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* The configuration constants below govern
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* the number of bits in the input sample and filter coefficients, the
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* number of bits to the right of the binary-point for fixed-point math, etc.
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/* Conversion constants */
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#define Amask ((1<<Na)-1)
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#define Pmask ((1<<Np)-1)
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/* Description of constants:
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* Npc - is the number of look-up values available for the lowpass filter
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* between the beginning of its impulse response and the "cutoff time"
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* of the filter. The cutoff time is defined as the reciprocal of the
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* lowpass-filter cut off frequence in Hz. For example, if the
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* lowpass filter were a sinc function, Npc would be the index of the
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* impulse-response lookup-table corresponding to the first zero-
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* crossing of the sinc function. (The inverse first zero-crossing
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* time of a sinc function equals its nominal cutoff frequency in Hz.)
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* Npc must be a power of 2 due to the details of the current
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* implementation. The default value of 512 is sufficiently high that
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* using linear interpolation to fill in between the table entries
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* gives approximately 16-bit accuracy in filter coefficients.
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* Nhc - is log base 2 of Npc.
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* Na - is the number of bits devoted to linear interpolation of the
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* filter coefficients.
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* Np - is Na + Nhc, the number of bits to the right of the binary point
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* in the integer "time" variable. To the left of the point, it indexes
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* the input array (X), and to the right, it is interpreted as a number
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* between 0 and 1 sample of the input X. Np must be less than 16 in
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* this implementation.
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* Nh - is the number of bits in the filter coefficients. The sum of Nh and
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* the number of bits in the input data (typically 16) cannot exceed 32.
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* Thus Nh should be 16. The largest filter coefficient should nearly
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* fill 16 bits (32767).
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* Nb - is the number of bits in the input data. The sum of Nb and Nh cannot
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* Nhxn - is the number of bits to right shift after multiplying each input
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* sample times a filter coefficient. It can be as great as Nh and as
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* small as 0. Nhxn = Nh-2 gives 2 guard bits in the multiply-add
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* accumulation. If Nhxn=0, the accumulation will soon overflow 32 bits.
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* Nhg - is the number of guard bits in mpy-add accumulation (equal to Nh-Nhxn)
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* NLpScl - is the number of bits allocated to the unity-gain normalization
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* factor. The output of the lowpass filter is multiplied by LpScl and
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* then right-shifted NLpScl bits. To avoid overflow, we must have