Analog Television - Digital Video and Audio Broadcasting Technology - page 15

the luminance signal so that a black/white receiver was undisturbed but a
color receiver was able to reproduce both color and black/white correctly.
If a receiver falls short of these ideals, so-called cross-luminance and
cross-color effects are produced.
In all three systems, PAL, SECAM and NTSC, the Red, Green and Blue
color components are first acquired in three separate pickup systems (ini-
tially tube cameras, now CCD chips) and then supplied to a matrix where
the luminance signal is formed as the sum of R + G + B, and the chromi-
nance signal. The chrominance signal consists of two signals, the color dif-
ference signals Blue minus luminance and Red minus luminance. Howe-
ver, the luminance signal and the chrominance signal formed must be
matrixed, i.e. calculated, provided correctly with the appropriate weighting
factors according to the eye's sensitivity, using the following formula
Y = 0.3 • R + 0.59 • G + 0.ll • B;
U = 0.49 • (B-Y);
V = 0.88 • (R-Y);
The luminance signal Y can be used directly for reproduction by a
black/white receiver. The two chrominance signals are also transmitted
and are used by the color receiver. From Y, U and V it is possible to reco-
ver R, G and B. The color information is then available in correspondingly
reduced bandwidth, and the luminance information in greater bandwidth
(“paintbox principle”).
To embed the color information into a CVBS (composite video, blan-
king and sync) signal intended initially for black/white receivers, a method
had to be found which has the fewest possible adverse effects on a
black/white receiver, i.e. keeps it free of color information, and at the same
time contains all that is necessary for a color receiver.
Two basic methods were chosen, namely embedding the information
either by analog amplitude/phase modulation (IQ modulation) as in PAL
or NTSC, or by frequency modulation as in SECAM. In PAL and NTSC,
the color difference signals are supplied to an IQ modulator with a reduced
bandwidth compared to the luminance signal (Fig. 2.8.) The IQ modulator
generates a chrominance signal as amplitude/phase modulated color sub-
carrier, the amplitude of which carries the color saturation and the phase of
which carries the hue. An oscilloscope would only show, therefore, if there
is color, and how much, but would not identify the hue. This would require
a vectorscope which supplies information on both.
In PAL and in NTSC, the color information is modulated onto a color
subcarrier which lies within the frequency band of the luminance signal