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70 Xilinx Development System
Xilinx System Generator v2.1 Reference Guide
• Traceback Length: Length of the traceback through the Viterbi trellis. Optimal
length is considered to be between 5 and 7 times the constraint length.
• Convolution Code 1: Used to decode data on input port din1. Length of
convolution code must be between 3 and 9 (inclusive).
• Convolution Code 2: Used to decode data on input port din2. Length of
convolution code must be between 3 and 9 (inclusive).
• Convolution Code 3: Used to decode data on input port din3. Length of
convolution code must be between 3 and 9 (inclusive). This parameter is only
available for encoder output rate of 3.
• Coding: Hard or Soft. Hard coding uses the Hamming metric to calculate
the difference between the input and the branches in the Viterbi trellis. Hard
coding requires the input data to be 1 bit wide. Soft coding uses the Euclidean
metric to cost the incoming data against the branches of the Viterbi trellis. When
using soft coding, the input port widths must be between 3 and 8 bits.
• Data Format: Signed Magnitude and Offset Binary (available for Soft Coding
only).
Other parameters used by this block are described in the Common Parameters section
of the previous chapter.
The Viterbi Decoder block cannot be placed in an enabled subsystem in System
Generator v2.1. See the Enabled Subsystems section (within the MATLAB I/O library
documentation) explanation for more details.
Xilinx LogiCore
The Viterbi Decoder block uses Xilinx LogiCORE: Viterbi v1.0.
The Core datasheet can be found on your local disk at:
%XILINX%\coregen\ip\xilinx\eip1\com\xilinx\ip\viterbi_v1_0\doc
\viterbi.pdf
This is a licensed core, available for purchase on the Xilinx web site at:
http://www.xilinx.com/ipcenter/viterbi
DSP
This library contains blocks that implement Digital Signal Processing (DSP) specific
functions.
CIC
Cascaded integrator-comb (CIC) filters are multirate filters used
for realizing large sample rate changes in digital systems. Both
decimation and interpolation structures are supported. CIC
filters contain no multipliers; they consist only of adders,
subtractors and registers. They are typically employed in
applications that have a large excess sample rate; that is, the
system sample rate is much larger than the bandwidth occupied
by the signal. CIC filters are frequently used in digital down-
converters and digital up-converters.