Specifications

8-VSB Agile Modulator Issue 1.02 December 9, 2008 Page 14 of 29
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Principle of operation
The input of 8-VSB modulator is a MPEG-2 Transport Stream (TS) based on MPEG-2 at
a fixed bit rate of 19.39265846Mbs. The TS has 188 byte packet structure. The 8-VSB
spectrum is contained in a 6 MHz channel.
The first byte (sync byte) of the MPEG-2 data packet is stripped and the remaining 187
bytes are randomized using a polynomial generator to prevent long strings of fixed
states (1 or 0) that may occur and could increase chances of error.
The Reed Solomon encoder operates on blocks of 187 data bytes to produce blocks of
207 bytes (data plus forward error correction bytes).
The data is then interleaved in order to spread the bits to make it more immune to burst
noise.
The trellis encoder converts each byte to four 8-level channel symbols. This is important
for the modulation stage.
The Mux inserts a sync byte after each 828 symbols. This is inserted to recover
synchronization of the MPEG packets as the MPEG sync bytes are removed.
The low level pilot is added to the baseband data. The low level pilot is 11.3 dB below
the average data signal power. The pilot is inserted 310 KHz from the lower band edge.
The low level pilot aids carrier recovery and is independent of data.
The VSB modulator processes symbols to produce a very short pulse occurring at
exactly the center of the symbol times but still maintains their proper 8-levels of
amplitude. These short pulses are then applied to a raised cosine filter (Nyquist Filter).
The signal is then up-converted to 44 MHz IF and then to the desired RF channel.
The output of RF channel coming from the modulator is amplified by the power amplifier.