Digital Signal Processing Reference
In-Depth Information
8VSB is no longer effected by means of a simple analog vestigial side-
band filter as it used to be in analog TV. Today a Hilbert transformer and
an IQ modulator are used (Fig. 23.9.). The 8VSB baseband signal is split
into two paths. One path is directly applied to the I mixer, the other one is
taken via a Hilbert transformer to the Q mixer. A Hilbert transformer is a
90° phase shifter for all frequencies of the band to be filtered. Together
with the IQ modulator, it acts as a single sideband modulator; part of the
frequencies of the lower sideband are suppressed. Vestigial sideband filter-
ing of modern analog TV transmitters today follows the same principle. A
vital prerequisite for the quality of vestigial sideband filtering is the correct
setting and operation of the IQ modulator. This means identical gain in the
I and Q paths; moreover, the carrier supplied to the Q path must have a
phase of exactly 90°. Otherwise the unwanted part of the lower sideband
will not be fully suppressed, so that a residual carrier is obtained at the
band center.
MPEG2
TS
IF
VSB
mod.
FEC
RF
MUX
Sync
gen.
Segment and
field sync
Fig. 23.10.
8VSB modulator and transmitter
23.1 The 8VSB Modulator
After discussing the principle of ATSC modulation, let us take a closer
look at the 8VSB modulator (Fig. 23.10.). The ATSC-conformant
MPEG-2 transport stream, including PSIP tables, MPEG-2 video elemen-
tary streams and Dolby digital AC-3 audio elementary streams, is fed to
the forward error correction (FEC) block of the 8VSB modulator at a data
rate of exactly 19.3926585 Mbit/s. In the baseband interface, the input
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