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Altera_Forum
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17 years ago

FIR Compiler - Output Number System?

Hello everyone! I need some help on trying to understand the Output Number System for the FIR Compiler 8.0. My filter design is giving me an output of 34 bits, which is way to much. How do I go ...
Altera_Forum's avatar
Altera_Forum
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17 years ago

Hi,

If your filter is LPF then it is easy to precalculate the maximum output value and this equals input maximum dc x (sum of coefficients). You will then know which MSBs are never used and discard safely.

You may choose to discard a further MSB if it is only occasionally used by peaks. This is undesirable but may be inevitable. The best way is to rescale the coefficients so that the output swings in the relevant MSBs.

Once you know how many MSBs to discard then you will know how many LSBs to truncate off. In this case rounding may be done but it is trivial.