Tektronix RFA300A, Руководство пользователя

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Reference
3- 14
RFA300A Measurement Set 8VSB
8VSB Overview
This section provides an overview of the 8VSB transmitter. For a complete
understanding of the transmitter, refer to Advanced Televisions Systems
Committee (ATSC) Digital Television Standard document A/53. This document
is available from the World Wide Web at ftp://atsc.org/pub/Standards.
The 8VSB standard is a RF modulation format used to broadcast Digital
Television (DTV) to the home. This format was proposed by the ATSC and
adopted by the FCC. The 8VSB transmitter generates the 8VSB signal as shown
in Figure 3--14. The transmitter must take the bit-stream output from a MPEG II
encoder and transmit it on a 6 MHz RF channel.
Data
Randomizer
Reed-Solomon
Encoder
Data
Interleaver
Trellis
Encoder
Pilot
Insertion
VSB
Modulator
RF
Upconverter
RFA300A
Input Signal
MUX
Segment Sync
Field Sync
Figure 3- 14: 8VSB exciter block diagram
To use the 6 MHz channel space with maximum efficiency, the 8VSB bit stream
must be randomized with a pseudo-random bitstream. This ensures a flat
noise-like spectrum and reduces interference to NTSC channels. A recurring
rhythmic pattern causes the RF energy content to cluster at certain areas in the
frequency spectrum. Therefore, without additional processing, certain parts of
the 6 MHz channel could be overused, while other areas are underused. The Data
Randomizer changes each byte value to a known pseudo-random binary
sequence (PRBS). The PRBS process is reversed in the television receiver to
recover the correct data value.
The Reed-Solomon (RS) encoder uses Forward Error Correction (FEC) to
correct received bit errors that can occur during transmission. The RS encoder
takes the 187-byte data block and adds 20 parity bytes. The receiver compares
the 187-byte block to the 20 parity bytes to determine if the data is valid. RS
encoding can correct up to 10 byte errors per one 187-byte data block (packet).
Data Randomizer
Reed-Solomon Encoder