L64777 A LSI Logic Corporation, L64777 A Datasheet - Page 14

• Manufacturer Part Number: L64777 A
• Description: Communications, DVB QAM Modulator
• Manufacturer: LSI Logic Corporation

Bytes to M-tuples Converter
Differential Encoder and QAM mapping
Square Root Nyquist Filter
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The values of the interleaver in the L64777 are: N = 204, B = 12, and
M = 17. You can switch off the interleaver. It is fully transparent with an
intrinsic delay of three clock cycles.
The main modules are a set of configured RAM-based delay lines to
implement the proper delay for individual data bytes, and a controller to
handle and generate the strobes needed by subsequent modules in the
data path.
This unit cuts down bytes to slices of m = 1, 2, 3, 4, 5, 6, 7, and 8 bits.
The programming parameter, mSize, must be set to m
the MSB of the oldest byte first. See the Digital Broadcasting Systems
for Television Sound and Data Services: Framing Structure, Channel
Coding and Modulation Cable Systems for a detailed specification. When
cutting six-bit symbols, it cuts three bytes into four symbols. The case of
four-bit symbols is trivial (eight bits are split into two sets of four bits
each).
This block performs differential encoding and mapping for 16 and 64
QAM, as specified in the Digital Broadcasting Systems for Television
Sound and Data Services: Framing Structure, Channel Coding and
Modulation Cable Systems , the baseline document, and its extensions.
The QAM 256 mapping is taken from the DVB document 1190.
The encoder performs differential encoding on the two most significant
bits of each symbol.
This pulse-shaper module implements a programmable square-root
raised cosine filtering function with a default of 15% roll-off factor. The
precision of the internal Nyquist filter computations, and the width of the
output data bus, are sufficient for QAM modulations up to 256. The filter
operates at four times the oversampling rate.
Each of the two I and Q branches has one filter, realized as polyphase
structures. Each filter consists of four filter branches, which compute
1-phase filter results at the symbol rate. Thus, the L64777 Nyquist filter
L64777 DVB QAM Modulator
1. The order is