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Yes. The (software) DSP guy has just explained that to me. The 12 muliplier decimator is working great thank you!!!!
I see I can apply a similar but easier method to interpolation just by ordering the coefficients correctly.
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pre add increases mult inputs by one bit and may lead to extra resource if mults don't support the increased bitwidth.
With interpolation there are no spare clock ticks but there are alternating zeros in input (physically or assumed) and so you arrange coeffs as polyphases by skipping prototype filter regularly. Pre-add may fail here due to loss of symmetry except for some values of interpolation.
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