Page 280 - Satellite Communications, Fourth Edition

260 Chapter Nine

transmitted within the luminance bandwidth by quadrature DSBSC
(as seen later), modulating them onto a subcarrier which places them
at the upper end of the luminance signal spectrum. Use is made of the
fact that the eye cannot readily perceive the interference which results
when the chrominance signals are transmitted within the luminance
signal bandwidth. The baseband response is shown in Fig. 9.5.
Different methods of chrominance subcarrier modulation are
employed in different countries. In France, a system known as sequen-
tial couleur a mémoire (SECAM) is used. In most other European coun-
tries, a system known as phase alternation line (PAL) is used. In North
America, the NTSC system is used, where NTSC stands for National
Television System Committee.
In the NTSC system, each chrominance signal is modulated onto its
subcarrier using DSBSC modulation, as described in Sec. 9.3. A single
oscillator source is used so that the I and Q signal subcarriers have the
same frequency, but one of the subcarriers is shifted 90° in phase to pre-
serve the separate chrominance information in the I and Q baseband sig-
nals. This method is known as quadrature modulation (QM). The I
signal is the chrominance signal which modulates the in-phase carrier.
Its bandwidth in the NTSC system is restricted to 1.5 MHz, and after
modulation onto the subcarrier, a single-sideband filter removes the
upper sideband components more than 0.5 MHz above the carrier. This
is referred to as a vestigial sideband (VSB). The modulated I signal
therefore consists of the 1.5-MHz lower sideband plus the 0.5 MHz
upper VSB.
The Q signal is the chrominance signal which modulates the quad-
rature carrier. Its bandwidth is restricted to 0.5 MHz, and after modu-
lation, a DSBSC signal results. The spectrum magnitude of the combined
I and Q signals is shown in Fig. 9.5.
The magnitude of the QM envelope contains the color saturation infor-
mation, and the phase angle of the QM envelope contains the hue, or tint,

Figure 9.5 Frequency spectra for the luminance and chromi-
nance signals.