Digital Signal Processing Reference
In-Depth Information
The field sync, like a data segment, starts with a data segment sync. In-
stead of normal data, however, this data segment sync contains a number
of pseudo random sequences, the VSB mode information, and some spe-
cial, reserved symbols. The VSB mode bits carry the 8VSB/16VSB mode
information. 16VSB was intended for cable transmission, but has not been
implemented in practice.
Data
segment
sync
Data
segment
sync
+7
+5
+3
+1
1
3
5
7
Level
before
pilot
addition
4 symbols
(372 ns)
4 symbols
Data segment
832 symbols
208 byte (77.3 s)
Fig. 23.18.
8VSB field sync
Terrestrial transmission employs the 8VSB mode. The pseudo random
sequences contained in the field sync are used as training sequences by the
channel equalizer in the receiver. Moreover, it is the pseudo random se-
quences by which the receiver detects the field sync and is thus able to
synchronize to the frame structure. During the field sync, the randomizer
block is reset in the modulator and in the receiver. The resulting 8VSB
baseband signal, consisting of field syncs and data segments, is taken to
the 8VSB modulator. Prior to amplitude modulation, a relative DC com-
ponent of +1.25 is added to the 8 level signal. Prior to this addition, the
8VSB signal has discrete amplitude stages of -7, -5, -3, -1, +1, +3, +5 and
+7. Adding the DC component shifts all 8VSB levels by a relative value of
+1.25.
Amplitude modulation of a baseband signal, no longer free of DC but
with a mixer signal actually free of carrier, however, produces a signal
with a carrier component. This carrier component is referred to as an
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