DEP function development on a low budget

An FIR filter has a very simple and regular structure, and hand coding a basic one is a very easy project. As a first step, create one with no more taps than your target FPGA has multipliers. (Hint : use the "Transpose Form")

Later you can learn tricks such as KCM (constant coefficient multipliers) and other ways to economise on hardware (many taps will have very small coefficients so their multiplications can reduce to a few additions).

Creating the filter coefficients is a bit more involved but you can use a little mathematical knowledge instead of the expensive tools. For example, the "window method" is as follows:

1) Decide the frequency response you want.

2) Convert that to an impulse response via Discrete Fourier Transform (DFT or Fast FT:FFT) or from knowledge of common results (perfect LPF = rectangle in frequency domain = sin(t)/t in time domain)

3) Shorten that impulse response to something finite, and with no. of time steps

In most cases, writing the VHDL to process the filter isn't the hard part, but getting the data in and out of the FPGA appropriately is.

Usually it is done in an FPGA for speed reasons, and the interfacing to get data in and out at high speed takes work.

Otherwise, for actual processing, look at the systolic array architecture.

-- glen

If your objective is to learn about DSP and FPGAs, then it is *better* to hand-code everything. The alternative is to merely fill in a few parameters and press a few buttons - after which all you know is how to fill in parameters and press a button.

Analogy: those people that have written a few assembler programs and looked at the output of a compiler have a much more useful understanding of what a computer is doing.

Once you know the basics, you are in a much better position to use the automated tools to best advantage.

For actual filter design, the tools are a godsend. No one sits around doing Remez exchanges and DFTs by hand. GNU Octave does most of what Matlab (basic) does, and is free.

Now you've got your filter coefficients, expressed in pure mathematical form. Start writing code; the machine generated stuff is crap anyhow and I know practically no one who uses it.

--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com 
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