is possible to apply inverted luminance to the cathodes and the R - Y and B - Y
signals directly to two of the grids so that the tube performs some of the matrixing. It is then only necessary to
obtain G - Y for the third grid, using the expression:
G - Y = 0.51( R - Y ) - 0.186( B - Y )
If a monochrome source having only a Y output is supplied to a colour display, R - Y and B - Y will be zero. It is
reasonably obvious that if there are no colour difference signals the colour signals cannot be different from one
another and R = G = B . As a result the colour display can produce only a neutral picture.
The use of colour difference signals is essential for compatibility in both directions between colour and
monochrome, but it has a further advantage that follows from the way in which the eye works. In order to produce
the highest resolution in the fovea, the eye will use signals from all types of cone, regardless of colour. In order to
determine colour the stimuli from three cones must be compared. There is evidence that the nervous system uses
some form of colour difference processing to make this possible. As a result the full acuity of the human eye is only
available in monochrome. Differences in colour cannot be resolved so well. A further factor is that the lens in the
human eye is not achromatic and this means that the ends of the spectrum are not well focused. This is particularly
noticeable on blue.
If the eye cannot resolve colour very well there is no point is expending valuable bandwidth sending high-resolution
colour signals. Colour difference working allows the luminance to be sent separately at a bit rate which determines
the subjective sharpness of the picture. The colour difference signals can be sent with considerably reduced bit
rate, as little as one quarter that of luminance, and the human eye is unable to tell.
The overwhelming advantages obtained by using downsampled colour difference signals mean that in broadcast
and production facilities their use has become almost universal. The technique is equally attractive for compression
applications and is retained in MPEG. The outputs from the RGB sensors in most cameras are converted directly to
Y , R - Y and B - Y in the camera control unit and output in that form. Whilst signals such as Y , R , G and B are
unipolar or positive only, it should be stressed that colour difference signals are bipolar and may meaningfully take
on levels below zero volts.
The downsampled colour formats used in MPEG were described in section 2.9 .
5.6 Progressive or interlaced scan?
Analog video samples in the time domain and vertically down the screen so a two-dimensional vertical/temporal
sampling spectrum will result. In a progressively scanned system there is a rectangular matrix of sampling sites
vertically and temporally. The rectangular sampling structure of progressive scan is separable which means that,
for example, a vertical manipulation can be performed on frame data without affecting the time axis. The sampling
spectrum will be obtained according to section 2.6 and consists of the baseband spectrum repeated as sidebands
above and below harmonics of the two-dimensional sampling frequencies. The corresponding spectrum is shown in
Figure 5.14. The baseband spectrum is in the centre of the diagram, and the repeating sampling sideband
spectrum extends vertically and horizontally. The vertical aspects of the star-shaped spectrum result from vertical
spatial frequencies in the image. The horizontal aspect is due to image movement. Note that the star shape is
rather hypothetical; the actual shape depends heavily on the source material. On a still picture the horizontal
dimensions collapse to a line structure. In order to return a progressive scan video signal to a continuous moving
picture, a two-dimensional low-pass filter having a rectangular response is required. This is quite feasible as
persistence of vision acts as a temporal filter and by sitting far enough away from the screen the finite acuity of the
eye acts as a spatial reconstruction filter.