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216 CHAPTER EIGHT
DSP can be used as a string of FIR registers and coefficient registers. Also, structures
are used to move data efficiently through the DSP chip as rapidly as possible. DSP pro-
grammers can take advantage of many library functions. Implementing a simple FIR
filter can be accomplished just by specifying the number of taps and the coefficients.
The DSP compiler takes care of the rest of the work.
Hardware
Well, enough ranting about software and hardware people. The sad truth is, we need
each other. Even the pure hardware implementation of FIR filters requires a significant
amount of software tools and programming. Prepackaged implementations of FIR fil-
ters are available, but not common. The most common way they are implemented is in
Application-Specific Integrated Circuits (ASICs) or FPGAs. FPGAs contain many reg-
isters and logic elements that can be configured using software. The software is typi-
cally written in higher-level languages like VHDL or Verilog. The VHDL code lines
engender tap registers, coefficient registers, and Multiply and Accumulate (MACs). The
entire FIR filter structure is visible right in the code itself. When the VHDL code is
compiled and loaded into an FPGA, the FIR filter takes on a physical instantiation.
Here are some web sites describing FIR filter design in such languages:

www.doulos.com/fi/vhdl_models/model_9605.html
www.item.uni-bremen.de/research/papers/paper.pdf/Helge.Bochnik/nato93/
boc9301.pdf
www.altera.com/support/examples/verilog/ver_base_fir.html

Testing FIR Filters
Several easy tests can be run on a FIR filter design when it is first tested. Some tests
are so simple they can be built right into the physical implementation. This allows the
test to be executed at a later time. The FIR filter tests are as follows:
Coefficient test Feed the FIR filter a series of data points consisting of all
zeroes with a single full value in the middle of the stream. As the full value hits
each FIR filter tap along the way, the output will be a serial stream equal to all the
coefficients right in order.
Frequency sweep To test any filter, analog or DSP, sweep it with a series of pure
sine waves. The frequency response curve should be similar to that shown in the
DSP design software. Further, if we continue the sine wave sweep above the
Nyquist frequency, we should observe the effects of the antialias filter. If we