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FIR Filter Artifacting

Question asked by ChadK on Nov 30, 2012
Latest reply on Dec 12, 2012 by SachinV

I've implemented an FIR filter on the BF518 EzBoard to filter the audio and send it back out, both via the SSM2603 codec (24-bit in & out at 96kSa/s).  The filter looks good, except I find that I have a spectral artifact around 3*f (18 kHz), where f is the input frequency in the passband (6 kHz).  I'm at a loss as to what is causing this.  I've tried bit shifting the accumulator output to make sure I was catching an overflow, but that only dropped the output amplitude of the whole signal.  When I shift the input signal up to 3*f, it is effectively suppressed.  Attached is a capture of the output (blue trace) with an overlay of the spectrum of the input signal.  Is there something simple I'm missing?