ref: fa894e6a3fc1f13f494aaae3089b457eda96dfba
dir: /src/resampl.h/
/* * FILE: resample.h * BY: Julius Smith (at CCRMA, Stanford U) * C BY: translated from SAIL to C by Christopher Lee Fraley * (cf0v@andrew.cmu.edu) * DATE: 7-JUN-88 * VERS: 2.0 (17-JUN-88, 3:00pm) */ #define MAXNWING 5122 #define MAXFACTOR 4 /* Maximum Factor without output buff overflow */ /* 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. */