Digital Terrestrial TV to North American ATSC Standard - Digital Video and Audio Broadcasting Technology - page 432

With a double-sided spectrum, the vectors representing the upper and
the lower sideband (each starting from the tip of the carrier vector) rotate
in opposite directions, thus varying the length of the carrier vector, i.e.
modulating the carrier. The carrier vector itself remains on the I axis. Even
if the carrier is suppressed, the sum vector yielded by the upper and the
lower sideband still remains on the I axis (Fig. 23.6.).

Q

Q

AM with
suppressed
carrier

Upper sideband Upper sideband

Carrier
vector

I

I

Lower sideband Lower sideband

Resulting vector always on I axis

Resulting vector always on I axis

Fig. 23.6. Vector diagram showing amplitude modulation with and without carrier

Q

Q

I

I

Upper sideband

8VSB constellation diagram
after vestigial sideband filter

I and Q compents caused
by vestigial sideband filtering

Fig. 23.7. Vector diagram and constellation diagram of an 8VSB signal

However, if one sideband is suppressed in part or completely, the result-
ing vector will swing about the I axis. Vestigial sideband filtering produces
a Q component. Such a Q component is also contained in analog vestigial
sideband filtered TV signals (Fig. 23.7.). Analog TV test receivers usually