| /* |
| * The configuration constants below govern |
| * the number of bits in the input sample and filter coefficients, the |
| * number of bits to the right of the binary-point for fixed-point math, etc. |
| * |
| */ |
| |
| /* Conversion constants */ |
| #define Nhc 8 |
| #define Na 7 |
| #define Np (Nhc+Na) |
| #define Npc (1<<Nhc) |
| #define Amask ((1<<Na)-1) |
| #define Pmask ((1<<Np)-1) |
| #define Nh 16 |
| #define Nb 16 |
| #define Nhxn 14 |
| #define Nhg (Nh-Nhxn) |
| #define NLpScl 13 |
| |
| /* Description of constants: |
| * |
| * Npc - is the number of look-up values available for the lowpass filter |
| * between the beginning of its impulse response and the "cutoff time" |
| * of the filter. The cutoff time is defined as the reciprocal of the |
| * lowpass-filter cut off frequence in Hz. For example, if the |
| * lowpass filter were a sinc function, Npc would be the index of the |
| * impulse-response lookup-table corresponding to the first zero- |
| * crossing of the sinc function. (The inverse first zero-crossing |
| * time of a sinc function equals its nominal cutoff frequency in Hz.) |
| * Npc must be a power of 2 due to the details of the current |
| * implementation. The default value of 512 is sufficiently high that |
| * using linear interpolation to fill in between the table entries |
| * gives approximately 16-bit accuracy in filter coefficients. |
| * |
| * Nhc - is log base 2 of Npc. |
| * |
| * Na - is the number of bits devoted to linear interpolation of the |
| * filter coefficients. |
| * |
| * Np - is Na + Nhc, the number of bits to the right of the binary point |
| * in the integer "time" variable. To the left of the point, it indexes |
| * the input array (X), and to the right, it is interpreted as a number |
| * between 0 and 1 sample of the input X. Np must be less than 16 in |
| * this implementation. |
| * |
| * Nh - is the number of bits in the filter coefficients. The sum of Nh and |
| * the number of bits in the input data (typically 16) cannot exceed 32. |
| * Thus Nh should be 16. The largest filter coefficient should nearly |
| * fill 16 bits (32767). |
| * |
| * Nb - is the number of bits in the input data. The sum of Nb and Nh cannot |
| * exceed 32. |
| * |
| * Nhxn - is the number of bits to right shift after multiplying each input |
| * sample times a filter coefficient. It can be as great as Nh and as |
| * small as 0. Nhxn = Nh-2 gives 2 guard bits in the multiply-add |
| * accumulation. If Nhxn=0, the accumulation will soon overflow 32 bits. |
| * |
| * Nhg - is the number of guard bits in mpy-add accumulation (equal to Nh-Nhxn) |
| * |
| * NLpScl - is the number of bits allocated to the unity-gain normalization |
| * factor. The output of the lowpass filter is multiplied by LpScl and |
| * then right-shifted NLpScl bits. To avoid overflow, we must have |
| * Nb+Nhg+NLpScl < 32. |
| */ |
| |