Digital Terrestrial TV to North American ATSC Standard - Digital Video and Audio Broadcasting Technology - page 441

8VSB pilot signal, and is found exactly at the center of the 8VSB modula-
tion product before it is subjected to vestigial sideband filtering. As a dou-
ble-sided spectrum, the modulation product would occupy bandwidth at
least as wide as the symbol rate. The symbol rate is 10.76 MS/s, so the
minimum required bandwidth is 10.76 MHz. The channel bandwidth in the
North American ATSC TV system is, however, only 6 MHz. As in analog
TV, therefore, the 8VSB signal is vestigial sideband filtered after ampli-
tude modulation, i.e. the major part of the lower sideband is suppressed.
This could be done by means of a conventional analog vestigial sideband
filter; this method is today no longer employed however, not even by mod-
ern analog TV transmitters. Instead, the 8VSB baseband signal with its pi-
lot DC component is split into two signals: One is taken directly to an I
mixer and the other first to a Hilbert transformer and then to a Q mixer
(Fig. 23.20.).

Pilot addition

DC +1.25

VSB
filter

+

+7

+5

+3

+1
-1

-3

-5

Carrier

-7

Fig. 23.19. 8VSB modulation with pilot

A Hilbert transformer is a 90° phase shifter for all frequencies of a band.
The Hilbert transformer in conjunction with the IQ modulator causes par-
tial suppression of the lower sideband, which is obtained due to the con-
figuration of the amplitudes and phases involved. The resulting 8VSB
spectrum only contains the upper sideband and a vestigial lower sideband.
Moreover, a spectral line is found at the previous band center, i.e. the band
center before vestigial sideband filtering. This spectral line results from the
added DC component and is referred to as pilot carrier. The 8VSB spec-
trum is Nyquist filtered with a roll-off factor of r = 0.115. After VSB
modulation, the signal is converted to RF. This conversion is today usually
effected by direct modulation simultaneously with VSB modulation. An