Truncating a filter removes coefficients from one or both ends. You might want to truncate a filter to:

• Reduce the number of coefficients to be some multiple of a decimation or interpolation ratio for purposes of efficient computation. For example, if a filter has 42 coefficients and you are decimating by a factor of 8, you might want to truncate two coefficients to leave 40 coefficients.
• Eliminate small-magnitude end coefficients which have a negligible effect on the filter's frequency response, to decrease the number of calculations and amount of memory required to implement the filter

Note that truncating a filter will alter its frequency response (unless the truncated coefficients just happen to be zero.) In general, truncating a filter will increase its passband ripple and/or decrease its stopband attenuation, so truncating can cause a filter which previously met its design specification to fail. Therefore, whenever you truncate a filter, be sure to examine its new frequency response to verify that the truncated response meets your needs.

The Truncate dialog allows you to truncate a filter based on several criteria. You can Truncate By:

• Quantity - the number of coefficients to be truncated.
• Magnitude value - a magnitude threshold value below which coefficients will be truncated. (Note that small-valued coefficients in the middle of the filter will not be truncated--only those on the ends.)
• Magnitude below peak, in dB - a magnitude threshold given in terms of dB below the peak coefficient magnitude.
• Magnitude below sum, in dB - a magnitude threshold given in terms of dB below the sum of all coefficients. (Note that the coefficient sum is the filter's DC gain, so this option truncates relative to the DC gain.)

You can Truncate At:

• Each End - to truncate coefficients at each end. (If you Truncate By Quantity, the specified number of coefficients will be truncated at both the beginning and end.)
• Beginning - to truncate coefficients only at the beginning.
• End - to truncate coefficients only at the end.