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DIGITAL SIGNAL PROCESSING (DSP) 207
DSP FILTERS
There’s no reason not to make an antialias filter using DSP techniques. We’ll be dis-
cussing how to synthesize a DSP filter next. Here are some good web sites and a PDF
file covering antialiasing filters:
www.alligatortech.com/why_low_pass_filtering_is_always_necessary.htm
www.dactron.com/pdf/appnote/aliasprotection.pdf
http://kabuki.eecs.berkeley.edu/ danelle/arpa_0697/arpa.html
http://members.ozemail.com.au/ timhoop/intro.htm

D/A Effects: Sinc Compensation

At the output of the DSP system, the D/A generates an output stream of analog values.
The D/A only outputs a series of analog values that look like a rectangular staircase of
constant voltages. Thus, the D/A inherently alters the output signal with the sinc func-
tion, which we’ll discuss again shortly. What’s needed within the DSP filter is an anti-
sinc compensation filter.
This antisinc precompensation filter can reside inside the DSP compute engine. Let’s
say the DSP compute engine generates D/A output values at a rate of N per second. The
antisinc predistortion computations are now added at the tail end of the DSP compute
engine. Just how this is done is up to the designer. Since all these systems are assumed
to be Linear Time Invariant systems, the antisinc filter can simply be added right into
the middle of the DSP calculations. The previous D/A results are fed into this new com-
pute block that runs computations for the antisinc compensation. The result is a new
compute block outputting a stream of D/A values at a rate faster than rate N. The D/A
will then run at a higher rate than normal. We smooth out the D/A values with a simple
low-pass filter at the D/A clock rate. The resulting output waveform will not be overly
distorted by the sinc effect. Note that running the D/A at a faster rate will mean higher
energy consumption.
Here are some PDFs further discussing sinc precompensation:

http://pdfserv.maxim-ic.com/arpdf/AppNotes/A0509.pdf
www.lavryengineering.com/pdfs/sample.pdf
www.ee.oulu.fi/ timor/EC_course/chp_1.pdf